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Influence of phonation on high-intensity sound transmission in the auditory system McBay, Heather Dorrelle 1971

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INFLUENCE OF PHONATION ON HIGH-INTENSITY SOUND TRANSMISSION IN THE AUDITORY SYSTEM by HEATHER DORRELLE McBAY B.Sc. University of B r i t i s h Columbia, 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Division of Audiology and Speech Sciences i n the Department of Paediatrics We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June, 1971 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Division of Audiology and Speech Sciences Department of Paediatrics  The University of British Columbia Vancouver 8, Canada Date J u n e > 1 9 7 1 TABLE OF CONTENTS Page ABSTRACT . . . . . . . . . . . . . . . . . i v LIST OF TABLES v i LIST OF FIGURES v i i i ACKNTOWlEDGMENT . x i Chapter 1. INTRODUCTION . . 1 Chapter 2. REVIEW OF THE LITERATURE . . . . . . . . . . . 4 2.1 Influence of Speech Production on Auditory System — E f f e c t on TTS . . . 4 • " 2.-11 Introduction 4 2.12 Temporary Threshold Shift (TTS). 4 2.13 Influence of Yowel Phonation on TTS . 10 2.2 Changes i n the Ear's Transmission System During Vocalization 11 2.21 Kiddle-Ear Muscles (MEM) . . . . 11 Anatomy 12 Acti v i t y of the MEM . . . . 12 MEM Act i v i t y i n Association with Vocalization . . . . 12 Effect of MEM Contraction on TTS . . . . . . . . . . . . 17 Effect of MEM Contraction on Transmission of Sound to Cochlea 18 2.22 Stapes Vibration . . 20 «• 2.3 Relation Between TTS and MEM Reflex Activation i n Individuals . . . . . . . ~ 22 Chapter 3. AIMS OF THE INVESTIGATION ... .... . . . . . 23 Chapter 4. EXPERIMENTAL APPARATUS AND PROCEDURES 26 - i i -Page 4.1 Experimental Plan 26 4.2 Instrumentation . . . . . . . . . . . 27 4.21 Instrumentation for TTS Experiments 27 Signal Production 27 Additional Apparatus 31 Arrangement 31 4.22 Instrumentation for Impedance Experiments 31 4.23 Calibration 35 4.3 Subjects . . . . . . . . . 35 Chapter 5 EXPERIMENTS 37 5.1 P i l o t Experiments 37 5.2 Main Experiments 39 5.21 TTS Experiments 39 Design . 39 Data Measurement 44 St a t i s t i c a l Treatment 46 Analysis and Results . . . . . . . 46 5.22 Impedance Experiments 91 Procedure 91 Analysis and Results 91 Chapter 6. DISCUSSION AND CONCLUSIONS . . . 95 BIBLIOGRAPHY . . . . . . . . . . : 109-APPENDIX 114 *. » - 1 1 1 -ABSTRACT This investigation studies the effect of phonation and of some act i v i t i e s e l i c i t i n g middle-ear muscle contraction on high-intensity sound transmission i n the normal human auditory system. For the most part i t i s concerned with the influence of phonation on TTS from a continuous pure-tone stimulus. The main experimental technique con-sisted of measuring subjects' hearing thresholds before and after a 5 min, 500-Hz, 118-dB SPL exposure, this exposure being sometimes accompanied by the performance of a specific activity such as phonation. Threshold tracings were obtained by using a Bekesy-type procedure, and "threshold was measured at 7 times after cessation of the exposure tone. Analysis of the results indicates that TTS from phonation (hum-ming) during exposure was significantly less than TTS from the exposure tone without any supplementary a c t i v i t y , for a variety of humming ac t i v i t i e s : humming at 125 Hz (males) or 250 Hz (females); humming loudly at these same frequencies; or humming at 250 Hz (males) or 500 Hz (females). TTS from humming loudly and humming at the higher fre-quencies was consistently,although not significantly,less than TTS from humming at the lower frequencies. For females, phonation (hum-ming during exposure was more effective i n decreasing TTS than for males. Repeatedly turning the head during exposure, which i s believed to e l i c i t MEM contraction, resulted i n less TTS than no act i v i t y during exposure. Similar slight decreases i n TTS were observed when the f o l -lowing a c t i v i t i e s which e l i c i t middle-ear muscle contraction were per— - i v -formed during exposure: chewing, smiling, swallowing. Listening to recorded humming during exposure did not significantly alter TTS from the exposure. The activity of exhaling after preparing to hum did not significantly alter TTS from the exposure. In addition to the TTS studies, measurements of acoustic impedance during the exposure tone and of acoustic reflex thresholds were obtained. Various hypotheses concerning causes for the reduced TTS from phonation during exposure are discussed. Attenuation provided by middle-ear muscle contraction during phonation does not appear suf-f i c i e n t to decrease TTS to the extent that humming does. Sound may be attenuated by inefficient stapes vibration during phonation and TTS may therefore be reduced. Two other p o s s i b i l i t i e s are suggested to account for the TTS decrease: interference between humming and the exposure tone; and interference (by humming) with the central control of middle-ear muscle activity. More evidence w i l l be necessary to satisfactorily determine which, i f any, of these mechanisms i s actually i n effect. -v-LIST OF TABLES Table Page 1. Sequence of presentation of exposure conditions to subjects participating i n the TTS experiments (See Section 5.21 Design for description of the exposure conditions. 42 2. Summary of ANOV. TTS as a function of sex, post-exposure time, and condition (N, , H^, H^) for 4 male and 4 female subjects. . . 48 3. Results of Newman-Keuls test for significance of di f -ferences between TTS for N, , H^, conditions (4 male and 4-female subjects ) 49 4. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, H L A, H^, conditions (4 male subjects) 50 5. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, H^A, H^, conditions (4 female subjects) 50 6. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, , H 2 A, conditions (4 male and 4 female subjects ) 52 7. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, , H 2 a ' H l b c o n c ^ - " t i o n s (4 male subjects). . . . . .53 8. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, , H 2 A, conditions (4 female sub j ects) 54 9. Results of Newman-Keuls test for significance of d i f -ferences between TTS for 7 postexposure times. (4 male and 4 female subjects) _ 55 10. Summary of ANOV. TTS as a function of post-exposure time and condition ( N, , H Q ) for 2 male and 6 female subjects 83 11. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, H^, H q conditions. (2 male and 6 female subjects) 83 - v i -Table Page 12. Summary of ANOV. TTS as a function of post-exposure time and condition ( N, H_^ a, T ) for 2 male and 6 fe-male subjects 84 13. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, , T conditions. (2 male and 6 female sub j ects) .* 84 14. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, H^, T conditions. (2 male and 6 female subjects). 85 15. Summary of ANOV. TTS as a function of post-exposure time and condition ( N, , L ) for 2 male and 6 fe-male subjects • 87 16. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, H^, L conditions. (2 male and 6 female subjects). 87 17. Summary of ANOV. TTS as a function of sex, post-exposure time, and condition ( N, Hj_a' ^ a ' ^ l b ' ^o' T, L ) for 2 male and 2 female subjects 88 18. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, H l a 5 t H^, H , T, L conditions. (2 male subjects) 89 19. Results of Newman-Keuls test for significance of d i f -ferences between TTS for N, , H 2 a, E ^ , H Q, T, L conditions. (2 female subjects^, . f 90 - v i i -LIST OF FIGURES Figure > Page 2.1 Temporary threshold s h i f t (ordinate) at various f r e -quencies (abscissa) 2 min a f t e r cessation of 3-min exposures to several pure tones at various levels (parameter) 6 2.2 Temporary threshold s h i f t observed one second a f t e r exposures of various duration at 62 dB SPL 6 2.3 Growth of TTS at 1400 Hz as a function of exposure l e v e l 7 2.4 Schematic i l l u s t r a t i o n of middle ear ligaments and stapedius muscle 11 2.5 Schematic i l l u s t r a t i o n of the tensor tympani muscle. 13 2.6 The movements of the stapes. In (a) i s shown the normal motion of the stapes to air-borne sound. When, 'as i n (b), the movement communicated by the incus i s v e r t i c a l , the stapes rotates about i t s long axis and does not produce a displacement of f l u i d i n the cochlea 21 4.1 Design and parameters of TTS experiments 28 4.2 Block diagram of instrumentation f o r TTS experiments. 29 4.3 Schematic diagram of the p r i n c i p a l components of the electroacoustic impedance bridge. . . • 32 4.4 Block diagram of instrumentation f o r impedance experiments 34 5.1 Bekesy record of a representative TTS experiment, f o r a male subject (exposure condition N) 45 5.2 Comparison of N vs. H^a f o r 16 subjects (4 males and 12 females) . 57 5.3 Comparison of N vs. H„ f o r 8 subjects (4 males and 4 females) 58 5.4 Comparison of N vs. H.., f o r 8 subjects C4 jnales and 4 females) 59 - v i i i -Figure ' Page 5.5 Comparison of N vs. H for 8 subjects C2 males and 6 females) °. • 60 5.6 Comparison of N vs. T for 8 subjects (2 males and 6 females) 61 5.7 Comparison of N vs. L for 8 subjects (2 males and 6 females) 62 5.8 Comparison of N vs. L for 2 male subjects. . . . . 63 5.9 Comparison of N vs. L for 6 female subjects. . . . 64 5.10 Comparison of N vs. for 4 male subjects. . . . 65 5.11 Ccmparison of N vs. for 12 female subjects. . . 66 5.12 Comparison of N vs. "^or subjects (4 males and 12 females) 67 5.13 Comparison of N vs. H 2 a for 4 male subjects. . . . 68 5.14 (Comparison of N vs. H 2 a for 4 female subjects. . . 69 5.15 Comparison of N vs. for 4 male subjects. . . . 70 5.16 Comparison of N vs. for 4 female subjects. . . 71 5.17 Comparison of N, T, for 8 subjects (2 males and 6 females) 72 5.18 Comparison of N, L, for 8 subjects (2 males and 6 females) 72 5.19 Ccmparison of H, vs. H Q for 8 subjects (4 males and 4 females). . . . . . . t a 73 5.20 Comparison of H, vs. H , for 8 subjects (4 males and 4 females). . . . . . . . 74 5.21 Comparison of L vs. for 8 subjects (2 males and 6 females) 75 5.22 Comparison of L vs. for 2 male subjects ~ 76 5.23 Comparison of L vs. for 6 female subjects. . . . 77 5.24 Comparison of vs. H 2 a for 4 male subjects. . . . 78 5.25 Comparison of vs. H 2 a for 4 female subjects. . . 79 - i x -Figure Page 5.26 Comparison of vs. f o r 4 male subjects. . . . 80 5.27 Comparison of H-^ a vs. f o r 4 female subjects. . . 81 6.1 Three photographs, taken with a fiberscope, of the g l o t t i s of a male speaker: Ca) exhaling Cb) ex-haling a f t e r preparing to hum Cc) Humming at ap-proximately 125 Hz 99 -x-ACKN0WI1IDGMEM' To "the many people who have helped to make this investigation possible, I would l i k e to express my sincere gratitude. Especially -To my advisor, Dr. A.-P. Benguerel, for his guidance at every stage of the investigation -And also to Dr. J.H. Gilbert for his interest and encouragement as well as for serving on my committee -To Dr. R.P. Gannon, not only for being on my committee, but also for permission to use the f a c i l i t i e s of the Audio-Vestibular Unit at Vancouver General Hospital -To Dr. M. Humphries for consultation on the s t a t i s t i c a l analysis -To G. McConnell for constructing the switch box -To a l l who helped i n a very important way by being subjects -To my friends and family for genuine interest i n my graduate career -To Donna Cumming for the many hours spent i n typing the manuscript To everyone who had a part i n this research, Thank-you. - X L -Chapter 1 INTRODUCTION During every day of a person's l i f e his ears are constantly being stimulated by sounds; the sound which perhaps recurs most often to man's ears throughout his lifetime i s the sound of his own voice. To date there exists a large body of information, the product of a century of intensive investigation, concerning the normal functioning of the auditory system. Despite a wealth of studies, however, and the resultant vast accumulation of knowledge about sound reception i n the human ear, neglectfully l i t t l e i s known about sound reception during the activity of speaking which so frequently affects the ear. Speech and hearing are closely related, i t i s well kncwn. By this i s usually meant that fluent i n t e l l i g i b l e speech i s dependent upon a well-functioning auditory system which enables a speaker to continually monitor his vocal output. Speaking i s therefore, i n this way, influenced by hearing, but i s the converse true? Is hearing influenced by speaking? Does speaking alter the functioning of the ear as well as depend upon i t ? What i s generally termed the sound reception .aspect of hearing encompasses the transmission of acoustic energy to the organ of Corti via the middle ear and cochlear f l u i d , the transformation of this energy into e l e c t r i c a l energy at the hair c e l l s , and f i n a l l y i t s transmission along the auditory nerve and higher-order neurons to the cortex. I f any l i n k i n the chain for transmitting auditory signals to the brain i s modified by speaking, hearing w i l l be affected. From observations of specific behavioural responses to acoustic stimulation i t i s sometimes possible to deduce the functional state of the auditory transmission system and of i t s -1--2-individual components: —middle ear, basilar membrane, hair c e l l s , cochlear nucleus, etc. For example, considerable information about the auditory mechanism for transmitting high-intensity continuous signals has come from auditory threshold measurements after intense acoustic stimulation (from several minutes to several hours). To find out how speaking influences this mechanism, investigators (Karlovich and Luterman, 1970. , 1969) asked people to carry out some type of voiced speech activity while their ears were being exposed to intense sounds. Subsequently they compared the subjects' postexposure auditory thresholds i n two cases: with and without phonation during exposure. Interestingly, they found that the postexposure reduction i n sensitivity was less with phonation than without for some exposure tone frequencies, but was greater for other exposure frequencies. Evidently speech production modifies transmission of high intensity signals i n the human auditory system. Just at what place along the pathway of these signals through the system this interference occurs, s t i l l remains to be solved. What has been demonstrated though i s that speaking apparently brings about alterations i n at least one portion of the sound transmission system— the middle ear. Bekesy (1960, p. 201) discovered that when a person speaks the vibration of the skul l causes the stapes to vibrate i n a manner different from i t s usual pattern during auditory stimulation. Also, as several independent researchers have recently reported (Djupesland, 1967; Salomon and Starr, 1963; Shearer and Simmons, 1965), the middle-ear muscles of man contract during phonation. Whatever the mechanism, by interfering at some stage(s) i n sound transmission, speaking affects hearing. One can learn about the la t t e r by controlling the former, for, i n a l l probability, there exist feedback -3-loops between the larynx and the ear. It is a worthwhile endeavour to evaluate in more detail the characteristics of the influence of phonation on sound transmission. With more information about this phenomenon, such as the extent of transmission alteration during speaking, the frequencies whose transmission is most affected, and the nature of the speech activities associated with greatest effect, i t becomes possible to enlarge our understanding of the physics and physiology of the auditory system. Besides contributing to auditory theory, such a study may yield information of practical or clinical significance. Practically, i t is of value to animals, including man, to attenuate the bone-conducted feedback of their own vocalizations and yet s t i l l remain maximally sensitive to environmental sounds (Bekesy, 1949). The clinical relevance of studying the influence of phonation on fatigue of the auditory system by continuous intense stimulation lies in the relation between auditory fatigue and permanent noise-induced hearing loss. Objective. The general objective of this project is to study the effect of phonation on sound transmission in the human auditory system, with particular attention to the influence of voice intensity and fundamental frequency; and to determine whether phonation alters the sound transmission properties of the auditory mechanism perhaps by creating an increased acoustic transfer impedance in the ear as a consequence of the contraction of the middle-ear muscles. The main experimental technique wi l l consist of «ieasuring subjects' hearing thresholds before and after an intense pure-tone exposure, this exposure being at times accompanied by the performance of some activity, such as phonation.. Chapter 2 REVIEW OF THE LITERATURE 2.1 Influence of Speech Production on Auditory System—Effect on TTS 2.11 Introduction Recent studies by ^Karlovich and Luterman (Karlovich and Luterman, 1970 v 1969; Luterman and Karolvich, 1969) have indicated that transmission of sound i n the human auditory system appears to be influenced by the activity of speaking. Karlovich and Luterman (1969) reported on preliminary investigations which revealed that temporary threshold shifts (henceforth TTS) after exposure of the ear to a high intensity (90 dB SL (sensation level) 4000-Hz tone were consistently greater when subjects read a passage aloud during exposure as compared with a condition i n which they read the same passage s i l e n t l y . The greater TTS for the reading-aloud condition seemed to imply an enhance-ment of the transmission of the 4000-Hz tone to the cochlea during this speech activity. , In a later study (1969) i n which subjects were exposed to a 2000-Hz tone of high (90 dB SL) intensity, Luterman and Karlovich found that the condition of reading aloud during the tonal exposure produced consistently, but not significantly, less TTS than the conditions of reading s i l e n t l y , reading s i l e n t l y while articulating, or engaging i n quiet reverie during exposure. Before further examination of the investigations of Karlovich and Lutermaa, a review of pertinent information about TTS i s appropriate. 