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Speech discrimination in noise for listeners with normal hearing and listeners with noise-induced hearing… Mack, Brenda Elisabet 1978

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SPEECH DISCRIMINATION IN NOISE FOR LISTENERS WITH NORMAL HEARING AND LISTENERS WITH NOISE-INDUCED HEARING LOSS by BRENDA ELISABET MACK B.A., University of B r i t i s h Columbia, 1972 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES ( DEPARTMENT OF PAEDIATRICS ) ( DIVISION OF AUDIOLOGY AND SPEECH SCIENCES ) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August, 197S (c) Brenda Elisabet Mack, 1978 In presenting th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of th is thesis fo r f inanc ia l gain sha l l not be allowed without my writ ten permission. Department of Aoifolo^u <_\pteck Sciences The Univers i ty of B r i t i s h Colximbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date CJLuqasl - i i -ABSTRACT The present study investigates the effect of low-pass noise on the speech discrimination performance of IB subjects with normal hearing and 18 subjects with noise-induced high-frequency hearing loss. W-22 word l i s t s , low-pass f i l t e r e d at 2000 Hz, were presented in sound f i e l d with a pink noise masker at three stimulus levels and three signal-to-noise ratios. Results indicated that the word discrimination performance of both groups deteriorated with increasing levels of noise and with increasing stimulus intensity levels, with the hearing-impaired group performing at a lower level throughout. While noise was shown to have a differential effect on the speech discrimination of the two groups, a satisfactory explanation of the effect, based on the study of Kiang and Moxon ([Tails of tuning curves of auditory nerve fibres. Journal of the Acoustical Society of America, 1974> 55, 620-63Q~f was not supported using the present experimental conditions. - i i i -TABLE OF CONTENTS Chapter Page ABSTRACT i i TABLE OF CONTENTS . i i i LIST OF TABLES . . v LIST OF FIGURES v i ACKNOWLEDGEMENT v i i 1. INTRODUCTION 1 2. REVIEW OF LITERATURE 3 2.1 The eff e c t of noise on word discrimination performance 3 2.2 Factors contributing to reduced speech discrimination i n noise 16 2.3 Physiological basis 24 3 . OBJECTIVES 28 4 . METHOD 29 4.1 Design 29 4 .2 Subjects 29 4 .3 Stimuli 31 4.3.1 Description 31 4 . 3 . 2 Preparation of Stimulus Tapes 37 4 . 4 Equipment 38 4 .5 Procedure 40 - i v -5. RESULTS 43 6. DISCUSSION AND CONCLUSIONS 55 REFERENCES 68 - V LIST OF TABLES Table Page 1 Means and ranges of pure-tone thresholds of the Normal group and the Patient group at the test frequencies 32 2 Means, ranges, and standard deviations of Word Discrimination Scores for 18 Normal subjects 44 3 Means, ranges, and standard deviations of Word Discrimination Scores for 18 Patients with high-frequency hearing loss 45 4 Slopes representing the regression functions of the mean Word Discrimination Scores achieved by Normal and Patient groups versus the 3 S/N ratios at the 3 stimulus levels . . . 51 5 Summary of 3-way analysis of variance of the results of the present study 52 6 Summary of 2-way analysis of variance of the results of the present study 54 - v i -LIST OF FIGURES Figure Page Mean pure-tone thresholds in dB HL (ANSI 1969) of the Normal group (/_\) and the Patient group (A) at the test frequencies. Also shown is the slope of the sk irt of the low-pass f i l t e r used to process the speech stimuli and noise ( ) 33 The one-third octave band analysis of the pink noise masker measured at the ear-level of the subject in position in the test booth . ••. . . . . 41 Mean WDS's of the a) NormaL group (open symbols), and b) Patient group (closed symbols) at 3 S/N ratios for stimulus levels of 60 dB (triangles), 70 dB (c irc les) , and 80 dB (squares). The regression lines of WDS's versus S/N ratios for each stimulus level are represented by for the 60 dB group, for the 70 dB group, and for the 80 dB group 46 Mean WDS's of the Normal and Patient groups at 3 S/N ratios for stimulus levels of a) 60 dB, b) 70 dB, and c) 80 dB. Symbols and regression lines as defined in Figure 3 48 Mean WDS's of the Normal group (open symbols) and Patient group (closed symbols) at the 3 stimulus levels for S/N ratios of 1-19 dB (triangles), +12 dB (c i rc les) , and +5 dB (squares) . . . . . . . 60 - v i i -ACKNOWLEDGEMENT I would like to express ray thanks to a l l who had a part in this thesis: Dr. David Chung for a l l his guidance during the research and writing of the thesis. Miss Noelle Lamb for serving on my committee. Dr. P. Gannon and the personnel of the Hearing Branch of the Workers' Compensation Board of B.C. for their help and the use of their f a c i l i t i e s for carrying out the necessary research. My subjects for their kind cooperation. Bob for a l l his understanding and encouragement over the past two years. Susan for her patient work on the figures. My fellow students for their friendship and support. CHAPTER 1 1. INTRODUCTION Patients with high-frequency noise-induced hearing loss frequently report d i f f i cu l t ie s understanding speech in a noisy environment yet experience l i t t l e d i f f i cu l t y in a quiet l i s tening situation. People with normal hearing, on the other hand, rarely report any such problems in quiet or noisy situations. Since patients with high-frequency losses have normal hearing through the range usually accepted as important for speech i n t e l l i g i b i l i t y (500 to 2000 Hz), i t seems improb-able that they should react di f ferently to noise than do people with normal hearing. Nonetheless, several studies in the laboratory situation support their complaints. These studies have shown that word discrimination performance does deterior-ate more rapidly in noise for subjects with high-frequency sensorineural hearing loss than for normal l i s teners. Few satisfactory explanations, however, have been given for this phenomenon. One recent physiological study (Kiang and Moxon, 1974) does provide significant support for the complaints of these individuals. It proposes that neurons with a high character-i s t i c frequency, available to normal l isteners but absent in - 2 -l i s t e n e r s with high-frequency h e a r i n g l o s s , may p r o v i d e a d d i -t i o n a l i n f o r m a t i o n on speech which a i d s d i s c r i m i n a t i o n i n n o i s e . The f i n d i n g s o f t h i s study p r o v i d e a promising b a s i s f o r the a n a l y s i s o f speech d i s c r i m i n a t i o n i n n o i s e . The p r e s e n t study, t h e r e f o r e , was undertaken to determine whether n o i s e does d i f f e r e n t i a l l y a f f e c t the word d i s c r i m i n a -t i o n performance o f normal l i s t e n e r s and l i s t e n e r s w i t h n o i s e induced h e a r i n g l o s s , and i f so, to provide a s a t i s f a c t o r y e x p l a n a t i o n f o r t h i s d i f f e r e n c e . - 3 -CHAPTER 2 2. REVIEW OF LITERATURE 2.1 THE EFFECT OF NOISE ON WORD DISCRIMINATION PERFORMANCE Word recognition tests are commonly used by audiology c l in ics to assist in the d i f ferent ia l diagnosis of various hearing impairments. Many individuals with high-frequency sensorineural hearing loss, however, show no decrease in performance on