2.12 Temporary Threshold Shift (TTS). Following exposure to any sound of significant duration and intensity the sensitivity of the ear may be -4--5-reduced, as manifested by an increase i n the threshold of hearing or a reduction i n the loudness of suprathreshold stimuli ( E l l i o t t and Fraser, 1970; Hirsh, 1952; Ward, 1970, 1967b, 1963). I f the sensitivity index i s a temporary s h i f t i n the ear's absolute threshold (TTS), the change i n sensitivity i s termed auditory fatigue. Ward (1963) defines auditory fatigue as, ". . . one of a number of terms used to describe a temporary change (usually, but not always, a decrease) i n threshold sensitivity following exposure to another auditory stimulus." (Ward, 1963, p. 241) To measure TTS, the normal threshold i s f i r s t determined, then the ear i s exposed to fatiguing stimulation, and f i n a l l y the postexposure threshold i s measured. The TTS, the difference between the pre- and postexposure thresholds, i s an index of the amount of auditory fatigue. I t i s influenced,in the case of pure-tone stimulation, by the interacting factors of frequency, duration, and intensity of the fatiguijig stimulus as well as the frequency of the threshold test signal and the time between cessation of the stimulation and measurement of the postexposure threshold. In general, the higher the frequency of a pure-tone fatiguing stimulus, the greater w i l l be the TTS produced (Ward, 1963) (See Fig. 2.1). Other things being equal, TTS increases i n proportion to the logarithm of the duration of the fatiguing exposure (except for frequencies below 2000 Hz where the increase i s more gradual) ( E l l i o t t and Fraser, 1970) (See Fig. 2.2). This i s essentially an exponential process i n which a limiting value of TTS i s approached asymptotically (Botsford, 1971, Ward, 1970). In general, the higher the level of sound stimulation, the greater the TTS at any instant and the longer i t s duration (Davis, et a l . , 1950) (See Fig. 2.3). Following exposures to tones of low and moderate -6-I 1.4 2.0 2.8 4.0 FREQUENCY (KC) Figure 2.1 Temporary threshold s h i f t (ordinate) at various frequencies (abscissa) 2 min a f t e r cessation of 3-min exposures t o several pure tones at various l e v e l s (parameter). Figure t i t l e and Figure from Ward, 1970. 1 • 1 \tarious i i i r — T — — T ~ on limes wiTh 12-see rests 1 e 32 sec on ond 32 sec oft 100 sec on ond 60 sec off / 32 sec on, (.5 sec off, etc.,for 28min / -ft -• I » 1 1 ' ( } . t 32ms 100ms 320ms 1.0s 3.2s 10s-. 32s E X P O S U R E D U R A T I O N Figure 2.2 Temporary threshold s h i f t observed one second a f t e r exposures of various duration at 62 dB SPL. Figure from S e l t e r s , 1964. -7-M X o o m -o. oo UJ > LU or 2;0 1 O TTS20sec • T T S i min I A T T Sz m in 1 0 0 A • O 5 0 8 0 ' 110 S P L O F E X P O S U R E S I G N A L (dB) Figure 2.3 Growth of TTS at 1400 Hz as a function of exposure le v e l . Figure from McPherson, 1966. -8-intensity (below 70 dB SPL (Sound Pressure Level)) TTS i s of short duration, disappears i n about 1 minute, and i s maximum at the frequency of the exposure tone. After high-intensity stimulation (approximately 80 dB SPL and above) TTS i s more longlasting and has a maximum value one-half octave or more above the exposure frequency. Possibly the lower intensity exposures produce neural changes and/or cochlear metabolic changes which recover f a i r l y rapidly, whereas the higher intensity exposure are responsible for more serious tissue changes requiring longer times for recovery. As Ward (1970) notes, "There i s certainly no dearth of places at which fatigue processes could be found, a l l the way from the round window to the cortex. . . Hcwever, there seems to be l i t t l e question •that the bulk of the phenomena associated with after-effects of exposures above 80 dB SPL, i.e. effects lasting an hour or more, reflect cochlear events of one sort or another". (Ward, 1970, pp. 104-105) Four possible cochlear physiological mechanisms to account for TTS have been suggested by Misrahy and Associates (1958). These are: (1) "obvious mechanical injury"; (2) "acoustic vibrations inducing change i n permeability of the basilar membrane allowing ionic potassium to leak out from scala media and block hair cells and nerve endings"; (3) "hypoxia producing diffusereversible or irreversible changes not necessarily accompanied by obvious structural changes"; and (4) "other biochemical changes, such as variations i n carbon-dioxide tension, pH, and accumulation of metabolites". (Ward, 1970, p. 110) TTS measured one-half an octave above the exposure frequency seems more directly dependent on exposure level than does TTS measured at the exposure frequency (McPherson, 1966). The fact that high-intensity exposures produce maximum TTS one-half octave above the exposure frequency seems to implicate motion of the basilar membrane i n the TTS phenomenon, since "the locus of maximum amplitude associated with a -9-given frequency occurs at a point sl i g h t l y basalward of the hair ce l l s responsible for i n i t i a t i o n of the neural message to the higher centers". (Ward, 1970, p. 106). After cessation of fatiguing stimulation TTS gradually decreases, i n an exponential fashion, rapidly i n the f i r s t few seconds, then gradually to zero, at a f a i r l y constant rate which does not seem to'depend on the parameters of the fatiguing stimulus (Ward, 1963). Graphical analysis of existing data on recovery<;of TTS led Botsford (1971) to postulate that at least three independent exponential processes are involved, with half-lives of 15 s, 6-9 min , and 171 min , as well as an exponential sensitization process. Evidence for the r e a l i t y of a sensitization process i s the phenomenon of the "bounce" i n TTS recovery, a reversal i n the direction of the recovery slope between 1 min and 3 min after termination of the fatiguing stimulus, especially for stimuli of high frequency, moderately short duration (4 min or less (Selters, 1963)), and moderately high intensity (80-100 dB. SPL (Jerger, 1956)). In fact, i f the i n i t i a l TTS i s not large, the postexposure threshold at about 1 min. may "overshoot" (i.e. become less than) the original threshold ( E l l i o t t and Fraser, 1970). There i s a wide range of variation among individuals i n the magnitude of the TTS following a given exposure. This variation appears not to be related to differences i n the auditory threshold at the exposure frequency (Ward, 1961). Intra-individual variation i n TTS produced by a certain exposure i s , however, small. E l l i o t t and Fraser (1970), for example, observed for exposures of 2000 Hz at 105 dB SL very high test-retest r e l i a b i l i t y coefficients (on the order of r = .90 for 12 subjects). To assess the effect of repeated exposure to high-intensity sound Riach, et a l . (1964) administered these same exposures 20 times to 12 subjects -10-over a period of several weeks. No change i n the pre-exposure thresholds (at 2800 Hz) was noticed, but there was apparent a trend toward a smaller TTS at the 1-min postexposure time. I f , however, ah ear i s exposed to fatiguing stimulation while i t i s s t i l l under the influence of previous fatigue processes, cumulative effects occur. Workers exposed to factory noises for many years, for instance, may exhibit smaller TTS as well as permanent hearing losses (Nixon and Glorig, 1962). 2.13 Influence of Vowel Phonation on TTS A recent investigation by Karlovich and Luterman (197:0 ) explored further the relations between speech production and sound transmission i n the auditory system. During exposure to a 110 dB SPL 1000-Hz tone, subjects were required to produce a voiced vowel (/a/ or / i / ) or " a si l e n t articulatory gesture representing the vowel without voicing or whispering" (ibidem, p. 511). Significantly less TTS (at 1414 Hz) resulted when the voiced vowels were produced during the exposure tone than when the non-voiced gestures were performed. According to Karlovich and Luterman, "these results imply that transmission of the fatigue stimulus [ to the cochlea J was attenuated during voiced vowel production". (ibidem, p. 513). As Ward believed, "one cannot reduce either [TTS or noise-induced permanent threshold s h i f t J by any other, means than reducing the sound reaching the ear". (Ward, 1967b,p.l20). In an ear with a pure conductive hearing loss, for example, fatiguing stimulation tends to produce less TTS than i n a normal ear because the effective level of sound reaching the cochlea i s reduced (Ward, 1963). As possible mechanisms for the attenuation of lew frequency fatiguing tones during phonation, Karlovich and Luterman postulated changes i n the middle ear, either contraction of the middle-ear muscles (henceforth MEM) or i n e f f i c i e n t -11-vxbration of the stapes. Either of these two mechanisms seem probable. The MEM definitely contract when persons speak, and the i r contraction has been demonstrated to decrease TTS and to impede sound transmission to the cochlea. Similarly, during vowel phonation, the stapes apparently vibrates i n a manner less e f f i c i e n t for transmission of pressure changes to the cochlear f l u i d . The literature pertaining to these subjects w i l l be considered now i n more d e t a i l . 2.2 Changes i n the Ear's Transmission System During Vocalization 2.21 Middle-Ear Muscles (MEM) Superior l i g o m e n f of Figure 2.4 Schematic i l l u s t r a t i o n of middle ear ligaments and stapedius muscle. Figure and Figure t i t l e from Zemlin, 1968, p.380. -12-Anatomy. The stapedius muscle (6.3 mm) originates within a canal on the posterior wall of the middle ear cavity and attaches to the posterior margin of the head of the stapes. Contraction of the stapedius draws the stapes posteriorly j at right angles to the direction of movement of the ossicular chain (See Fig. 2.4).The muscle acts almost parall e l to the two ligaments holding the stapes i n the oval window and has an effect similar to reinforcing the looser upper ligament (Miller, 1961). A branch of the f a c i a l nerve innervates the stapedius (Jepsen, 1963; Zemlin, 1968, p. 383, 384). The tensor tympani (25 mm) originates within a bony canal paral l e l to the Eustachian tube, passes across the middle ear cavity and inserts on the manubrium of the malleus. Contraction of the tensor tympani draws the malleus medially and anteriorly, almost at right angles to the direction of rotation of the ossicular chain, increasing the tension on the tympanic membrane (See Fig. 2.5).A branch of the trigeminal nerve supplies the tensor tympani (Jepsen, 1963; Zemlin, 1968, p. 383). The stapedius and tensor tympani act i n direction opposite each other; by stiffening the articulation of the ossicles they act to damp their vibrations. Contraction of these muscles increases the acoustic impedance of the eardrum but probably does not change i t s position owing to the antagonistic activity of the two muscles (Kryter, 1970, p. 96). ' A c t i v i t y of the MEM. The acoustic reflex (henceforth AR) can be described as reflex contraction of the stapedius muscle, e l i c i t e d i n response to acoustic stimulation of sufficient intensity. The AR i s b i l a t e r a l , i . e . , a loud sound presented to one ear of the listener produces reflex a c t i v i t y i n both middle ears. The superior olivary -13-- l i l -nucleus i n the pons i s believed to be the center for the AR; the afferent neurons of the reflex arc are located i n the cochlear nuclei, and the efferent neurons are i n the motor nucleus of the f a c i a l nerve. In man the AR i n response to pure tones f i r s t occurs, on the average, at levels o f stimulation of about 82 dB SL (mean AR threshold for pure tones 125 Hz - 4000 Hz) (Franzen and L i l l y , 1970). Higher frequencies, i t appears, are more effective than lower frequencies i n e l i c i t i n g the AR, and random noise i s more effective than pure tones (mean reflex threshold for white noise i s approximately 46 dB SL (Franzen and L i l l y , 1970). These differences may be due to the relative loudnesses of these s t i m u l i — stimuli of equal loudness are apparently equally effective i n e l i c i t i n g the reflex (Ross, 1968). The latency period of the AR depends on the frequency and level of the stimulus tone and varies from about 35 to 150 ms (Zemlin, 1968, p. 387). As the sound becomes more intense, the greater i s the degree of contraction of the stapedius, u n t i l a maximum i s reached. Following intense acoustic stimulation Hie stapedius contracts and remains contracted throughout the stimulation, but the degree of contraction decreases as the sound continues u n t i l a resting state i s reached ( i f then a tone of a different frequency i s presented, the contraction i s restored to original strength) (Jepsen, 1963). The reflex relaxation i s not, therefore, the result of muscular fatigue but must reflect adaptation processes at other levels of the reflex arc. " I t i s conceivable that the reflex gradually relaxes during continued stimulation i n order to compensate for, or because of, a gradual decrease i n the loudness of sounds with long duration stimulation ..." (Kryter, 1970, p. 95). -15-MEM reflexes can also be e l i c i t e d by non-acoustic methods. The stapedius muscle contracts reflexly i n response to mechanical or e l e c t r i c a l stimulation of the skin of the external ear and surrounding area (Klockhoff, 1961; Macrae, 1970); the tensor tympani reflexly contracts when a i r i s blown towards the o r b i t a l region (Djupesland, 1964> 1967; Klockhoff, 1961). Contraction of one or both of the MEM occurs with somatic motor a c t i v i t i e s such as tight eye closure (Djupesland, 1967), yawning (Klockhoff, 1961), swallowing (Djupesland, 1967; Wersall, 1958),. opening the mouth (Djupesland, 1967), clenching the teeth (Djupesland, 1967), contraction of numerous muscles of the head and neck, such as for smiling forcefully (Djupesland, 1967), and bodily movements such as l i f t i n g or tairning the head (Carmel and Starr, 1963; Salomon and Starr, 1963). MEM A c t i v i t y i n Association with Vocalization. Salomon and Starr (1963) undertook an investigation on human MEM ac t i v i t y during general motor a c t i v i t i e s , using electromyography to record MEM contraction i n two patients, whenever the patient talked or hummed, contractions of the tensor tympani were registered. This muscular act i v i t y preceded, usually by 40 ms to 300 ms, the onset of the speech sound, or else occurred at the same time as the sound. Throughout the period of vocalization and for about 300 ms after cessation, the tensor tympani remained active. These time relations, Salomon and Starr noticed, corresponded quite closely to those of ^ the i n t r i n s i c laryngeal muscles -Faaborg-Andersen (1957) had shewn that the laryngeal muscles become active up to 500 ms before the onset of sound and continue i n the contracted state u n t i l a short time after the sound has ended. In a second patient Salomon and Starr studied stapedius ac t i v i t y . Movements -16-of the pharyngeal and laryngeal muscles during vocalization and coughing always gave ris e to increased stapedius a c t i v i t y , which followed a similar time course to that already noted for the tensor tympani. Evidence for the existence of a non-acoustic reflex connection between the larynx and MEM has come from the work of McCall and Rabuzzi (1970). These investigators e l e c t r i c a l l y stimulated the internal branch of the superior laryngeal nerve of the cat and observed, i n association with contraction of the cricothyroid muscle, b i l a t e r a l contractions of the stapedius and tensor tympani muscles. The cat, l i k e man, demonstrates MEM activity i n association with vocalization. Carmel and Starr (1963) observed, by electromyography, that MEM action potentials preceded the onset of the cochlear microphonics from the cat's vocalization by 75-500 ms. During vocalization MEM activity continued, generally stopping with the termination of the round-window response. In his 1964 paper Simmons included some electromyographical studies on MEM activity i n cats.. MEM contractions associated with the cats' vocalizations (meow) were observed, beginning about 100 ms before any actual sound could be heard and lasting slightly longer than the vocalizations. The degree of contraction, as assessed by the magnitude of the impedance changes,seemed to be proportional to the anticipated intensity of vocalization. Simmons also related results of studies of speech-related changes i n acoustic impedance i n human ears. A patient with unilateral f a c i a l nerve paralysis apd, consequently, one non-functional stapedius muscle, served as a subject. The patient's normal ear showed impedance changes i n conjunction with speech production (the words, "one, two, three") whereas i n the paralyzed ear the patient's -17-speech a c t i v i t i e s e l i c i t e d no impedance change. Shearer and Simmons (1965) observed changes i n acoustic impedance associated with moderate-intensity whispering and vowel (/a/) phonation. Always the impedance changes occurred either at the same time as the speech sounds, or, more often, about 65-100 ms before. "The occurrence of acoustic impedance change s l i g h t l y before speech output would indicate that the stapedius muscle under these circum-stances i s activated concurrently with the speech musculature ... Impedance change was never i n i t i a t e d after the onset of speech, which would have been the case i f the reflex were acoustically triggered." (Shearer and Simmons, 1965, p.206) It would seem that the mechanism which causes contraction during speech production may be a neural mechanism, "part of the neurological pattern i n the production of speech." (ibidem, p.206). In Djupesland's 1967 monograph, there appeared the information that increased a c t i v i t y , as recorded by electromyography, occurred i n the stapedius and tensor tympani muscles of a l l subjects (48 patients about to undergo ear surgery) whenever they said the words, " j a " and "nei". The increased a c t i v i t y of the muscles started always 30-450 ms before the speaking voice was recorded and persisted for up to 300 ms after the speaking had stopped. The MEM a c t i v i t y seemed to increase with the intensity of the speech. Effect of MEM Contraction on TTS. Studies have demonstrated that voluntary- and AR- contraction of the MEM affect TTS (Kryter, 1970, p. 100). A similar effect can be postulal^ed for MEM contraction i n association with vocalization. When subjects able to voluntarily contract their MEM did so during successive exposures to high intensity clicks they exhibited up to 20 dB less TTS (at 4000 Hz) than for the same -18-exposure without voluntary contraction (Fleer, 1963; cited i n Kryter, 1970, p. 103.) Activation of subjects' AR during auditory fatiguing stimulation also generally brought about a decrease i n TTS (Karlovich, •et. a l , , 1970 Loeb and Fletcher, 1961). After an ear was exposed to a 1000-Hz fatiguing stimulus together with an intense pulsed noise i n the contralateral ear, Karlovich , et _ a l . , (1970 ) noted that less TTS was manifested than when the 1000-Hz tone alone was presented. The TTS decrease was directly proportional to the sound pressure level of the noise i n the contralateral ear (above a minimum effective level). Presumably, according to Karlovich and Luterman, activation of the AR by the noise attenuated transmission of the fatiguing tone. Transmission of a 4000-Hz tone, however, was not affected by the AR-arousing noise. Loeb and Fletcher (1961), who carried out a similar experiment, reported that when AR-activating stimuli (110 dB SL clicks or 100 dB narrow band (200 Hz), noise centered at 2100 Hz) were presented 150 ms before rounds of machine-gun f i r e , there occurred significantly less TTS from the gunfire exposure than that following the same f i r i n g without AR activation. Effect of MEM Contraction on Transmission of Sound to Cochlea. Evidence that contraction of the MEM measurably affects the transmission of sound through the middle ear can be found i n studies on both animals and man. Wiggers (1937) studied cochlear microphonic recordings to determine the effect of spontaneous contraction of the MEM of anesthetized guinea pigs on the transmission of tones of various ^ frequencies. When contracted, the MEM were found to significantly reduce the efficiency of transmission of tones below 1000 Hz; the lower the tone the greater the reduction, with a maximum attenuation of the equivalent of 45 dB at 100 Hz. Using -19-similar techniques on cats, other researchers have reported less transmission loss attributable to the AR approximately 20 dB for frequencies below 1000 Hz (Jepsen, 1963). MEM contraction, however, seems to s l i g h t l y improve transmission (in guinea pigs, cats) of some high frequency tones (about 1500 Hz), a phenomenon which possibly can be accounted for by the muscles' shifting upward the frequency of the natural resonances of the tympanic membrane and ossicles (Jepsen, 1963; Wiggers, 1937). Transmission of tones above 2000 Hz appears not to be affected by MEM contraction. Evidently the stapedius muscle i s more important i n transmission attenuation than the tensor tympani. Galambos and Rupert (1959) and Simmons (1959) cut either the tensor tympani or the stapedius muscle of cats and then, while the one intact muscle was being activated by intense sounds, measured the round window cochlear microphonic response to tonal stimulation; they discovered that only when the stapedius had been cut did the MEM attenuation of the microphonics disappear (cited i n Ward, 1961). MEM effects on sound transmission through cats' middle ears were examined by Garmel and Starr (1963) by analysis of simultaneous MEM electromyographical recordings and cochlear microphonic measurements i n response to a prolonged (2-hour), steady, high-intensity (85 dB SPL) white noise. These effects were found to be: "a) the i n i t i a l attenuation of round-window response shortly after onset of white noise: b) the gradual r i s e thereafter; *• and c) the intervening decreases associated with bodily movements". (Carmel and Starr, 1963, p.600) Direct studies of the influence of the MEM on sound transmission i n the human auditory system are, understandably, scarce. Reger (1960) -20-recorded shifts i n absolute threshold of some subjects before and during voluntary contraction of their MEM. Contraction lowered the threshold of hearing for low tones (125 - 1000 Hz), the reduction being the greatest (approximately 30 dB) for the frequencies of 125 Hz and 250 Hz. 2.22 Stapes Vibration In addition to MEM contraction, another change i n the peripheral auditory transmission system which i s known to occur during vocalization i s an alteration i n the vibration pattern of the stapes. Bekesy found that for moderate air-borne sound stimulation the stapes rotates about a ve r t i c a l axis near the posterior edge of the footplate of the stapes. During vowel phonation, however, when the v e r t i c a l vibration of the sku l l i s great, the stapes begins to rotate about a longitudinal axis that runs through the footplate. As a result, the motion of the cochlear fluid: i s minimal (from one edge of the foot-plate to the other) and there i s much less f l u i d displacement than when the stapes rotates around a v e r t i c a l axis i n a piston-like movement (See Fig. 2.6). A similar s h i f t of rotational axes occurs when the ear i s stimulated by intense sounds at low frequencies. It i s a means of limiting transmission of excessive vibrations to the cochlear f l u i d (Bekesy, 1960, p.201). The MEM could possibly be involved.in the s h i f t of the vibration axis of the stapes, although Bekesy does not make mention of them. M i l l e r (1961) explained how stapedius contraction seems to favour a rotation of the stapes about a longitudinal axis through^ the footplate by reinforcing the upper, looser ligament suspending the stapes i n the oval window. The conditions e l i c i t i n g the special stapes vibration mode - loud sounds and speaking - are conditions which provoke stapedius a c t i v i t y (See -21-l Figure 2.6 The movements of the stapes. In (a) i s shown the normal motion of the stapes to air-borne sound. When, as i n (b), the move-ment communicated by the incus i s v e r t i c a l , the stapes rotates about i t s long axis and does not produce a displacement of f l u i d ' i n the cochlea. (Figure and Figure t i t l e from Bekesy, 1960, p.202) * « -22-section 2.21). In any case, the vibration of the stapes during vocalization i s such that the efficiency of the middle ear as a sound "transmitter i s reduced. 2.3 Relation Between TTS and MEM Reflex Activation i n Individuals Since the MEM are known to influence TTS (Section 2.21), probably because they attenuate transmission of low frequency sounds (section 2.21), various investigators have sought to uncover significant cor-relations, among individuals, between the degree of MEM activation and the amount of TTS. " I t seems reasonable that those subjects showing the greatest reflex activation should shew the least TTS." (Ward, 1961, p.1043). Most of these attempts, unfortunately, have met with l i t t l e success. No significant correlations were found by Ward (ibidem) between subjects' TTS's produced by various octave bands of intense (100-130 dB SPL) high frequency noise and their MEM activity (as determined by AR thresholds). In a later study by Durrant and Shallop (1969) MEM a c t i v i t y during exposure to a 105dB SPL narrow band noise centered at 1000 Hz was assessed by the difference between measurements of the acoustic compliance of the ear taken before and one-min after onset of the noise. Rank-order correlation coefficients between these compliance shifts and the TTS from 4 min. of the noise were non-significant. None of the correlations were negative, as might have been expected; i n fact, a sl i g h t l y positive correlation was noted between TTS and compliance s h i f t . Brasher, et a l . (1969) found no significant correlations between TTS from 1000-Hz and 4000-Hz octave bands of noises and the following MEM responses: AR threshold; degree of AR contraction ( as measured by magnitude of acoustic impedance change ) at onset of the two noises; and degree of AR contraction after 2 min continuous exposure to the noises. Chapter 3 AIMS OF THE INVESTIGATION The present investigation i s designed to study certain aspects of the influence of speech act i v i t y on high-intensity sound transmission i n the normal human auditory system. For the most part, i t i s devoted to studying the influence of phonation on TTS from a continuous pure-tone stimulus. I t has been established (Chapter 2) that when the ear i s stimulated by a 1000-Hz tone and, concurrently, vowels are produced, TTS i s reduced. TTS from a 4000-Hz tone i s increased i f the subject reads aloud during exposure, whereas TTS from a 2000-Hz tone, with the subject reading aloud,is s l i g h t l y but not significantly decreased. The implication i s that phonation interferes with the transmission of these fatigue stimuli i n the auditory system. How speaking affects transmission of other frequencies i s not known. There i s very l i t t l e information about the speech parameters having most effect on transmission; i t i s only known that whispering seems ineffective and voicing necessary. Both mechanisms which have been proposed to account for the transmission alteration, MEM contraction and i n e f f i c i e n t stapes vibration, are plausible, but perhaps there are alternative explanations. In any event, no definitive experiments have been performed to delineate the actual mechanism. The MEM do contract when persons speak, and*their contraction i s known to al t e r sound transmission, whether MEM ac t i v i t y i s f u l l y responsible, • however, for the transmission alterations observed by Karlovich and Luterman has not been determined. . In this respect, i t should be of -23-- 2 4 -interest to observe how phonation affects the transmission of a 500-Hz tone for example, a frequency which i s known to be considerably attenuated by the MEM, more so than higher frequencies. There appears to be a trend for MEM act i v i t y to increase as the intensity of vocalization increases, but no studies have directly investigated the effects of the intensity of phonation on high-intensity sound transmission. Certainly, relatively l i t t l e i s known about the effects of speech on the auditory transmission system. More s p e c i f i c a l l y , the purpose of this research i s : I (a) To investigate the effect on TTS (resulting from exposure to a 500-Hz tone) of phonation (huirming) during the tonal exposure, i n particular: - the effect of varying the intensity of the voice; - the effect of varying the fundamental frequency of the voice. (b) To determine whether these effects are different i n males and females. (c) To determine the effect on TTS (from a 500-Hz tone) of the physical sound wave, divorced from the sound production (humming). (d) To investigate the effect on TTS (from a 500-Hz tone) of vocal f o l d approximation without voice during the tonal exposure. (e) To investigate the effect on TTS (from a 500-Hz tone) of MEM acti v i t y (non-acoustically elicited) during the tonal exposure. (f) To compare the effects of phonation on TTS with the effects of MEM ac t i v i t y (non-acoustically elicited) on TTS. -25-(a) to assess the effect of phonation (humming) on middle ear impedance, i n particular: - the effect of varying the intensity of phonation; - the effect of varying the fundamental frequency of phonation.-(b) To assess middle ear impedance: - during a continuous 500-Hz tone; - during a continuous 500-Hz tone accompanied by phonation (humming): - during a continuous 500-Hz tone accompanied by a c t i v i t i e s known or believed to e l i c i t the MEM reflex. (c) To determine whether significant correlations exist between TTS (from a 500-Hz tone) and: - acoustic stapedius reflex thresholds; - rate of decrease of middle ear impedance during a continuous 500-Hz tone. Chapter 4 EXPERIMENTAL APPARATUS AND PROCEDURES 4.1 Experimental Plan In general, the technique for investigating the influence of phonation and other a c t i v i t i e s on sound transmission i n the auditory system consisted of exposing a subject's ear to a TTS-producing stimulus while, at the same time, the subject performed a vocal (or non-vocal) task, and then measuring the resultant TTS. Examination of the subject's TTS's for these various task conditions, as well as for the condition of no a c t i v i t y during the TTS-producing stimulus, enabled certain inferences to be drawn concerning the relative effectiveness of sound transmission i n the ear for, each condition. TTS, as defined for these experiments, i s the difference between a subject's mean threshold of hearing for pulsed pure tone before exposure to an intense TTS-producing stimulus and his threshold for the same tone measured at a particular time after cessation of the exposure. Because Bekesy audiometry permits continuous recording of a threshold which i s rapidly changing i n time (recovering), i t i s the preferred method of study of TTS. A Bekesy audiometer produces a pure tone stimulus whose intensity i s controlled by the subject. Whenever the subject hears a tone he presses a switch activating an attenuator which reduces the intensity of that tone u n t i l i t i s below his threshold of hearing. As soon as he no longer hears the tone he releases the switch and the intensity of the tone i s automatically increased. The automatic record of these intensity adjustments enables estimation not only of the subject's -27-threshold but also of the v a r i a b i l i t y about his average threshold, namely the magnitude of the excursions, i n decibels, between a just-heard tone and a just-not-heard tone. Diagrammatically represented i n Figure 4.1 i s the basic form of the TTS experiments i n this investigation. Each TTS experiment on a given ear consisted of three essential parts: a pre-exposure period, an ' exposure period, and a post-exposure period. During the 2 minute pre-exposure period and the 4 minute post-exposure period the subject tracked his threshold, by Bekesy audiometry, for the single frequency of 700 Hz. During the exposure period the subject received a 5 minute monaural exposure to a 500-Hz tone of 118 dB SPL (henceforth called the exposure tone). The subject performed one of the following tasks throughout the duration of the exposure tone: no task (N); humming at 250 Hz (females) or 125 Hz (males) (H,) at a comfortable level (H-. ) or a loud l e v e l (H.,, ): 1 l a lb humming at 500 Hz (females) or 250 Hz (males) (M^) at a comfortable level (H 0 ); exhaling with the intention of humming (H ); turning the head from side to side (T); listening to recorded humming (L). In addition to the TTS studies, measurements of MEM a c t i v i t y during the exposure tone and of acoustic reflex thresholds were obtained. Contraction of the MEM was studied by recording changes i n the acoustic impedance of the ear. 4.2 Instrumentation 4.21 Instrumentation for TTS Experiments Signal Production. Figure 4.2 shows the signal-producing and observer response-recording equipment. A Grason-Stadler Model E800 -28-Threshold racking 700Hz 2 min Exposure f e = 500 Hz SPL = 118 dB t2 = 5 min. Activity One of Following: N H l a H l b H 2 a H 0 T L Threshold Tracking f t = 700 Hz t3 = 4 min. PRE- POST EXPOSURE EXPOSURE PERIOD EXPOSURE PERIOD PERIOD Figure 4.1 Design and parameters of TTS experiments. - 2 9 -THRESHOLD TRACKING Recording Attenuator Oscillator Interrupter CHANNEL 2 EXPOSURE Oscillator Step 2 Attenuator CONTROL ROOM TEST ROOM Figure 4.2 Block diagram of instrumentation for TTS experiments. -30-Bekesy Audiometer supplied the pulsed test tones used for determining hearing thresholds. These tones, interrupted approximately 2.5 times per second ( r i s e - f a l l time: 25 msec), were generated by a frequency o s c i l l a t o r which remained at a single frequency for plotting threshold as a function of time for TTS study, or which could continuously vary i n frequency over a range of 100 - 10,000 Hz for standard audiogram determination. By means of a switch controlling a recording attenuator, the subject could adjust the intensity of the tone, at a rate of either 5 dB/sec. or 2.5 dB/sec, i n 0.25 dB steps, according to whether he just heard the signal or whether he just ceased to hear i t . The attenuation rate, determined by the position of the motor speed switch on the audiometer was 5 dB/sec. for standard audiograms and 2.5 dB/sec. for single frequency TTS audiograms. The operator also had access to an additional fixed attenuator which increased or decreased the intensity of the signal by 20 dB. Throughout the course of these experiments a 20 dB attenuation was used because of the acute hearing of many of the subjects. The continuous 500-Hz TTS-producing stimulus (exposure tone) was generated by the frequency o s c i l l a t o r of a Madsen Audiometer (Model OB 60). The intensity of this signal, adjustable by the operator (in 5 dB steps) by means of an attenuator, was routinely set at the maximum of 118 dB SPL. Oscillator 1 of Channel 1 was the source of the exposure tone for a l l but one experimental condition. The exception, when the exposure tone arose from Oscillator 2 of Channel 2, occurred^ for the condition of dual presentation of the exposure tone and recorded humming. In this case the input from the tape recorder to Channel 1 pre-empted tonal input from o s c i l l a t o r 1. The outputs of the two audiometers were connected to the earphones -31-v i a an external switch box which allowed the signal from either audiometer to be delivered to either the right or the l e f t earphone (Madsen TDH-39 earphones i n MX-41/AR cushions set into insulated plastic mountings on a light-weight headband). Additional Apparatus. Auxiliary equipment consisted of a Madsen AS 50 Tape Recorder i n the control room and a microphone (Sennheiser, Type MD 420-2) at a distance of 1.5 inches from the l i p s of the subject i n the test room. Recordings were made at a tape speed of 7.5 inches per •second at a recording level of approximately 0 dB as registered on the VU meter of the recorder. A connection between the recorder playback output and the Madsen audiometer allowed the recorded material to be played back to the subject at a level adjusted via the Madsen attenuator. Both a tape signal, i n Channel 1, and the pure tone exposure signal, i n Channel 2, could be directed to the same earphone after having been passed through the internal mixer of the Madsen audiometer. Arrangement. The stimulus-generating and observer response-recording instruments were i n a separate control room, adjacent to the sound-insulated audicmetric test room i n which were located the earphones, the subject control switch for the Bekesy recording attenuator, and the subject, (see Fig. 4.2). A double-glass window permitted the operator to see the test subject. By means of a wall switch, located i n the control room, the lights i n the test room and the control room could be turned on and off. 4.22 Instrumentation for Impedance^.Experiments The instrument used to measure acoustic impedance of the ear was the Madsen electroacoustic impedance bridge (Model Z0 70). As indicated i n Figure 4.3, the principal components of the instrument, which are located i n a small probe unit attached to a headband, are: a 220-Hz o s c i l l a t o r -32-^S. Bfidg* Circuit Figure 4.3 Schematic diagram of the principal components of the electro-acoustic impedance bridge. (Figure and Figure t i t l e from Jerger, 1970, p.313) -33-connected to a receiver; a microphone; and a pressure system consisting of an air-pump and a manometer. Three f l e x i b l e tubes lead from these components to a probe t i p which i s secured i n the external meatus to form an airt i g h t cavity terminated by the tympanic membrane. With this arrangement the 220-Hz probe tone can be conveyed to the sealed cavity and i t s sound pressure level i n the cavity monitored by the probe microphone. The microphone signal i s fed to a balance meter and i s registered as a visual deflection on the meter. By adjustment of the •potentiometer which governs the probe tone intensity, the balance meter can be nulled. When this occurs the SPL i n the closed ear canal i s exactly 95 dB. The sound pressure i n the canal i s dependent on: "a) the acoustic impedance of the volume of the space just i n front of the tympanic membrane, b) the acoustic impedance of the ear measured at the tympanic membrane, and c) the acoustic impedance of the acoustic part of the bridge." (Djupesland, 1967, p. 32). When the measuring bridge i s balanced, any change i n either the position of the tympanic membrane . (to a l t e r the volume of the sealed cavity) or i n the acoustic impedance of the ear measured at the tympanic membrane, such as occurs with MEM contraction, w i l l bring the bridge out of balance and cause a deflection on the balance meter. For determination of MEM acti v i t y during the exposure tone and of acoustic reflex thresholds a Maico c l i n i c a l audiometer (H IB) was used i n conjunction with the impedance meter. The audiometer, the source of the AR-arousing pure tones, was connected to a Maico earphone (TDH-39) attached to the opposite end of the headband holding the impedance probe (see Fig. 4.4). Because a suff i c i e n t l y intense sound i n one ear w i l l activate the AR i n both ears, a reflex-arousing sound was introduced to - 3 4 -Audiometer Earphone Probe Attenuator Oscillator Impedance Meter Graphic Recorder Figure 4.4 Block diagram of instrumentation for impedance experiments. -35-one ear while the reflex MEM contraction was detected by observation of impedance changes i n the opposite ear. A graphic recorder, connected to the impedance bridge, monitored the signal from the probe microphone to produce a permanent record of the impedance changes occurring with MEM contraction. 4.23 Calibration Prior to the commencement of the experiments the acoustic output of the audiometer-earphone units and the frequency response of the earphones were determined using a Bruel and Kjaer Type 2203 Precision Sound Level Meter with Type 1613 octave f i l t e r set, a Bruel and Kjaer A r t i f i c i a l Ear (Type 4152) with a standard 6 cc. NBS Coupler, and a Bruel and Kjaer Beat Frequency Oscillator (Type 1022). Throughout the data collection period, daily measurements of the exposure tone level at the earphones were obtained. The mean of a l l daily measurements (32) was 118.38 dB SPL (SD=0.20). The calibration of the Bekesy audiometer (500-Hz reference) was also checked daily. On days when MEM activity was studied with the impedance meter the calibration of the Maico audiometer was checked, also with a 500-Hz reference. Background noise i n the test room, measured with the Bruel and Kjaer equipment before and after data collection, was found to be 25dBA. Octave f i l t e r measurements revealed that frequencies below 250 Hz contributed largely to this noise. The level of the noise i n the frequencies of 500 - 16,000 Hz never exceeded 15 dB SPL^ . 4.3 Subjects The subjects were twelve female and four male unpaid volunteers, i n good health, ranging i n age from 14 years to 56 years, with a median age -36-of 23 years. A l l subjects had normal hearing as evidenced by absence of history of ear pathology and by air-conduction hearing threshold levels within 25 dB of 0 dB (ISO Standard, 1964) for pure tones of frequencies 125 -10,000 Hz. In addition, only those persons were included i n this study who, at 1 minute after cessation of the 5-minute exposure tone, manifested a TTS at 700 Hz (approximately half an octave above the ex-posure tone) of more than 1 dB. (16 of the 18 persons screened met this criterion). Two-thirds of the subjects were university undergraduates or graduates, predominantly i n the fields of Audiology and Speech Sciences and Education. The rest were diversely occupied as home-maker, high-school student, or s k i l l e d tradesman. One-half of the subjects had had previous experience with threshold tracking by Bekesy audiometry and with experimentally induced auditory fatigue; for the others these auditory procedures were a new experience. Five of the eight experienced subjects had been unfamiliar with audiometric tasks u n t i l having served i n a short p i l o t investigation one month prior to the experimental series. Chapter 5 EXPERIMENTS 5.1 P i l o t Experiments A number of preliminary experiments were carried out about one month before the main experimental series i n order to determine the general form of the TTS experiments outlined i n Chapter 4 (See Fig. 4.1). These p i l o t studies provided the information for specifying the parameters of the exposure tone (frequency, intensity, duration), as well as the conditions for pre- and post-exposure threshold tracking. I t was decided to choose the exposure frequency from a range i n which the MEM effectively attenuate transmission of tones —namely below 1000 Hz. Thus., selection of the exposure tone had to be from one of the following frequencies (fixed frequencies of Madsen audiometer): 125, 250, or 500 Hz. I t was necessary that the intensity of the tone be not uncomfort-able to the subject and that i t s duration be reasonably short, while producing a TTS large enough to be measured at least 1 min after exposure, even i f i t should be reduced by an activity such, as phonation. Preferably, an i n i t i a l TTS of about 20 dB was sought. Experiments revealed that to produce a TTS of this size exposures to 125 Hz or 250 Hz, at the limits of the audiometer, would have had to considerably exceed 5 min duration. Exposure for 5 min to a tone of 500 Hz, however, at 118 dB SPL (maximum output of audiometer) produced an average of 25 dB i n i t i a l TTS. This level was tolerable to the stabjects and 500 Hz was a frequency which humming could sa t i s f a c t o r i l y accompany. (Hearing 500 Hz while humming at the same frequency or at one or two octaves below was not d i f f i c u l t for subjects). Several subjects were exposed for 5 min to the - 3 7 --38-500-Hz 118-dB tone and then were asked to track their thresholds, at 5 dB/s, across the frequency range of 125-10,000 Hz. (This procedure requires 3 min 20 s). I t was found that maximum TTS occurred between 500 Hz and 1000 Hz. There were considerable differences among subjects, however. An average "best" frequency (i.e. giving maximum TTS) seemed to be approximately one-half octave above 500 Hz. Since Davis ,...et al ' . : , (1950)(See Section 2.12) have reported that maximum TTS i s generally measured one-half octave above the exposure frequency, i t was decided to use a 700-Hz tone for pre- and post-exposure threshold tracking. The p i l o t study also served to select experiments that would be interesting and profitable to pursue i n more detailed study. To determine the influence of phonation on TTS, one subject was asked to perform various a c t i v i t i e s (on separate days) while her ear was exposed for 5 min to'a 500-Hz, 118-dB SPL tone. I t was observed that humming at 250 Hz or 500-Hz, comfortably or loudly, decreased considerably the TTS. The lower fundamental frequency and the stronger intensity of humming resulted i n s l i g h t l y greater reduction of TTS. Performance of Toynbee's manoeuvre,"'' which reduces the efficiency of the middle-ear transmission mechanism, decreased TTS almost to the extent that the less 2 effective hunmrLng tasks did. Phonation i n the "creaky voice" mode decreased TTS, but not as much as did humming. Listening to recorded humming at 500 Hz (subject's own) i n the contralateral ear, or applying an external voice source (electrolarynx^ at the throat throughout exposure did not appear to significantly a l t e r TTS. On the basis of these 1. Toynbee's manoeuvre consists of swallowing with the mouth and nostrils closed; the result i s reduction i n middle-ear pressure (Ballin, 1926, p. 58). 2. Creaky voice:glottis vibrates to produce double pulses, following closely i n time. -39-experiments i t was decided to examine more thoroughly the influence on TTS of the humnung a c t i v i t i e s which decreased TTS most—namely humming at 250 Hz and humming loudly; as well as an act i v i t y which does not seem to influence TTS—listening to humming.' Similar experiments were conducted on three other subjects to assess the influence on TTS of various a c t i v i t i e s e l i c i t i n g MEM contraction. The a c t i v i t i e s which were investigated were: swallcwing; clenching the teeth; smiling forcefully while tightly closing the eyes; chewing; and stimulating the skin i n the v i c i n i t y of the external ear. A l l a c t i v i t i e s were repeated at a rate of approximately twenty to t h i r t y times per minute. Of the a c t i v i t i e s investigated, chewing, smiling,and swallcwing during exposure ( 5 min, 500 Hz, 118 dB SPL) produced less TTS than no ac t i v i t y during exposure. The decrease i n TTS was very slight, however (about 3 dB). '.It was not known, i n the cases of swallowing or chewing, whether the TTS alteration could be attributed to MEM contraction; possibly i t resulted from changes i n middle-ear pressure (swallowing) or from changes i n the pressure i n the external meatus (chewing). Smiling forcefully and closing the eyes was an objectional task to subjects. For these reasons, then, none of the MEM-eliciting a c t i v i t i e s described were examined i n more detail. . 5.2 Main Experiments 5.21 TTS Experiments  Design. The TTS paradigm consisted of a^ .2 min pre-exposure threshold track-ing at 700 Hz, a 5 min monaural exposure to a 118-dB SPL 500-Hz fatigue stimulus, and a 4 min post-exposure threshold tracking at 700 Hz. The subjects were instructed to perform one of the following a c t i v i t i e s during the exposure period: -40-(a) Condition N: to s i t s i l e n t l y and do nothing i n particular. (b) Condition H-^: to hum comfortably at 125 Hz (males) or 250 Hz (females), the duty cycle being 7-8 s of humming and 2-3 s of rest. The periods for humming and rest were signalled by the l i g h t i n the test room ( on = hum; o f f = inhale ), under the control of the experimenter v i a a switch i n the control room. The intensity of phonation corresponding to "comfortable" was specified as 60 dB SPL at a distance of 54 + 2 inches from the subject. A sound l e v e l meter (with f i l t e r corresponding to the frequency of humming) was placed i n the test room at this distance so that the subject would produce and maintain as constant as possible the required intensity of humming. (c) Condition t o •'lum c o m f o r r t a h l y at 250 Hz (males) or 500 Hz (females); duty cycle and intensity as i n (b). (d) Condition H-^: to hum loudly at 125 Hz (males) or 250 Hz (females); duty cycle as i n (b). "Loud" humming was specified as 70 dB SPL registered on the sound level meter approximately 54 inches from the subject. (e) Condition H q : to approximate the vocal folds as i f about to hum but to exhale without producing voice; duty cycle as i n (b). (f) Condition T : to turn their heads from l e f t to right and back approximately fiv e times per minute (self-paced). One complete cycle consisted of : head l e f t for 3 s, straight ahead for 3 s, right for 3 s, straight ahead** for 3 s. (g) Condition L : to s i t s i l e n t l y and l i s t e n monaurally to the fatigue tone mixed with t h e i r humming, the l a t t e r recorded under condition and played back at ]_ Bodily movement such as turning the head reportedly e l i c i t s MEM contraction (Carmel S Starr, 1963; Salomon £ Starr, 1963). -41-85 dB HTL (Hearing Threshold Level), a l e v e l which approximated the loudness . of the actual humming. Routinely, subjects were requested to refrain from excessive bodily movements or unnecessary clearing of the throat, coughing, yawning, or swallowing during the fatigue exposure — a l l of which a c t i v i t i e s may e l i c i t MEM contraction. Recordings were made of a l l subjects' humrning so that the actual frequencies hummed could be checked i f the need arose. ConditionsN and H-^ were mandatory for a l l 16 subjects. In addition, 4 subjects (2 males, 2 females) participated i n the humming conditions of H 2 a and H^; another 4 subjects ( a l l females) were given the L and H Q conditions; and 4 female subjects took part i n the T condition as well as i n the impedance experiments (to be reported i n section 5.22). The remaining 4 subj ects (2 males, 2 females) completed a l l the conditions. As a p a r t i a l control against possible comulative and/or sequence effects no two subjects were submitted to the same sequence of conditions. Scheduling of conditions for the three groups of 4 subjects undergoing 4 conditions each was i n accordance with the Latin square design. With "this design each condition occurred equally often f i r s t , second, third, or l a s t (Table l a ) . A Latin-square design, however, was not possible for the other group of 4 subjects required to completie, i n addition to the 7 exposure conditions ( (a) through (g) ), another N condition and one impedance testing session. The sequence of experiments, i n this case, was randomized independently for each subject, with these restrictions: (1) that N and H ^ occur i n the f i r s t 4 sessions, i n orders corresponding to those of the other Latin squares; (2) that the number of sessions separating the 2 N conditions be fixed ( a i 4 sessions); and -42-Table J. Sequence of presentation of exposure conditions to subjects participating i n the TTS experiments (See Section 5.21 Design for description of the exposure conditions). Table K a ) - SESSION 1 2 3 t Group I 1 N " l a H2a "lb SUBJECT 2 3 H l a H l b **lb H2a N H l a V N 4 H2a N Hlb. "la 1 2 3 1 Group II SUBJECT 1 2 N la l a H o L N "o . 1, 3 H o L " l a N 4 L N H o "ja i 1 2 3 4 Group III 1 N la I T SUBJECT 2 3 la T T I N "la I N ! 4 I N T " l a Table Kb) 1 2 3 4 5 6 7 8 9 Group IV 1 N la T • L N H2a H r\ I \jSUBJECT 2 3 la I H o N H l a H2'a N T H2a H l b L I H o N _ T - L N 4 H o N H2a l a T I N L H l b -43-(3) that the same conditions do not have the same session number. (Table lb). Data collecting sessions for each subject were scheduled to allow at least 24 hours between sessions. In most cases sessions were separated by 48 hours or more; often a week intervened between sessions. Procedure. At the start of the experimental series, each subject was i n -formed of the general nature of the investigation. He was then asked to designate which ear he preferred to have fatigued (on that day and i n future sessions). Fortuitously, i n the group of 16 subjects, 8 right and 8 l e f t ears were chosen. When the subject was comfortably seated i n the sound-treated booth he was given a standard set of instructions for Bekesy audio-metry (Jerger, 1960). For practice he tracked his threshold by sweep-frequency Bekesy audiometry, once for each ear. Then followed the experi-mental procedure which was standard for a l l sessions: (a) Sweep-frequency tracking of threshold for the ear contralateral to the exposure ear; (b) 2 min of single-frequency (700 Hz) threshold tracking for the contralateral ear; (c) Sweep-frequency threshold tracking for the exposure ear; (d) 2 min of single-frequency (700 Hz) threshold tracking for the exposure ear (pre-exposure period of TTS paradigm); (e) Exposure to 500-Hz, 118-dB SPL tone for 5 min, with appropriate a c t i v i t y (exposure period); (f) 4 min of single-frequency (700 Hz) threshold tracking for the exposure ear (post-exposure period). (See Appendix A for instructions which subjects received for procedures (a) - (f) ). Hearing thresholds were determined at 700 Hz i n the contralateral ear to provide a measure of session-to-session threshold fluctuations due to -44-practice, state of health, attentiveness, etc. The pre-exposure threshold measurements at 700 Hz were examined each session before administration of fatiguing exposure. I f a subj ect's threshold for the exposure ear was noticeably worse than i n his f i r s t experimental session (before subjection to intense stimulation), and i f threshold for the' contralateral ear was unchanged or better, i t was f e l t to be inadvisable to continue the experi-ment on that day. The sweep-frequency trackings for both ears provided the subjects with practice i n Bekesy audiometry and served to alert the experimenter against possible threshold increases (over a range of f r e -quencies) resulting from repeated intense exposure. In the course of the experiments, however, no such changes were observed. Instructions concerning the act i v i t y a subject was to perform during exposure were given usually after pre-exposure tracking, for the humming conditions, arid at the beginning of the session for the others. Ten niinutes, or less, of practice usually sufficed for the subject to learn to perform the a c t i v i t i e s acceptably. Most subjects succeeded i n carrying out the humming activies with ease. Some, however, i n i t i a l l y experienced d i f f i c u l t y i n producing the desired fundamental frequency, but with practice succeeded. Data Measurement. Reproduced i n Fig. 5.1 i s the Bekesy record of a rep-resentative TTS experiment. Pre-exposure threshold was determined from the pre-exposure tracing (on the l e f t ) by averaging visually estimated lines-of-best-fit drawn through the peaks and the troughs of the last minute of the tracing. To measure post-exposure thresholds, an average curve was f i t t e d through the midpoints of the peaks and troughs of the post-exposure Bekesy tracing (on the right). Where this curve inter-sected lines corresponding to specific post-exposure times, thresholds O S B £ K £ S Y AUDIOMETER GRASON - STADLER COMPANY, INC MODEL NO. E 8 0 O -10 ' FIXED FREQUENCY co LU CO • o LU Q LU > U J o x CO LU Qi X K— O < LU X CO LU ID _ J < > O CO »—• •3-C^ TONE 20 rJB dB/SEC l o o Hz — 2.5 NAME ,pKV\ SEX. .AGE LI DATE_14l3hl TIME 8:00P.M. RY Hlj, Figure 5.1 Bekesy record of a representative TTS experiment, f o r a male subject (exposure condition N). -46-were determined. The post-exposure times considered were: 5s, 10s, 15s, 30s, 1 min, 2 min, and 4 min. These were represented as distances on a specially constructed measuring card prepared on the basis of accur-ately-timed fixed-frequency Bekesy tracings. TTS, i n dB, at each of the 7 times, was obtained by subtracting the pre-exposure threshold. S t a t i s t i c a l Treatment. The TTS data obtained were submitted to several analyses of variance. The data were c l a s s i f i e d by: (1) post-exposure time at which TTS was measured (TIME); (2) ac t i v i t y of the subject during exposure (CONDITION); and (sometimes) (3) sex of the subject (SEX). The analyses of variance which were performed were two-factor (TIME x CONDITION) or three-factor (SEX x TIME x CONDITION) repeated-measures analyses (Winer, 1962, pp. 298-349). When indicated, following a significant overall F, •the Newman-Keuls method was used to probe the nature of the differences between the means for the task conditions. This involved testing for significant differences between pairs of condition means. Hypotheses about the differences between pairs of condition means were also tested separately from the analysis of variance, by means of t-tests for cor-related observations (Winer, 1962, pp. 39-43). Analysis and Results. The analysis of variance (ANOV) model used was one which takes into consideration the variance due to subjects. This model identifies and eliminates inter-subject differences as a source of error, thereby increasing the precision of the analysis (Lindquist, 1953, p. 156, 237; Winer, 1962, pp. 319-337). It was believed that this r e l a t i v e l y e f f i c i e n t analysis would reveal any systematic alteration of TTS due to a c t i v i t y performed during exposure, and would indicate whether this alter-ation was a function of time of TTS measurement and/or sex of the subject. Since not a l l subjects performed a l l a c t i v i t i e s , the data were partitioned -47-into groups, each containing subjects who completed the same a c t i v i t i e s : several separate ANOV's were therefore conducted i n l i e u of one large one. Subject ReliaTality. Condition N was repeated, after 4 sessions, on 4 subjects. Test-retest correlation coefficients on the TTS scores at each of the 7 post-exposure times were high: 0.98 for t = 5s, 10s, 15s, 30s; and 0.89 for t = 1 min and t = 2 min. These results indicate that although TTS from the exposure tone differed considerably among the subjects (TTS at t = 5s ranged from 18 to 43 dB), each subject exhibited a consistent TTS from the exposure. Humnung. The f i r s t analysis was a SEX (male, female) by TIME (5s - 4 min) by CONDITION (N, IL^  , H 2 a, H^) design i n which 8 subjects (4 males, 4 females) were tested under each condition. Table 2 i s a summary of this analysis. Considerable variance i s associated with subjects, which i s not surprising since i t has long been known that the TTS produced by a given exposure varies greatly among individuals. The ANOV revealed that SEX had a non-significant effect on TTS, whereas CONDITION and TIME each had a highly significant effect, beyond the .01 level. There were no significant interactions between SEX and CONDITION or between SEX and TIME, and no significant overall interaction between the three factors. A Newman-Keuls probe of the condition effect was made to determine the significance of differences between the N, H^a, H 2 a, and con-ditions across a l l post-exposure times and both sexes. A summary of this analysis i s given i n Table 3. (See also Fig.'s 5.2, 5.3, 5.4). The results indicated significant differences between TTS from N and TTS from each of the humming conditions; for the 8 subjects the differences between H 2 a and N, and PL^ and N were more significant than the d i f f e r -ences between H, and N. None of the hunming conditions significantly -48-SEX x TIME x CONDITION Table 2. Summary of ANOV. TTS as a function of sex, post-exposure time, and condition ( N, H^, , ) for 4 male and 4 female subjects. SOURCE OF VARIATION df MS F Between Subjects 7 Sex 1 95.91 0.14 Subjects 6 688.16 Within Subjects 216 Time (T) .' 6 1379.99 ft* 47.32 Sex x T 6 2.11 0.07 T x Subjects 36 29.16 Condition (C) 3 521.71 ft* 8.53 Sex x C 3 51.18 0.84 C x Subjects 18 61.17 T x C 18 13.98 ** 5.47 - Sex x T x C J.8 1.86 0.73 T x C x Subjects 108 2.56 * • p < .05 ** p < .01 -49-differed from each other i n their effect on TTS. (See also Fig.'s 5.19, 5.20). The. significance of differences between conditions across a l l post-exposure times was determined separately for males and females. The results of these Newman-Keuls probes, summarized i n Tables 4 and 5, re-vealed a difference i n the effect of the conditions i n males and females. For females, a l l three humming conditions differed very significantly (at the .01 level) from N. For males, however, only the condition of humming loudly (H-^) was significantly different from N. I t appears then that there i s more reduction i n TTS when females hum during fatiguing exposure "than when males hum during exposure. Table 3. Results of Newman-Keuls test for significance of differences between TTS for N, H^, , conditions (4 male and 4 female subj ects). Cond \ Ition ^ Mean / l b 2a l a 9.88 11.66 12.13 17.01 9.88 H 2 a 11.66 R, 12.13 l a 1.78 2.25 7.13 0.47 5.35 * 4.89 S^g-q.99 (r,18) > 4.25 4.91 5.32 * p < .05 ** ' P < -01 In order to obtain more information on the effects of exposure con-dition at specific post-exposure times the Newman-Keuls probe was made on the effects of the conditions at t = 10s, t = 15s, and t = 1 min; and t-tests were performed at a l l 7 post-exposure times. The results of these -50-Table 4. Results of Newman-Keuls test for significance of differences . between TTS for N, H, , H , H , conditions. (4 male subjects). Condition ^ Mean H l b H2a H l a ' N 9.74 11.16 12.19 14.96 H l b 9 ' 7 4 H 2 a 11.16 H l a 12.19 1.42 2.45 5.22* 1.03 3.80 2.78 q.99 (r,18) 4.25 4.91 5.32 Table 5. Results of Newman-Keuls test for significance of differences between TTS for N, E. , H , E., conditions. (4 female subjects). Condition — : ^ N / Mean H 1 K E H N lb l a 2a 10.02 12.0 6 12.15 19.06 10.02 H l a 12.06 H 2 a 12.15 2.04 2.13 9.04"" 0.09 6.99"""' 6.90 S^g- q.99 (r,18) 4.25 4.91 5.32 * p < .05 ** p < .01 -51-tests for males and females together are presented i n Table 6 and Fig.'s 5.2, 5.3, 5.4, 5.19, 5.20; for males i n Table 7 and Fig.'s 5.10, 5.13, 5.15, 5.24, and 5.26; and for females i n Table 8 and Fig.'s 5.11, 5.14, 5.16, 5.25, 5.27. I t may be seen that differences between the humming conditions and the N condition were most significant at the ea r l i e r post-exposure times. For example, at t = 1 min for males and females together, according to results of the Newman-Keuls test, there were significant differences only between TTS from the humming condition and TTS from N; at.t = 10s and t = 15s, however, the differences between a l l three humming conditions and N were highly significant (.01 level). Apparently there i s a trend, as the tests indicate, towards dependence of the CONDITION effect on SEX and on TIME. The trend for SEX x CONDITION interaction, however, should not be over-emphasized because the ANt>V, i t w i l l be recalled, revealed no significant interaction between SEX and CONDITION or between SEX, CONDITION, and TIME, although the TIME x CONDITION interaction was found to be highly significant. I t was expected, and the ANOV revealed, that time of TTS measure-ment had a highly significant effect on TTS. The reason for this effect i s the recovery of TTS which i s occurring at a f a i r l y rapid rate, especially i n the f i r s t minute after exposure. In order to further examine the differences i n TTS due to time, over a l l 4 conditions and both sexes, the Newman-Keuls test depicted in. Table 9 was carried out. For 5s - 30s post-exposure times i t was found that there were very significant (.01 level) differences i n TTS between any time and a l l other times except those immediately preceding and following. TTS for the 1 min, 2 min, and 4 min times were not significantly different from each other. -52-TAble 6. Results of Newman-Keuls test for significance of differences between TTS for N, H l a, I-L. . conditions. (4 male and 4 female subjects). Condi tion >^ Mean H l b H 2 a H l a N 15.03 16.53 18.63 24.75 t=10s H., 15.03 lb H 2 a 16.53 E, 18.63 l a 1.50 3.59 9.72*'""' 2.09 8.22*'""' 6.13 S^g- q.99 (r-,18) 4.25 4.91 5.32 Condi .tion > Mean - • • - - .. • * i H,, H. E, N lb 2a l a 11.91 14.16 15.19 20.75 t=15s H 1 K ' 11.91 lb H 2 a 14.16 H-. . 15.19 l a 2.23 3.28 8.85"" 1.03 6.6o"'" A A 5.56 S^g- q.99 (r,18) 4.25 4.91 5.32 Condi .tion ^ Mean H l b H l a " . H 2 a N 4.66 6.63 7.09 10.78 t=lmin E,. 4.66 lb a. 6.63 l a H 2 a 7.09 1.97 2.44 6.13"" 0.47 4.16 3.69 S^g- q.99 (r,18) 4.25 4.91 5.32 j'j.'j * p < .05 P < ' 0 1 -53-Table 7. Results of Newman-Keuls test for significance of differences between TTS for N, H, , H 0 , PL conditions. (4 male subjects). Condi \ tion ^ Mean H l b H2a H l a N 15.69. 16.00 18.81 22.38 t=10s H 1 K 15.69 lb H 0 16.00 2a Hn 18.81 l a 0.31 3.13 6.69"""' 2.81 6.38"'" 3.56 S^ pg- q.99 (r,18) 4.25 4.91 5.32 Condi .tion ^ ^ Mean lb 2a l a 12.44 14.00 15.44 18.63 t=15s H ' 12.44 lb H_ 14.00 2a Hn 15.44 l a 1.56 3.00 6.19*'" 1.44 4.63* 3.19 S^g q.99 (r,18) 4.25 4.91 5.32 Cond i t i o n ^ Mean lb l a 2a 4.31 6.69 7.00 8.94 t=lmin H l b 4 ' 3 1 IL 6.69 l a H 2 a 7.00 2.38 2.69 4.63*" 0.31 2.25 1.94 c q.99 (r,18) ^TTS • 4.25 4.91 5.32 * p < .05 ** p < . 01 -54-Table 8. Results of Newman-Keuls test for significance of differences between T T S for N, H , H „ , H,, conditions. (4 female subjects). Condition ^ Mean H l b • H2a " l a N 14.38 17.06 18.44 27.13 14.38 H. 18.44 la 2.69 4.06 12.75 1.38 10.07** ftft 8.69 S ^ r q.99 (r,18) 4.25 4.91 5.32 Condb Ltion >^ Mean H l b H l a H2a N 11.38 14.31 14.94 22.88 11.38 t=15s H l a 1 4 - 3 1 H 2 a 14.94 2.94 3.56 11.50** ft* 0.62 . 8 . 5 7 ft* 7.94 S^rg q.99 (r,18) 4.25 4.91 5,32 Cond s ition ^ Mean H l b H l a H2a ' N 5.00 6.56 7.19 12.63 5.00 H.. 6.56 t=lmin I a H 2 a 7.19 1.56 2.19 7.63*'"" ftft * 0.63 6.07 ft* 5.44 S^ g - q.99 (r,18) 4.25 4.91 5.32 * p<.05 ** p < . 01 -55-Table 9. Results of Newman-Keuls test for significance of differences between TTS for 7 postexposure times. (4 male and 4 female subjects). Time ~7 N J / Mean 30" 15" 10" 5" 5.72 7.84 7.29 10.49 15.50 18.73 23.09 4' 21 1' 30" 15" 10" 5.72 7.84 7.29 10.49 15.50 18.73 "2.13 1.57 4.77 2.65 3.20 9.78 13.02 17.38 7.66 10.89 15.25 8.21 11.45 15.80 5.01 8.24 12.60 3.23 7.59 4.36 S^g-q.99 (r,36) 3.65 4.17 4.49 4.71 4.88 5.03 * p <.05 ** p < . 01 -56-The following series of Figures., Figures 5.2 to 5.27, show TTS (measured at 700 Hz) at 7 different times ater cessation of 5 min exposure to a 500-Hz, 118-dB-SPL tone. TTS for various a c t i v i t i e s during exposure are compared (See Section 5.21 Design for a f u l l description of the exposure a c t i v i t i e s ) . -57-TTS (dB) 35-30-| 25 20-15-10-0-\-Y/—r m r~ 10" 15' -i * 4' 30" V 2' Fig. 5.2 Comparison of N vs. for 16 subjects (4 males and 12 females) 5- JLO" 25" . 30" 2' 2' 4' .TTS (N) 28.39 . 24.08 20.49 24.53 21.32 20.99 8.83 TTS(K^) 22.16 27.59 24.09 8.59 5.42 6.22 4.72 SD CN) 9.28 8.63 7.86 6.92 5.88 5.96 5.83 SD Oij^) 7.69 6.44 5.49 4.43 3.98 3.20 2.78 N 26 26 26 £6 16 16 16 t -4.05 -4.24 -4.42' -4.23 -3.86 -3.88 -3.02 P <0.0025 <0.0025 ^0i0025 <0.0025 ^0.0025 <0.0025 <0.01 -58-TTS (dB) 35 30-| 25 20-15-10-5-o-l y/-r— 5" —I 1— 10" 15" T " 4" 30" i V Fig. 5.3 Comparison of N vs. H 2 a for 8 subjects (4 males and 4 females) 5' 10" 15" 30" 1« 2« 4' TTS (N) 29.44 24.75 20.75 14.78 10.78 11.00 7.56 TTS ( H 2 a 20.03 16.53 14.16 9.78 7.09 8.28 5.72 SD (N) 8.73 7.89 6.97 5.92 4.18 3.17 : 3.16 SD ( H 2 a 8.54 7.85 7.20 5.47 4.45 3.45 3.70 N 8 8 8 8 8. 8 t -4.46 -4.18 -3.49 -3.01 -3.24 -2.83 -1.83 P < 0.005 <0.005 . < "0.01 < 0.02 < 0.02 <0.05 < 0.20 -59-Fig. 5.4 Comparison of N vs. H,, for 8 subjects (4 males and 4 females). 5" 10" 15" 30" l 1 i 2' 4' TTS (N) TTS (H^) SD (N) SD (H^) N t P 29.44 19.91 8.73 7.51 . 8 -4.67 < 0.005 24.75 15.03 7.89 7.32 8 - s . m cfO.0025 20.75 11.91 6.97 6.94 8 -4.89 <0-.005 14.78 7.59 5.92 5.24 * -4.55 <0.005 10.78 ! 11.00 4.66 ! 5.34 4.18. | 3.17 4.02 ! 2.93 8 i ' 8 -5.52 -4.92 CO. 0025 |<0.0025 I, , ....! 7.56 4.72 j 3.16 3.11 8 -1.91 < 0.10 -60-TTS (dB) 35-1 30-25-20-15-10-5-5" i r-10" 15' 30" V 4' Fig. 5.5 Comparison of N vs. H q for 8 subjects (2 males and 6 females). 5" 10" 15" 30" 1 ' TTS (N) T T S ( H ); SD SD N t P (N) ( H ): o 29.81 29.47 12.14 9.97 8 -0.18 > 0.20 25.19 25.16 11.11 9.50 8 -0.02 >0.20 21.13 21.78 9.96 8.71 8 0.46 > 0.20 15.03 15.59 8.27 6.94 « 0.49 >0.20 12.03 11.53 6.70 4.33 8 -0.34 y 0.20 11.44 10.84 5.82 6.12 8 -0.57 > 0.20 10.00 8.97 5.89 5.70 8 -1.47 > 0.20 -61-TTS (dB) 35-30 25-20-15-10-5-- T 0 | // i 5': l — 30" 2* i r 10" 15" V I 4' Fig. 5.6 Comparison of N vs. T for 8 subjects (2 males and 6 females). 5" 10" 15" 30" 1' 2' 41 TTS (N) 30.28 25.35 21.28 15.06 10.75 9.91 8.35 TTS (T) 26.34 21.22 17.84 12.47 8.41 \ 9.34 6.91 SD (N) 8.83 8.48 7.73 7.16 6.24 6.94 . 6.30 SD (T) 10.36 9.32 9.06 8.22 6.53 6.06 3.58 N 8 .8 8 8 8 8 t -1.97 -2.72 -2.49 -2.05 -1.75 -0.54 -1.24 P < 0.10 <0.05 < 0.05 <0.10 ^0.20 >0.20 >0.20 Fig. 5.7 Comparison of N vs. L for 8 subjects (2 males and 6 females). 5" 10" 15" 30" 1' 2' 4' TTS (N) 29.81 25.19 21.13 .15.03 12.03 11.44 10.00 TTS (L) 28.25 23.38 19.81 13.63 9.31 9.88 8.63 SD (N) 12.14 11.11 9.96 8.27 6.70 5.82 5.89 SD (L) 10.15 8.91 7.61 6.47 4.95 4.05 4.85 N 8 8 8 8 *• 8 8 8 t -0.83 -1.03 -0.80 -0.97 -1.55 -1.32 -0.98 P >o.20 >0.2Q >0;20 > 0.20 >0.20 >0.20 > 0.20 -63-TTS (dB) 35-j 30-25-20 15H 10-5H 5" 30' • i 10" 15" r -r 2' 4* Fig. 5.8 Comparison of N vs. L for 2 male subjects. 5" 10" 15" 30" 1' 2\ 4' TTS (N) 28.75 24.13 20.13 14.25 9.50 8.00 6.88 TTS (L) 21.50 17.50 14.75 10.25 7.50 7.75 6.00 SD (N) 14.85 14.32 13.97 12.02 8.13 5.66 5.13 SD (L) 13.08 11.67 11.31 9.90 7.42 4.95 6.01 N 2 2 2 2 2 2 \;\2. t -5.80 -3.55 -2.88 -2.67 -4.00 -0.50 -1.42 P < 0.20 C 0.20 > Q.20 ? 0.20 £ 0.20 > 0.20 > 0.20 -64-TTS (dB) 35-30-25-20-15-10-5-0 y / - T — 5" - i 1— 10" 15" -I t 4' 30' 2" Fig. 5.9 Comparison of N vs. L fo r 6 female subjects. 5" 10" 15" 30" l 1 2" 4' fTS (N) 30.17 25.54 21.46 15.29 12.88 12.59 11.04 PTS (L) 30.50 25.33 14.75 14.75 9.92 10.58 9.50 3D (N) 12.71 • 11.45 9.97 8.15 6.80 5.89 6.17 3D (L) 9.25 8.09 5.73 5.73 4.65 3.95 4.70 6 6 6 f> 6 6 6 t 0.17 -0.11 0.02 -0.31 -1.08 -2.00 -1.63 •P > 0.20 >0.20 > 0:20 > 0.20 >0.20 <0.20 < 0.20 -65-TTS (dB) 35 30-25-20-15-10-5-0 5" N •HLO -1 1— 10" 15" ~r~ r ~~l— t 4* 30" i 2* Fig. 5.10 Comparison of N vs. for 4 male subjects. 5" 10" 15" 30" l f 2» 4' TTS (N) 26.94 22.38 18.63 12.44 8.94 9.19 6.25 TTS (PL l a 1 23.19 18.81 15.44 9.94 6.69 6.94 4.31 SD (N) 10.25 9.58 8.99 7.62 4.75 3.56 3.09 SD ( H l a ) 10.05 8.83 7.77 5.95 4.23 4.23 4.77 N 4 4 4 4* 4 4 4 t -2.95 -3.07 -2.26 -2.27 -3.95 -1.76 -0.99 P < 0.10 < 0.10 < 0.20 < 0.20 < 0.05 < 0.20 > 0.20 -66-TTS (dB) 35-1 30 25-20-15-10-5-0 5" "T 1— 10" 15" B — « N " I t 4" 30" i V i 2" F i g . 5.11 Comparison of N vs. for 12 female subjects. 5" 10" 15" 30" 1' 2» 4' TTS (N) 28.88 24.65 21.11 15.23 12.11 11.58 9.69 TTS ( H l a • 21.81 17.19 13.65 8.15 4.98 5.98 4.85 SD (N) 9.37 8.66 7.78 6.88 6.19 6.59 . 6.36 SD (H, l a ' 7.26 5.88 4.87 4.03 4.00 2.96 2.06 N 12 12 12 I2 12 12 12 t -3.57 -3.85 -4.17 -4.00 -3.73 -3.67 ^2.89 P <0.005 CO. 005 < 0.0025 (0.0025 (0.005 <0.005 <0.02 -67-TTS (dB) 35-1 30H 25 20 15-10-5-5" -D N " A H 2 5 0 1 r~ 10" 15' l 30' ~i 1' -1 t 4' 2* Fig. 5.12 Comparison of N vs. ^2S0 ^ o r subjects (4 males and 12 females), 5" 10" 15" 30" V . 2' 4' TTS(N) 28.39 24.08 20.49 . 14.53 11.31 10.99 8.83 TTS(H 2 5 0 ) 20.95 16.89 13.73 8.58 5.48 6.45 4.89 SD (N) 9.28 8.63 7.86 6.92 5.88 5.96 5.83 SD ( H 2 5 0 ) 7.63 6.56 5.78 4.76 4.46 3.42 2.66 N 16 16 16 16 • 16 16 t -4.33 -4.47 -4.50 -4.02 -3.74 -3.57 -2.96 P < 0.0025 < 0.0025 <6.0025 <0.0025 ^0.0025 ^0.005 < 0.01 -68-TTS (dB) 35-1 30H 5" 10" 15" 30" 1" T 4' Fig..5.13 Comparison of N vs. H 9 for 4 male subjects. 5" 10" 15" 30" 1' 2' 4' TTS (N) 26.94 22.38 18.63 12.44 8.94 9.19 6.25 TTS 18.38 16.00 14.00 9.88 7.00 7.88 5.00 SD (N) 10.25 9.58 8.99 7.62 4.75 3.56 3.09 SD 9.30 9.34 8.95 7.11 6.06 4.75 4.45 N 4 4 4 4 4 4 4 t -2.17 -2.03 -1.68 -K27 -1.48 -0.95 -1.02 P < 0.20 < 0.20 < 0.20 > 0.20 > 0.20 > 0.20 > 0.20 -69-TTS (dB) 35-30-25-20-15-10 5H 0 I // i 5" 2' ~r~ 4" -1 r~ 10" 15' 30" r Fig. 5.14 Comparison of N vs. H 2 a for 4 female subjects. 5" 10" 15" 30" l 1 2' 4' ITS (N) 31.94 27.25 22.88 17.13 12.63 12.82 8.88 TTS ( H 2 a } 21.69 17.06 14.31 9.69 7.19 8.69 6.44 SD (N) 7.48 6.46 4.53 3.01 3.01 1.42 3.02 SD ( H 2 a ) 8.73 7.48 6.39 4.37 3.09 2.19 3.28 N 4 4 4 4 4 4 t -4.66 -4.16 -3.35 -3.41 -3.53 -4.01 -1.39 P < 0.02 < 0.05 < 0.05 < 0.05 \ < 0.05 < 0.05 > 0.20 -70-T T S (dB) 35-30-Fig. 5.'15 Comparison of N vs. H.., for 4 male subjects. 5" : 10" 15" 30" 1' 2' 4' TTS (N) 26.94 22.38 18.63 12.44 8.94 9.19 6.25 TTS OL^: 19.44 15.69 12.44 7.81 4.31 4.69 3.81 SD CN) 10.25 • 9.58 8.99 7.62 4.75 3.56 3.06 SD ( H ^ ) 9.60 9.57 9.39 7.34 5.36 2.82 1.00 N 4 4 4 4 4 4 4 t -2.18 -2.48 -2.49 -2.50 -3.26 -3.72 -2.26 P CO.20 ^0.10 < 0.10 <0.10 O . 0 5 C 0.05 < 0.20 -71-TTS (dB) 3 5 -5" 10" 15" 30" V 2' 4' Fig. 5.16 Comparison of N vs. H,, f o r 4 female subjects. 5" 20" 25" 30" V 2' 4' TTS CN) 31.94 27.25 22.88 17.13 12.63 12.82 8.88 TTS GL^) 20.38 24.38 21.38 7.38 5.00 6.00 5.63 SD CN) 7.48 6.46 4.53 3.01 3.01 1.42 3.02 SD CH^) 6.22 5.69 4.85 3.20 2.96 3.30 4.40 N 4 4 4 4 4 4 t -5.21 -7.01 -5.48 -4.95 -5.05 -3.50 -1.08 P < Q.02 <0.01 < 0.02 < 0.02 <0.02 < 0.05 > 0.20 -72-TTS (dB) 35-30 25 20-15-10-5-0+•/-!-5" —I 1— 10" 15" —l— 30" "T~ r -T -2' 4' Fig. 5.17 Comparison of N, T, H^a for 8 subjects (2 males and 6 females) TTS (dB) 35-30-1 25-20-15-10-5-0-fv/-r— 5" 10' ~~1 15' 3 0 " 2' -1 t 4' Fig. 5.18 Comparison of N, L, for 8 subjects (2 males and 6 females) -73-TTS (dB) 35-30-25-20-15-10-5-1 ° Q H ) o A A H 2 O 0 -hv-r-5" ~ i 1 — 10" 15" 30' T -2" i V ~T~ 4' Fig. 5.19 Comparison of H^a vs. H 2 a for 8 subjects (4 males and 4 females). 5" 10" 15" 30" 1' 2' 4' TTS(Hn ) l a 23.00 18.63 15.13 .9.81 6.63 6.75 4.88 TTS(H 2 ) 20.03 16.53 14.16 9.78 7.09 8.28 5.72 SD (H l a) 8.92 7.71 6.46 4.79 3.58 3.03 3.54 SD (H 2 a) 8. 54 7.85 7.20 5.47 4.45 3.45 3.70 N 8 8 8 8 8 8 8 t -0.99 -0.85 -0.53 -0.02 0.35 1.13 0.61 P > 0.20 >0.20 >0'.20 > 0.20 > 0.20 >0.20 >0.20 -74-TTS (dB) 35-30-25-20-15-10-a ° Hla A A Hlb -A-01// i 5" —I 1 — 10" 15" ~T~ 30' T " 2' 4' Fig. 5.20 Comparison of vs. for 8 subjects (4 males and 4 females) TTS(H l a) TTSGi^) SD (H I a) SD (K^) N t P 5" 23.00 19.91 8.92 7.51 8 -1.40 > 0.20 10" 18.63 15.03 7.71 7.32 8 -2.11 < 0.10 15" 15.13 11.91 6.46 6.94 8 -2.44 < 0.05 30" 9.81 7.59 4.79 5.24 8 -2.06 < 0.10 1' 6.63 4.66 3.58 4.02 8 -1.63 < 0.20 2' 4' 6.75 4.88 5.34 4.72 3.03 3.54 2.93 3.11 8 8 -1.48 -0.11 < 0.20 >0.20 -75-TTS (dB) 35-30H • 25H X 20H 15-10-o D H t o A A L s X N. X X o. x. x V A A~., 0-{-//—\ — i 1 1 1 — | 1 t 5" 10" 15" 30" 1' 2' 4" Fig. 5.21 Comparison of L vs. H, for 8 subjects (2 males and 6 females). 5" 10" 15" 30" 1' 2' 4' TTS (L) 28.25 23.38 19.81 13.63 9.31 9.88 8.63 TTS (PL l a > 23.84 19.09 15.34 9.16 4.97 5.66 4.59 SD (L) 10.15 8.91 7.61 6.47 4.95 4.05 4.85 SD (H l a) 7.41 6.34 5.90 5.35 4.94 3.35 3.02 N 8 8 8 8 8 8 8 t 1.87 1.86 2.24 2*. 68 3.44 4.35 3.84 P < 0.20 < 0.20 < -o.io < 0.05 < 0.02 <0.005 £ 0.01 -76-TTS (dB) 35-30-25-20-15-10-5-A A. J. A . ••, 5" 10" 15" 30" T 2' i 4' Fig. 5.22 Comparison of L vs. for 2 male subjects. 5" 10" 15" 30" 1' 2' 4' TTS (L) 21.50 17.50 14.75 .10.25 7.50 7.75 6.00 TTS(Hn ) l a 26.13 21.63 18.38 12.13 6.88 5.88 3.63 SD (L) 13.08 11.67 11.31 9.90 7.42 4.95 6.01 SD (H l a) 11.84 11.14 10.08 8.31 7.25 5.83 6.19 N 2 2 2 2 2 2 2 t -5.26 -11.16 -4.13 -1^66 4.50 3.03 17.00 P < 0.20 < 0.10 < 0.20 0.05 < 0.20 > 0.20 O . 0 5 TTS (dB) 35 30 25-20-15-10-5-5"' \ \ "I 1 10" 15' ° D H , 0 A A L a. -1 • 4' 30" r 2" Fig. 5.23 Comparison of L vs. for 6 female subjects. 5" 10" 15" 30" 1' 2« 4' ITS (L) 30.50 25.33 14.75 14.75 9.92 10.58 9.50 TfS(H l a) 23.08 18.25 14.33 8.17 4.33 5.58 4.92 SD (L) 9.25 8.09 .5.73 5.73 4.65 3.95 4.70 SD (H l a) 6.78 5.29 4.84 4.64 4.67 2.97 2.15 N 6 6 6 6 6 6 6 t H. 24 3.75 5.69 5 «31 4.26 4.50 3.42 P < 0.01 < 0.02 <0.0025 C0.005 < 0.01 <0.01 <0.02 -78-TTS (dB) 35-30-25-20-15-10-5-0 * A H2a '•a 5" r T t 10" 15' 30" i 2' Fig. 5.24 Comparison of vs. H 2 a for 4 male subjects. 5" 10" • 15" 30" 1' 2' 4' TTS(H l a) 23.19 18.81 15.44 9.94 6.69 6.94 4.31 TTS(H 2 a) 18.38 16.00 14.00 9.88 7.00 7.88 5.00 SD (H l a) 10.05 8.83 7.77 5.95 4.23 4.23 4.77 SD (H 2 a) 9.30 9.34 8.95 7.11 6.06 4.75 4.45 N 4 4 4 4 4 4 4 t -1.03 -0.76 -0.46 -0.03 0.17 0.35 0.25 P > 0.20 >0.20 >0.20 >0.20 >0.20 >0.20 >0.20 -79-T T S °.Hi„ 4 H 2 o D F r 5" 10'" 15" 30" V T 4' Fig. 5.25 Comparison of H vs. H"2 for 4 female subjects. 5" 10" 15" 30" 1' 2' 4' * .TTS (H., ) l a 22.81 18.44 14.94 9.69 6.56 6.56 5.44 TTS(H 2 ) 21.69 17.06 14^31 9.69 7.19 8.69 6.44 SD (PL ) l a 9.20 7.79 . 5.85 4.25 3.47 1.87 2.38 SD (K 2 a) 8.73 7.48 6.39 4.37 3.09 2.19 3.28 N 4 4 4 4 4 4 4 t -0.27 -0.36 -0.23 otoo 0.28 1.84 0.93 P > 0.20 >0.20 > 0.20 >0.20 > 0.20 > 0.20 > 0.20 35-1 30-25H V- •.. A 2<H 5-0- - / / - i 1 1 r -80-TTS (dB) 35-30-25-20-15-10-5-° - 0 HIo V H,b 0-|-//~r— 5" .":2 ~ I t 4" I i 10" 15' 30' i V i 2' Fig. 5.26 Comparison of vs. for 4 male subjects. 5" 10" 15" 30" TTS(H l a} 23.19 TTS (PL,) 19.44 SD (PL ) 10.05 l a i SD (PL, ) lb N t P 9.60 4 -1.30 > 0.20 18.81 15.69 8.83 9.57 4 -1.46 > 0.20 15.44 12.44 7.77 9.39 4 -1.51 > 0.20 9.94 7.81 5.95 7.34 4 -1.31 > 0.20 6.69 4.31 4.23 5.36 4 -1.38 > 0.20 6.94 4.69 4.23 2.82 4 -1.43 > 0.20 4.31 3.81 4.77 1.00 4 -0.22 > 0.20 -81-TTS (dB) 35H 30-25-20-15-10-5-D. A HL B - • A - — — 0 1 // i 5" —| 1 r— 10" 15" 30' 1 — 2" i 4" Fig. 5.27 Comparison of H l a vs. f o r 4 female subjects. TTS(H l a) TTSCH^) SD (H l a) SD (H^) N t P 5" 22.81 20.38 9.20 6.22 4 -0.65 > 0.20 10" 18.44 14.38 7.79 5.69 4 -1.38 > 0.20 15" 14.94 11.38 . 5.85 4.85 4 -1.70 < 0.20 30" 9.69 7.38 4.25 3.20 -1.31 >0.20 6.56 5.00 3.47 2.96 4 -0.80 >0.20 6.56 6.00 1.87 •3.30 4 -0.50 >0.20 5.44 5.63 2.38 4.40 4 0.08 >0.20 -82-Exhaling After Preparation for Hurarning (HQ). The second analysis was a TIME by CONDITION (N, H^, H ) ANOV with 8 subjects (2 males, 6. females). Table 10 i s a summary of this analysis. Once again, CONDITION and TIME were significant, CONDITION at the .05 level and TIME at the .01 level. There was no significant CONDITION x TIME interaction. Table 11 presents a summary of the Newman-Keuls probe of the condition effect across a l l post-exposure times. As expected, H ^ was significantly different from N. and H q were also significantly different; There was no significant difference between N and Ho. (See also Fig.'s 5.5 and 5.2). Turning (T). The third analysis was a TIME by CONDITION (N, H ^ / T ) ANOV with 8 subjects ( 2 males, 6 females). The results of this analysis are presented i n Table 12. CONDITION and TIME each were found to have a highly significant effect; also there was a highly significant CONDITION x TIME interaction. Table, 13 i s a summary of the Newman-Keuls probe of the condition effect across a l l post-exposure times. I t may be seen that the Ej^ humming condition was significantly different, at the .01 le v e l , from the N condition ; but the T and N conditions were not significantly different, nor were the T and H ^ conditions. (See also Fig. 5.17). The ANOV, however, indicated a significant CONDITION x TIME inter-action, so information about the effects of the exposure conditions at various post-exposure times was sought. Accordingly, Newman-Keuls tests were made on the effects of the conditions at t = 10s, t = 15s, and t = 1 min. The results, i n Table 14, indicate that E^ was more s i g n i f i -cantly different from N at the e a r l i e r times (10s and 15s) than at the later ( 1 min ) time. (See also Fig's 5.17 and 5.2). The difference -83-TIME x CONDITION Table 10. Summary of ANOV. TTS as a function of post-exposure time and condition ( N,H1 ,H ) for 2 male and 6 female subjects. 11 1 SOURCE OF VARIANCE df MS F Total 167 -Subjects 7 . 682.09 ft Condition (C) 2 651.29 5.74 C x Subjects 14 113.48 Time (T) 6 1440.24 A A 30.61 T x Subjects 42 47.05 T x C 12 1.73 -o.61 T x C x Subjects 84 -2.85 Table 11. Results of Newman-Keuls test for significance of differences between T T S for N, H L A , H q conditions. (2 male and 6 female sub j ects). Condition ^ N / Mean H • H N l a o 11.81 17.62 17.80 R, 11.81 l a H 17.62 o ^ 5.8l" 6.00*' 0.18 Sppg-q.99 (r,14) 5.99 6.96 * i p < .05 ** p < .01 -84-TIME x GONDTION Table 12. Suinmary of ANOV. TTS as a function of post-exposure time and condition C N,H_ ,T ) for 2 male and 6 female subjects. SOURCE OF VARIANCE i df MS F Total 167 Subjects 7 675.03 Condition (C) 2 388.69 A A 6.72 C x Subjects . 14 57.87 Time (T) 6 1381.37 * * 42.99 T x Subjects 42 32.14 T x C 12 5.93 A 3.26 T x C x Subjects 84 1.82 Table 13. Results of Newman-Keuls test for significance of differences between TTS for N, ^  , T conditions. (2 male and 6 female subjects). Condition • — > Hn T l a N I Mean 12.01 14.65 17.28 Hi l a T 12.01 14.65 * 2.63 5.27 • 2.64 <r,14) . 4.28 4.97 * p < .05 **• p < .01 -85-Table 14. Results of Newman-Keuls test for significance of differences between TTS for N, , T conditions. (2 male and 6 female subjects). Cond. Ltion ^ , Mean PL T N l a 18.28 21.22 25.35 t=aos PL 18.28 l a T 21.22 2.94 7.06"" A 4.13" S ^ g q.99 (r,14) 4.28 4.97 Cond: \ / Ltion y f Mean PL T N l a 15.09 17.84 21.28 t=15s H l a 15.09 T 17.84 2.75 6.19*"""" 3.44 S ^ q . 9 9 (r,14) 4.28 • 4.97 Cond Ltion ^ f Mean PL T N l a 6.28 8.41 10.75 t=lmin PL 6.28 l a T 8.41 2.13 4.47*" 2.35 S^g- q.99 (r,14) 4.28 4.97 * p < .05 * * p < . 01 between T and N was. significant, at the .05 le v e l , at time t s 10s only, accorxiing to the Newman-Keuls test, and at t = 15s as well as t = 10s, according to the t-tests (Pig. 5.6). At the other times this difference was not significant but was sometimes close to the .05 level. Listening C D . The fourth analysis CTable 15) was a TIME by CONDITION CN, H l a, L) ANOV with 8 subjects C2 males, 6 females), i n which CONDITION and TIME were found to be highly significant (. 01 level) and the CONDITION x TIME interaction non-significant. A Newman-Keuls probe of the condition effect over a l l times CTable 16) revealed that, as expected from the second analysis, the difference between and N was highly significant. H^a was also significantly different from L; N and L were not significantly d i f -ferent. CSee Fig.'s 5.18, 5.7, 5.8, 5.9, 5.21, 5.22, 5.23). A l l Conditions. The la s t analysis was a SEX (male, female) by TIME (5s -4 min) by CONDITION CN, R^, H 2 a, H^, H , T, L) ANOV i n which 4 subjects C2 males, 2 females) were tested under each condition. Presented i n Table 17 i s the outcome of this analysis. I t may be seen that SEX was not sig-nificant, as was found also i n the f i r s t ANOV (Table 2) and that CONDITION and TIME each had a highly significant effect. Also, and here the results d i f f e r from those of the f i r s t ANOV, there was a significant interaction between SEX and CONDITION and between TIME, SEX, and CONDITION, as well as a significant interaction between TIME and CONDITION. The condition effect was also analyzed separately for males and for females at three post-exposure times, t*= 10s, t = 15s, and t = 1 min. Table 18 summarizes these Newman-Keuls tests for males and Table 19 contains the results, of the test for females. The results were essentially i n agreement with the previous four analyses. One difference, hcwever, was that i n the male data H. was the only humming condition significantly -87-TIME x CONDITION Table 15. Summary of ANOV. TTS as a function of post-exposure time and condition ( N,H, ,L ) for 2 male and 6 female subjects. SOURCE OF VARIANCE df MS F Total 167 Subjects 7 785.76 Condition (C) 2 535.91 A A 7.28 C x Subjects 14 73.66 Time (T) 6 1421.90 67.31 T x Subjects 42 21.13 T x C 12 0.71 0.20 T x C x Subjects 84 3.46 Table 16. Results of Newman-Keuls test f o r significance of differences between TTS for N, , L conditions. (2 male and 6 female subjects). Condition > N/ Mean " l a L N 11.81 16.13 17.80 R. 11.81 l a L 16.13 A A A ^ 4.3l" 5.99"" 1.68 S^g q.99 (r,14) 4.83 5.61 * P < .05 ** p < .01 -88-SEX x TIME x CONDITION Table 17. Summary of ANOV. TTS as a function of sex, post-exposure time, and condition ( N, R ^ , H^, R ^ J H , T, L') for 2 male and 2 female subjects. SOURCE OF VARIATION df MS F Between Subjects 3 Sex 1 646.87 0 .28 Subjects 2 2295.33 Within Subjects 192 Time (T) 6 1838.70 35. A .t. «« *\ 69 . Sex X T 6 20.04 0. 39 T X Subjects 12 51.52 Condition (C) 6 106.80 5. A A «•» 15 Sex X C 6 85.47 4. 12 * C X Subjects . 12 20.75 T X C 36 3.36 1. 95 Sex X T X C 36 5.94 3. 44 T X C X Subjects 72 1.73 *. p < .05 p <.01 -89-Table 18. Results of Newman-Keuls test for significance of differences between T T S for N, H , H_ , H 1 K , H , T, L conditions. (2 male , . . \ _La /a _LD o subjects). C o n d i t i o n — ^ t=10s Mean H 2 a H 0 H-, H, lb l a N 17.50 18.25 20.13 21.50 21.63 23.00 24.13 L 17.50 0.75 2.63 4.00" 4.13" 5.50"" 6.63 H 2 a 18.25 1.88 3.25 3.38 ft 4.75 5.88 H o 20.13 1.38 1.50 2.88 . 4.00 "it 21.50 0.13 1.50 2.63 21.63 1.38 2.50 T 23.00 1.13 S ^ q.99 (r,l<2) 3.72 4.34 4.73 5.03 5.25 5.44 Co n d i t i o n — ^ t=lmn . . Mean H l a H l b H 0 H 2a o N 6.: 7.25 7.50 7.50 8.88 9.50 11.00 H l a 6.88 0.38 0.63 0.63 2.00 2.62 4.13 H l b 7,25 0.25 0.25 1.63 2.25 3.75 L 7.50 0.00 1.38 2.00 3.50 H 2 a 7.50 1.38 2.00 3.50 H 8.88 * 0.63 2.13 o N 9.50 - 1.50 ~ o q.99 (r,12) 3.72 4.34 4.73 5.03 5.25 5.44 ^ns * p<.05 < .01 -9.0-Table 19. Results of Newman-Keuls test for significance of differences between T T S for N, H, , H„ , H_. , H , T , L conditions ( 2 female subjects). Condition —^ t=10s Mean V H.,, H-, lb l a H 2 a T H N 18.50 20.00 23.13 26.25 29.00 30.13 30.88 H l b 18.50 1.50 4.63"" 4\ 4 7.75 10.50"' ' 11.63"" 12.38" H l a 20.00 3.13 AJ 6.25"' 9.00*" 10.13*""" 10.88" H 2 a 23.13 - 3.13 A. 4\ 4 5.88 ft* 7.01 7.75" T 26.25 2.75 3.88 ft 4.63 H o 29.00 1.13 1.88 N 30.13 0.75 a.99 (r,12) 3.72 4.34 4.73 5.03 5.25 5.44 xTS Condition-^ t=Imin Mean lb l a 7.00 7.25 7.75 H 2a N H o 9.38 10.88 12.38 14.00 lb la 2a 7.00 7.25 7.75 9.38 10.88 12.38 0.25 0.75 0.50 2.38 3.88 5.38 7.00 2.13 3.63 5.13*" 6.75* 1.63 3.13 4.63*" 6.25 1.50 3.01 4.63 «• 1.51 3.13 1.62 4.73 5.03 5.25 5.44 S^g q.99 (r,12) 3.72 4.34 p < .05 < .01 -91-different from N; at t = 10s this difference was significant at the .01 le v e l , but at t = 15s and t - a min the difference lost significance. In the female data, as i n the f i r s t analysis (Table 2) a l l three humming conditions were significantly different from N. 5.22 Impedance Experiments Procedure. Acoustic stapedius reflex thresholds at 250, 500, and 1000 Hz were determined for the exposure ears of a l l subjects (4 males, 12 females) by standard procedures (jerger, 1970, p. 312). ' Impedance during exposure for 5 min to the 500-Hz, 118-dB SPL tone was monitored i n the non-exposure ears of 6 subjects (2 males, 4 females). Because of the b i l a t e r a l nature of the acoustic reflex, contraction of the MEM of the exposure ear i s reflected i n impedance changes i n the opposite ear. Impedance was also measured on one subject who hummed loudly during the 5-min exposure (condition H.^). To investigate MEM ac t i v i t y during the various a c t i v i t i e s performed by the subjects i n the TTS experiments, impedance was measured on two subjects (1 male, 1 female) while they performed each one of the 5 activ-i t i e s performed i n conditions H^ , E^a, H^, H , and T (See Section 5.21 Design). Then these same two subjects performed the above a c t i v i t i e s a second time while receiving the exposure tone for a period of 30s for each activity. 2 or 3 min intervened between a c t i v i t i e s while the subjects prepared for the next activity. Analysis and Results. Acoustic Stapedius Reflexes (AR). Owing to d i f f i c u l t y i n achieving an air-tight seal of the probe t i p i n the external canals of 4 subjects, AR data was available for only 12 subjects (4 males, 8 females). The following information was obtained: (1) AR (SL): AR threshold at 500 Hz, -92-i n Sensation Level; (2) AR (HTL): AR threshold at 500 Hz, i n Hearing Threshold Level (ISO, 1961 standard): and (3) TTS from the 5-min, 118-dB SPL exposure tone, as characterized by (3a) TTS at t = 0: extrapolated value of TTS at 0 post-exposure time, and (3b) Tj : the time required for decrease of TTS to one-half i t s value at t = 0. Pearson product-moment correlation coefficients were calculated between: (1) and (3a); (1) and (3b); (2) and (3a); (2) and (3b). These coefficients were: TTS at t = 0 T, h r-0.03 -0.21 -0.15 -0.44 A l l correlation coefficients were small and negative, the coefficients between AR thresholds and Ta being greater than those between AR thresh-olds and TTS at t = 0. Although small, the correlation coefficients between TTS and the AR thresholds i n Hearing Threshold Level were greater than those between TTS and the AR thresholds i n Sensation Level. Impedance During Exposure. When the exposure tone was presented to the subjects, acoustic impedance f i r s t rapidly increased, reaching a maximum 5 - 20 seconds after exposure onset. Then, with continuing stimulation, impedance gradually decreased. As a result, Z(5min), the impedance at the end of the 5 minute exposure period, was on the average 60% of Z(15s), the impedance 25 seconds after commencement of the exposure. Va r i a b i l i t y among the 6 subjects was large and Z(5mm) ranged from 27% to 86% of Z(15s). HumTidng loudly while receiving the exposure tone resulted i n a slower decrease of impedance during exposure. The one subject who performed this ac t i v i t y exhibited an impedance at 5 min of 93% of that at 15s, whereas AR (SL) AR (HTL) - 9 3 -this same subject showed a 45% decrease i n impedance between 15s. and 5 min from the fatiguing exposure without any- activity. To determine whether there was any relation between rate of decrease of impedance during the fatiguing exposure and TTS produced by the expos-ure, Spearman rank-order coefficients of correlation were calculated between: CD TTS at t = 0 and the rat i o , i n percent, of g^Ss)"^' Z i C Siniri) (2) TTS at t = 15s and the ra t i o , i n percent, of %( i 5 S ) ' **an*:ec^ data were used to calculate the correlations because of uncertainty of a linear relation between recorded voltage and acoustic impedance. The correlation coefficients were negative and f a i r l y high: TTS at t = 0 TTS at t = 15s by rank by rank zSS)) b y r a n k "°-43 -°-77 This suggests that a slower decrease i n impedance, reflected i n a ^(^Ss)^ r a t i o closer to 100%, i s associated with less TTS, at t = 0 and t = 15s. Effect of Ac t i v i t y on Impedancei For both subjects (male and female), huriming comfortably at either a low or high frequency or humming loudly resulted i n a larger impedance than that without humming or any other act i v i t y . This result was observed whether the humming was performed with the exposure tone (for 30s) or without i t . For the male subject, the impedance for humming high and for humming loudly was larger than' the impedance for humrning low and comfortably, whether the humnring was performed with the exposure tone (for 30s) or without i t . There were no noticeable differences between the impedance for humniing high and hunnning loudly for this subject. For the female subject, the impedance for hum-ming loudly was larger than the impedance for humrning high and for hum-- 9 4 -jttlng low and comfortably, whether the humming was. performed with the exposure tone (for 30s) or without i t . The la t t e r two hurcming a c t i v i t i e s had similar effects on impedance for this subject. For both subjects, ex-haling after preparation for humming (as i n H ) appeared not to alt e r impedance, while turning the head (as i n T) produced intermittent i n -creases i n impedance. These results were observed whether the H q and T ac t i v i t i e s were performed with the exposure tone or without i t . Chapter 6 DISCUSSION AND CONCLUSIONS The above results concerning the effect of phonation on TTS are essentially i n agreement with those of Karlovich and Luterman (1970 ). Ccmmon to their study with a 1000-Hz tone and to the present study with a 5Q0-Hz tone are the findings that: (1) phonation during fatiguing exposure consistently reduces TTS from the exposure at each post-exposure time for both males and females; and (2) TTS i s a function of the post-exposure time at which i t i s measured (because of recovery of TTS). In addition, i t was found i n the present study that an increase i n fundamental frequency or i n intensity of phonation seems to further decrease TTS, although not significantly. The jiu\nimal tendency noted by Karlovich and Luterman for females to exhibit greater TTS differences than males between the phonation and silence conditions was also apparent i n the present study. These male/female differences were, i n fact, more noticeable i n the present study than i n Karlovich and Luterman's. In this respect mention should be made of Ward's (1966) observation that the difference between TTS from octave-band noise presented i n 1-second pulses and TTS from the same noise continuously- presented was significantly greater for females than for males. The greater TTS decrease for females, he suggested, might be the result of females perhaps having more ef f i c i e n t MEM than males. Their MEM, contracting rapidly i n response to the noise, should effectively reduce i t s intensity. A similar situation, i n which the MEM are periodically re-activated by intermittent acoustic stimulation, can be expected to occur during the humming a c t i v i t i e s of the present experiments and during the vowel-production a c t i v i t i e s of the experiments of Karlovich and Luterman. -95-The male/female differences observed i n both of these investigations involving phonation may, therefore, be accounted for by Ward's hypothesis of a more e f f i c i e n t MEM system i n females. Certain observations, however, suggest caution against accepting this as the f u l l explanation for these differences. F i r s t , the MEM reflex i s activated acoustically by the exposure tone; i f females possess more e f f i c i e n t MEM than males, less TTS should be produced i n females' ears by this tone (condition N) owing to the better "protection" resulting from the reflex. This result was, i n fact, obtained by Ward (1966), but i n the present investigation as well as i n that of Karlovich and Luterman the females shewed s l i g h t l y , but not significantly, greater TTS magnitudes than the males. Secondly, the exposure frequency used i n the present TTS experiments (500 Hz) i s known to be attenuated to a greater degree by the MEM than is the frequency of Karlovich and Luterman's exposure tone (1000 Hz); yet the decrease i n TTS for the phonation conditions of both investigations was very similar. I f TTS reduction during phonation were entirely the result of MEM action, one would expect phonation to reduce TTS from a 500-Hz exposure more than TTS from a 1000-Hz exposure. There i s s t i l l another reason for doubting that MEM action i s wholly responsible for the reduction of TTS observed for the phonation conditions. In the present experiments phonation (and, presum-ably, MEM activity) occurred for a greater percentage of the exposure period than i n Karlovich and Luterman's experiments (70% as opposed to 50%), but i n both studies the TTS reduction brought about by phonation was similar. Perhaps MEM activity has less of an effect on TTS than hitherto believed. The present study indicated that a non-acoustic ac t i v i t y such as turning, which i s believed to e l i c i t MEM contraction, reduces TTS, s i g -n i f i c a n t l y at t = 10s, less significantly at other tiroes. The TTS from turning, however, i s never reduced to the extent that i t i s from hurnidng. Similar slight reductions i n TTS occur for other a c t i v i t i e s which e l i c i t MEM contraction: swallowing, chewing, and smiling with forceful eye clos-ure. To assess more f u l l y the effect of the MEM further experiments w i l l be necessary i n which MEM a c t i v i t y i s e l i c i t e d i n various ways, for example by cutaneous stimulation, swallowing, yawning, etc. It has been suggested that the sound of the voice triggers MEM con-traction during phonation. This hypothesis, however, does not seem to hold i n the ligh t of recent electromyographic evidence that MEM ac t i v i t y pre-cedes voice onset (Djupesland, 1967; Shearer and Simmons, 1965) Neither i s i t well supported by the present study. I f the acoustic energy of hum-ming were sufficient to trigger MEM contraction, then TTS from listening to humrcrihg during exposure (condition L) should have been of the order of that observed for the T condition. In actual fact, TTS from the L con-dition was greater than TTS from the T condition (See Fig.'s 5.17, 5.18). Unlike the situation for T, TTS from L was never significantly different from N. The phonation experiments of the present study, however, also supply an observation compatible with acoustic MEM activation by the voice, namely that humming loudly during exposure (H-^) produced less TTS than humnung at a lower level (H^a) • The more intense the voice the greater the expected MEM contraction and, therefore, the smaller the TTS. Obviously, more experiments are necessary to sa t i s f a c t o r i l y resolve this issue. Several studies have proposed that MEM contraction during phonation may be neurologically associated with laryngeal activity. As Shearer and Simmons (1965) suggest, the occurrence of MEM ac t i v i t y before voice onset seems to indicate that the MEM are activated concurrently with the laryn-geal musculature. Perhaps a non-acoustic reflex arc exists between the -98-larynx and the MEM, making possible the contraction of the MEM during pho-nation. McCall and Rabuzzi (1970) found evidence for this type of reflex arc i n the cat when they recorded reflex a c t i v i t y i n the MEM of both ears i n association with contraction of the cricothyroid muscle of the larynx. One of the exposure a c t i v i t i e s i n this study, the activity of preparing to hum but exhaling without phonation (H ), results i n approximation of the vocal folds, as can be seen by inspection of the photographs i n Figure 6.1. These photographs show the state of the larynx of a male subject as he: a) exhaled; b) exhaled without phonation after preparing to hum (H Q); and c) hummed at approximately 125 Hz. I t may be seen that during the H Q a c t i v i t y there was p a r t i a l adduction of the vocal folds and constriction of the g l o t t i c sphincter brought about by contraction of muscles of the l a r -ynx. I f the laryngeal muscle action of the H q activity reflexly e l i c i t s MEM contraction, TTS from the H q condition should be similar i n magnitude to TTS from a condition such as turning during exposure (T),'which i s believed to e l i c i t MEM contraction. TTS from H , however, was greater than for T and not essentially different from N. A second mechanism that can be postulated to explain contraction of the MEM during phonation i s that the MEM are stimulated to contract concurrently with the laryngeal mus-culature by neural impulses directly from the motor cortex. Evidence upon which to base a decision between this last mechanism and a reflex mechanism might be produced by experiments on laryngectomees. If TTS reduction should occur when a laryngectomee "thinks about humming" during the exposure tone, i.e. when he concentrates on humming, without a larynx and without being required to produce sound (provided this i s possible), the hypothesis of MEM activation by neural impulses from the cortex would seem the more promising, since laryngeal excision prevents a larynx-MEM reflex. -99-Figure 6 . 1 Three photographs, taken with a fiberscope, of the g l o t t i s (a) exhaling (b) exhaling after preparing to hum (c) humming at approximately 125 Hz. -100-In the bat, contraction of the MEM shortly before emission of high-intensity orientation vocalizations attenuates these vocalizations, and the subsequent relaxation of the MEM restores hearing sensitivity for echoes (Henson, 1965). Contraction of the MEM of man during phonation may prevent man from hearing his own voice too loudly. Or, as Carmel and Starr (1963) have suggested, MEM contraction i n association with phonation and with other somatic motor a c t i v i t i e s such as swallowing or yawning may not have an auditory function but may merely be a concomittant of complex motor acts such as speaking which involves cranial nerves V, VII, IX, X, XI, and XII. Phonation during exposure seems to be required for a marked decrease i n TTS: i t i s not enough that subjects merely hear their humming during exposure, as i n condition L. Although contraction of the MEM appears to reduce TTS, MEM activity cannot f u l l y account for the TTS decreases observed for the phonation conditions. There i s no question that MEM ac t i v i t y reduces the amplitude of sound transmitted across the middle ear. For example, i n cats whose MEM have been deactivated by anaesthesia, the cochlear microphonic responses to tones of one to five minute durations are larger than i n the same cats, awake and with normally-muscled ears (Galambos and Rupert, 1959 i n Galambos, 1960). In attempting to find causes for reduced TTS i t i s reasonable to seek mechanisms which attenuate sound transmission to the cochlea. TTS i s a function of the level of the exposure tone: increases i n the level o^.the exposure tone increase TTS and vice versa (See Fig. 2.3). I f the attenuation provided by MEM contraction i s not sufficient to decrease TTS to the extent that humming did, then there must be other mechanisms to account for reduction of TTS by phonation. A mechanism which might account for TTS decrease when phonation -101-accompanies tonal exposure has been suggested by Karlovich and Luterman (1970); namely that, as Bekesy- observed, vibration of the stapes i s altered during phonation so as to attenuate transmission of sound to the cochlea. (See Section 2.22). During phonation the v e r t i c a l vibration of the s k u l l causes the stapes to vibrate i n a manner less e f f i c i e n t for transmitting pressure changes to the cochlear f l u i d . According to Bekesy, what causes the large v e r t i c a l vibrations of the s k u l l during phonation are the v e r t i c a l vibrations of the edges of the vocal folds and the vibrations of the lower jaw and s k u l l that result from the sound pressure i n the mouth (Bekesy, 1960, p. 189). The present study does not rule out this mechanism of sound attenuation during phonation as an explanation for reduction of TTS: TTS from Hstening to humming during exposure (condition L) was i n general significantly different from TTS from hum-ming during exposure, but not significantly different from TTS from the exposure tone without a supplementary act i v i t y (condition N). Since air-borne played-back hunming of condition L i s unlikely to cause the head to vibrate i n a predcminantly v e r t i c a l motion, the vibration of the head i n condition L i s not expected to be the same as during the phonation conditions, and, therefore, the stapes vibration mode i n condition L i s also most l i k e l y not the same as during the phonation conditions. An experiment i n which a v e r t i c a l vibration of the s k u l l (similar to that occurring during phonation) can be produced would help to c l a r i f y this point. Thus far, explanations for TTS decrease with phonation have concentrated on mechanisms which reduce transmission of sound to the cochlea. Auditory fatigue i s a complex phenomenon and can be influenced by many factors. The source of fatigue seems to be between the level of -102-electrocochlear a c t i v i t y and early neural processes. Fatigue seems to be absent i n the cochlear microphonics since these potentials i n experimental animals do not decrease i n magnitude during a sustained tone, as long as the MEM have been rendered non-functional and the tones are not extremely intense (Wever, 1949, p. 325). Although the part of the e l e c t r i c a l a c t i v i t y (click-evoked, i n this case) recorded at the round window of cats, which i s attributed to the cochlear microphonics, does not change msasureably after several minutes' exposure to intense tones, the components of the round-window response which are though to reflect e l e c t r i c a l a ctivity i n the auditory neurons are reversibly depressed (Rosenblith, 1950). Conceivably, any events which modify physiological activity i n the area(s) involved with TTS w i l l alter TTS. The po s s i b i l i t y should be considered that interactions between the complex sound of humnring and the exposure tone may, i n some way, lessen the effect on the hair c e l l s and auditory neurons of prolonged stimulation by the exposure tone and therefore decrease TTS. "Two-tone inhibition" or "interference" has been described i n audiological literature as a reduction i n the rate of response to one tone by the addition of a second tone. Studying this phenomenon i n the cat, Sachs and Kiang (1968) discovered that "two-tone inhibition occurs for a l l auditory-nerve fibers i n the cat" and that "the general characteristics of this inhibition are similar for a l l fibers", (p. 1128). Two-tone inhibition was found to be a function of the frequency and level of the two stimuli: the nearer i n frequency and higher i n level the interfering tone, i n relation to the tone whose neural response was being measured, the greater was the reduction i n neural response. For the phonation conditions, however, one cannot speak of true "two-tone" inhibition, as only the exposure stimulus -103-i s a pure tone, whereas humrrdng i s a complex sound. The fact that, i n the phonation experiments, TTS decrease tended to become greater as the fundamental frequency of phonation increased from 125 Hz to 250 Hz to 500 Hz (See Fig.'s 5.10, 5.12, and 5.14) could be explained i n at least two different ways. The f i r s t explanation considers that the strongest component of the spectrum of humming i s the fundamental. Thus, since two-tone inhibition increases as the two tones approach each other i n frequency, one would expect this inhibition to become greater as the fundamental frequency of humrrdng rises. The second explanation considers that the most important component. of the spectrum of humrciing i s the harmonic closest to the exposure tone, 500 Hz i n this case. Thus one could expect the 500-Hz harmonic to be stronger when i t i s the second harmonic than when i t i s the fourth. Another po s s i b i l i t y which may be more r e a l i s t i c i s that the two previous explanations are complementary. There i s also evidence from the present study which leads one to question the interference hypothesis as an explanation for reduced TTS during phonation: listening to humming did not reduce TTS as phonation did. These two conditions, however, d i f f e r i n the mode of transmission of the two humrrdng sounds to the cochlea. Because humming i s transmitted predominantly via bone-conduction and the recorxiing of humming predominant-l y v i a air-conduction, amplification or attenuation of the various harmonics w i l l most l i k e l y be different for the two sounds. Before accepting an interference, hypothesis to account for TTS decrease during phonation, more electrophysiological evidence w i l l be required, evidence which can be correlated with.psychophysical data concerning TTS and phonation. An additional experiment on a laryngectomee might also provide information about the relative importance of a sound--104-interference hypothesis for reduced TTS during phonation conditions. For example, i f oesophageal speech production (or oesophageal humming, i f possible) should result i n TTS reduction similar to that observed i n the phonation conditions with normal subjects, then the interference hypothesis becomes more tenable. During oesophageal speech production, the head (and therefore the stapes) w i l l most l i k e l y vibrate differently than during normal phonation, so Bekesy's stapes-vibration hypothesis w i l l probably not explain TTS decreases. The MEM hypothesis, as has been pointed out, does not seem able to f u l l y account for the decrease i n TTS observed i n the phonation experiments. Therefore, by elimination, the hypothesis of interference between the voice sound and exposure tone w i l l be more promising. Recently, the question has been raised as to whether there are central influences on auditory fatigue. This possibility was f i r s t suggested by Wernick and Tobias (1963) who reported that when subjects were required to perform mental arithmetic during exposure to a 4000-Hz fatiguing stimulus, greater TTS resulted than when they were not required to perform any task during the same exposure. Since then a number of researchers have investigated the effect that performance of specific a c t i v i t i e s during fatigue exposure has on TTS (Bell and Stern, 1964; Collins and Capps, 1965; Durrant and Shallop, 1969; Pricke, 1966; Melnick, 1968; Smith and Loeb, 1968). Although most of the results of these investigations are equivocal, some of the. experiments indicate that attention may affect TTS. In view of these findings one wonders whether the decreases i n TTS i n the present experiments,. when humming accompanied tonal exposure, may be explained i n terms of an attentional effect. Certainly the humrning a c t i v i t i e s were attention-demanding: the subject -105-was required to watch the d i a l of the sound level meter and to start and stop humming when the lights were turned on and off. The H q condition, however, a non-humiung condition whose timing was also controlled by the lights, similarly demanded the subject's attention, yet did not result i n significant reduction of TTS. Prolonged stimulation by sounds of high intensity, such as the exposure tone of the present experiments, causes sustained contraction of the MEM, as implied by raised acoustic impedance throughout the exposure duration. One of the few studies which has investigated i n any d e t a i l MM ac t i v i t y during prolonged sound stimulation i s the study by Carmel and Starr (1963). When Carmel and Starr exposed cats for 2 hours to high-intensity (85 dB SPL) white noise, they observed that electromyographic ac t i v i t y of the MEM increased sharply at f i r s t as the noise was introduced, then decreased rapidly during the f i r s t few minutes of exposure, and, with continuing exposure, decreased gradually to eventually reach a plateau i n approximately 1 5 - 7 5 min. Increased EMG responses occurred when the animals moved around or vocalized. Under the same exposure conditions, e l e c t r i c a l activity at the cats' round windows and at stations higher i n the auditory pathway than the round window approximately mirrored MEM act i v i t y (Starr and Livingston, 1963). There was a very rapid i n i t i a l decrease of the round-window response, then a f a i r l y rapid r i s e i n amplitude of the response, followed by a slow 30-90 min continuing r i s e i n amplitude after which a plateau was reached. Movements and vocali2ation were associated with reductions i n the e l e c t r i c a l responses. I f the tendons of both MEM were sectioned there was no i n i t i a l attenuation of the round window response and the responses did not gradually r i s e as i n normal animals (Carmel and Starr, -106-1963). In the present study i t appeared that the more rapid the decrease i n MEM ac t i v i t y during the exposure tone (as assessed by rate of decrease of impedance) the greater the TTS produced by the exposure. I f MEM act i v i t y were to decrease more slowly, there would be more effective attenuation of the exposure tone. And, with more effective attenuation by. the MEM, less TTS should result, as was observed. Although Brasher, et a l . (1969) did not find a significant correlation between AR contraction after 2 min of continuous exposure to octave-band noises and TTS from these noises, their results are not contradictory to the above results. Brasher, et a l . correlated with TTS the impedance after 2 min of stim-ulation, whereas, i n the present study, a rank-order correlation was calculated between TTS and the r a t i o of the impedances: z(l5s)^ ' both studies no significant correlations were observed between AR thresh-olds Cat 500 Hz) and TTS. Carmel and Starr (1963) present evidence that the sustained cont-raction of the MEM i n response to steady noise i s regulated by complex central processes. Intercollicular decerebration, they observed, which leaves the AR centres and the MEM intact, resulted i n a very rapid i n i t i a l attenuation of the round window response, but a subsequent rapid r i s e to a stable plateau within 10 niin. On the basis of their results, they concluded: "The peripheral neuromuscular control[of the e l e c t r i c a l activity] i s obviously capable of responding rapidly to acoustic transients, but . . . during prolonged sound there appears to be a s h i f t from control via the middle-ear muscles to control operating along the central auditory pathway." (p. 611). In the present study, acoustic impedance (MEM activity) decreased more slowly when the subject performed a humming ac t i v i t y during the 5 min -107-exposure (condition H^) than when she received the exposure without performing any activity. The resultant TTS was also less for the humming condition. I f decrease i n MEM act i v i t y i s under central control, humming may interfere with this control. I t would be preiimture to postulate how or where the interference arises, since so l i t t l e i s known about the central control of the MEM. Possibly by studying the effect of phonation (and other a c t i v i t i e s ) on the relaxation of the AR and/or on TTS, we w i l l gain more knowledge about this phenomenon. Summary. This investigation was designed to study the effect of phonation and of some ac t i v i t i e s e l i c i t i n g middle-ear muscle contraction on high-intensity sound transmission i n the normal human auditory system. For the most part i t was concerned with the influence of phonation on TTS from a continuous pure-tone stimulus. The results of the study indicated that: (1) There were significant diffeiences i n TTS according to the post-exposure time at which threshold was measured. (2) TTS from the exposure tone accompanied by phonation (humming) was consistently and significantly less than TTS from the exposure tone without any supplementary activity. Humrrdng at a low or high frequency or at a high intensity (lew frequency) significantly decreased TTS. There were no significant d i f -ferences i n TTS between the above humming a c t i v i t i e s . (3) In general, there were no significant differences i n TTS between sexes. There was a greater difference, however, for females than for males, between TTS from phonation (hum-ming) during exposure and TTS from the exposure without phonation. *** (4) The differences between TTS from phonation (humming) during exposure and TTS from the exposure without an a c t i v i t y were most significant when post-exposure threshold was measured early (eg. 10s, 15s) after exposure cessation. (5) Repeatedly turning the head during exposure decreased TTS, significantly at 10s post-exposure. Similar slight decreases i n TTS were observed when the following activities which e l i c i t MEM contraction were performed during exposure: -108-chewing, smiling, swallowing. (6) listening to recorded humming during exposure did not sig-n i f i c a n t l y a l t e r TTS from the exposure. (7) The ac t i v i t y of exhaling after preparing to hum during exposure did not significantly alter TTS from the exposure. In addition to the TTS studies, measurements of MEM ac t i v i t y during the exposure tone and of acoustic reflex thresholds were obtained. Contraction of the MEM was studied by recording changes i n the acoustic impedance of the ear. • (1) Impedance markedly increased upon presentation of the exposure tone. With continuing stimulation, impedance gradually de-creased. (2) A slower decrease i n impedance during exposure was associated with less TTS from that exposure. (3) Whenever phonation (humming) was produced, the impedance became larger; whenever phonation (humrjiing) accompanied exposure, impedance became larger. (4) Exhaling after preparing to hum appeared not to alter impedance. (5) Turning the head caused impedance interndttently to become larger. • (6) There were no significant correlations between threshold of the acoustic reflex at 500 Hz and TTS from the exposure tone. Attenuation provided by middle-ear muscle contraction during phonation did not seem sufficient to decrease TTS to the extent that humming did. Sound may be attenuated by i n e f f i c i e n t stapes vibration during phonation and TTS may therefore be reduced. Two other possib-i l i t i e s were suggested to account for the TTS decrease: interference between humming and the exposure tone, and interference (by humming) with the central control of middle-ear muscle a c t i v i t y , but more evidence w i l l be necessary to sat i s f a c t o r i l y determine which, i f any, of these mechanisms i s actually i n effect. -109-BIBLIOGRAPHY Ba l l i n , M.J. (1926). Politzer's Text-Book of the Diseases of the Ear ( B a i l l i i r e , T i n d a l l , and Cox, London). von Bekesy, G. (1949). ""The Structure of the Middle Ear and the Hearing of One's Own Voice by Bone Conduction," J. Acoust. Soc. Amer. 21, 217-232. von Bekesy, G. (1960). Experiments i n Hearing (McGraw-Hill Book Co., New York). B e l l , D.W. , and Stern, H.W. (1964). "Effects of Extraneous Tasks on . Auditory Fatigue," J. Acoust. Soc. Amer. 36, 1162-1166. Botsford, J.H. (1971). "Theory of Temporary Threshold Shift," J. Acoust. Soc. Amer. 49, 440-446. Brasher, P.F., Coles, R.R.A. , Elwood, M.A. , and Ferres, H.M. (1969). "Middle-Ear Muscle Activity and Temporary Threshold Shift," Int. Audiol. £, 579-584. Carmel, P.W. , and Starr, A. (1963). "Acoustic and Nonacoustic Factors Modifying Middle-Ear Muscle Ac t i v i t y i n Waking Cats," J. Neurophysiol. 26_, 598-616. Collins, W.E., and Capps, M.J. (1965). "Effects of Several Mental Tasks on Auditory Fatigue," J. Acoust. Soc. Amer. 37_, 793-796. Davis, H., Morgan, CT., Hawkins, J.E. J r . , Galambos, R. , and Smith, F.W. (1950). "Temporary Deafness Following Exposure to Loud Tones and Noise," Acta Oto-laryngol. Suppl. 88. Djupesland, G. (1964). "Middle Ear Muscle Reflexes E l i c i t e d by Acoustic and Nonacoustic Stimulation," Acta Oto-laryngol. Suppl. 188. Djupesland, G. (1967). Contractions of the Tympanic Muscles i n Man (Universitetsforlaget, Oslo). Durrant, J.D., and Shallop, J.K. (1969)*: "Effects of Differing States of Attention on Acoustic Reflex Activity and Temporary Threshold Shift," J. Acoust. Soc. Amer. 46, 907-913. E l l i o t t , D.N., and Fraser, W. (1970). "Fatigue "and Adaptation," i n Foundations of Modern Auditory Theory, J.V. Tobias, Ed. (Academic Press Inc., New York). -110-Faaborg-Andersen, K. (1957). "Electromyographic Investigation of Intrinsic Laryngeal Muscles i n Human," Acta Physiol. Scand. 41, Suppl. 140. Fleer, R. (1963). "Protection Afforded Against Impulsive Noise by Vol-untary Contraction of the Middle Ear Muscles," Rept. 576. U.S. Army Med. Res. Lab. Fort Knox, Kentucky. Franzen, R.L., and L i l l y , D.J. (1970). "Threshold of the Acoustic Reflex for Pure Tones," J. Amer. Speech Hearing Assoc. 12, 435. Fricke, J.E. (1966). "Auditory Fatigue and Mental Activity," J. Auditory Res. 6_, 283-287. Galambos, R. (1960). "Studies of the Auditory System with Implanted Electrodes," i n Neural Mechanisms of the Auditory and Vestibular ' Systems,G.L. Rasmussen and W.F. Windle, Eds. (Charles C. Thomas, Springfield, I l l i n o i s ) , p.137. Galambos, R., and Rupert, A. (1959). "Action of the Middle Ear Muscles i n Normal Cats," J. Acoust. Soc. Amer. 31, 349-355. Henson, O.W. J r . (1965). "The Activity and Function of the Middle Ear Muscles i n Echc^Locating Bats," J. Physiol. 180, 871-887. Hirsh, I.J. (1952). The Measurement of Hearing (McGraw-Hill Book Co., New York). Jepsen, 0. (1963).. "Middle-Ear Muscle Reflexes i n Man," i n Modern  Developments i n Audiology, J. Jerger, Ed. (Academic Press, Inc., New York). Jerger, J. (1956). "Recovery Pattern from Auditory Fatigue," J. Speech Hearing Dis." 21, 39-46. Jerger, J. (1960). "Bekesy Audiometry i n Analysis of Auditory Disorders," J. Speech Hearing Res. 3_, 275-287. Jerger, J. (1970). " C l i n i c a l Experience With Impedance AudLometry," Arch. Oto-laryngol. 92, 311-324. Karlovich, R.S., and Luterman, B.F. (1969). "Auditory Fatigue During Articulation," J. Acoust. Soc. Amer. 45, 786-787. Karlovich, R.S., and Luterman, B.F. (1970). "Application of the TTS Paradigm for Assessing Sound Transmission i n the Auditory System during Speech Production," J. Acoust. Soc. Amer. 47, 510-517. Karlovich, R.S., Luterman, B.F., and Abbs, M. (1970). "Influence of Pulsed Contralateral Noise upon TTS from Continuous Pure-Tone Fatigue Stimuli," J. Amer. Speech Hearing Assoc. 12_, 452. Klockhoff, I. (1961). "Middle Ear Muscle Reflexes i n Man," Acta Oto-laryngol. Suppl. 164. -111-Kryter, K.D. (1970). The Effects of Noise on Man (Academic Press Inc., New York). Lindquist, E.F. (1953). Design and Analysis of Experiments i n Psychology  and Education (Houghton M i f f l i n Co., Boston). Loeb, M., and Fletcher, J.L. (1961). "Contralateral Threshold Shift and Reduction i n Temporary Threshold Shift as Indices of Acoustic Reflex Action," J. Acoust. Soc. Amer. 33, 1558-1560. Luterman, B.F., and Karlovich, R.S. (1969). "Further Observations Con-cerned with Auditory Fatigue During Vocal and Nonvocal Speech A c t i v i t i e s , " J. Acoust. Soc. Amer. 46, 403-408. McCall, G.N., and Rabuzzi, D.D. (1970). "Reflex Contraction of Middle Ear Muscles Secondary to Stimulation of Laryngeal Nerves i n the Cat," J. Amer. Speech Hearing Assoc., 12, 446. McPherson, D.F. (1966). "Relation of Post-Exposure Temporary Threshold Shift to Post-Exposure Temporary Loudness Shift," (unpublished doctoral dissertation, Purdue University). Macrae, J.H. (1970). "A Discussion of Non-Acoustic Methods of E l i c i t i n g the Middle-Ear Muscle Reflexes," J. Oto-laryngol. Soc. Aust. _3, 102-105. McRobert, H., 'Bryan, M.E., and Tempest, W. (1969). "The Effect of Middle Ear Muscle Contraction on Sound Transmission Through the Human Ear," Int. Audiol. 8_, 557-561. Melnick, W. (1968). "TTS with Auditory Task During Exposure," J . Acoust. Soc. Amer. 43, 635-636. Misrahy, G.A., Shinabarger, W.E., and Arnold, J.E. (1958). "Changes i n Cochlear Endolymphatic Oxygen A v a i l a b i l i t y , Action Potential, and Microphonics During and Following Asphyxia, Hypoxia, and Exposure to Loud Sounds," J. Acoust. Soc. Amer. 30_, 701-704. MeSller, A.R. (1961). "Network Model of the Middle Ear," J. Acoust. Soc. Amer. 33, 168-176. Mi l l e r , A.R. (1962). "Acoustic Reflex i n Man," J. Acoust. Soc. Amer. 34, 1524-1534. Nixon, J . C , and Glorig, A. (1962). "Noise-induced Temporary Threshold Shift vs. Hearing Level i n Four Industrial Samples," J. Auditory Res. 2_, 125-138. Reger, S.N. (1960). "Effect of Middle Ear Muscle Action on Certain Psycho-physical Measurements," Ann. Otol. Rhinol. Laryngol. 69, 1179-1198. -112-Riach, W.D., E l l i o t t , D.N., and Frazier, L. (1964). "Effect of Repeated Exposure to High-Intensity Sound," J. Acoust. Soc. Amer. 36, 1195-1198. Rosenblith, W.A. (1950). "Auditory Masking and Fatigue," J. Acoust. Soc. Amer. 22, 792-800. Ross, S. (1968). "On the Relation Between the Acoustic Reflex and Loud-ness," J. Acoust. Soc. Amer. 43, 768-779. Sachs, M.B., and Kiang, N.Y.S. (1968). "Two-Tone Inhibition i n Auditory-Nerve Fibers," J. Acoust. Soc. Amer. 43, 1120-1128. Salomon, G., and Starr, A. (1963). "Electromyography of Middle Ear Muscles i n Man During Motor A c t i v i t i e s , " Acta Neurol. Scand. 39, 161-168. Selters, W. (1963). "Threshold Recovery During the F i r s t Thirty Seconds," J. Auditory Res. 3_, 1-7. Selters, W. (1964). "Adaptation and Fatigue," J. Acoust. Soc. Amer. 36, 2202-2209. Shearer, W.M., and Simmons, F.B. (1965). "Middle Ear Ac t i v i t y During Speech i n Normal Speakers and Stutterers," J . Speech Hearing Res. 8_, 203-207. Simmons, F.B. (1959). "Middle Ear Muscle Ac t i v i t y at Moderate Sound Levels," Ann. Otol. Rhinol. Laryngol. 68, 1126-1144. Simmons, F.B. (1964). "Perceptual Theories of Middle Ear Muscle Function," Ann. Otol. Rhinol. Laryngol. 2 i , 724-739. Smith, R.P., and Loeb, M. (1968). "Several Experiments on Central Factors i n Auditory Fatigue," J. Auditory Res. 8_, 303-312. Starr, A., and Livingston, R.B. (1963). "Long-Lasting Nervous System Responses to Prolonged Sound Stimulation i n Waking Cats," J. Neurophysiol. 26_, 416-431. Ward, W.D. (1961). "Studies on the Aural Reflex. I. Contralateral Remote Masking as an Indicator of Reflex Activity," J. Acoust. Soc. Amer.33, 1034-1045. Ward, W.D. (1962). "Studies on the Aural Reflex. III. Reflex Latency as Inferred from Reduction of Temporary Threshold Shift from Im-pulses," J. Acoust. Soc. Amer. 34, 1132-1137. Ward, W.D. (1963). "Auditory Fatigue and Masking," i n Modern Developments  i n Audiology , J. Jerger, Ed. (Academic Press Inc., New York). Ward, W.D. (1965). "Temporary Threshold Shifts Following Monaural and Binaural Exposure," J. Acoust. Soc. Amer. 38_, 121-125. -113-Ward, W.D. (1966). "Temporary Threshold Shift i n Males and Females," J. Acoust. Soc. Amer.'40, 478-485. Ward, W.D. (1967a). "Further Observations on Contralateral Remote Masking and Related Phenomena," J. Acoust. Soc. Amer. 42, 593-600. Ward, W.D. (1967b). "Adaptation and Fatigue," i n Sensorineural. Hearing  Processes and Disorders, A.B. Graham, Ed. ( L i t t l e , Brown and Company, Boston). Ward, W.D. (1970). "Biochemical Implications i n Auditory Fatigue and Noise-Induced Hearing Loss," i n Biochemical Mechanisms i n  Hearing and Deafness, M.M. Paparella, Ed. .(Charles C. Thomas, Springfield, I l l i n o i s ) . Wernick, J.S., and Tobias, J.V. (1963). "Central Factor i n Pure-Tone Auditory Fatigue," J. Acoust. Soc. Amer. 35_, 1967-1971. Wersall, R. (1958)t "The Tympanic Muscles and their Reflexes," Acta Oto-laryngol. Suppl. 139. Wever, E.G. (1949). Theory of Hearing (John Wiley and Sons, Inc., New York). Wiggers, H.C.. (1937). "The Functions of the Intra-Aural Muscles," Am. J. Physiol. 120, 771-780. Winer, B.J. (1962). S t a t i s t i c a l Principles i n Experimental Design (McGraw-Hill Book Co., New York). Zemlin, W.R. (1968). Speech and Hearing Science. Anatomy and Physiology (Prentice-Hall, Inc., Englewood C l i f f s , New Jersey). -114-APPENDIX INSTRUCTIONS TO THE SUBJECTS There are four parts to the experiment: F i r s t you w i l l hear the pulsed tone i n your l e f t (right) ear. The tone w i l l start out quite low i n pitch and w i l l gradually become higher. Press the switch as soon as you hear the tone and l e t go as soon as i t disappears. When that i s over you are going to hear, i n the same ear, a pulsed tone that does not change i n pitch. Press the switch as soon as you hear the tone and l e t go as soon as i t disappears. Then, after you have had a short break, the same procedure w i l l be repeated on your right (left) ear. That i s , the tone that goes from low to high and then the tone that does not change i n pitch. Again, press, when you hear the tone, l e t go when you don't. The next part of the experiment consists of a continuous loud tone i n your right ( l e f t ) ear. This tone w i l l be on for five minutes. You w i l l have instructions about what to do during this loud tone. About 10 seconds before the tone i s to be turned off I w i l l alert you by jump-ing on the floor. As soon as the loud tone i s turned o f f the pulsed tone that does not change i n pitch w i l l be turned on. You should be able to hear this tone (in your right (left) ear) almost immediately after the loud tone stops. Be right ready to press the switch as soon as you hear the tone. Let go as soon as you don't hear i t , etc., as before. After four min-utes of this you w i l l be through for today. Do you have any questions? 

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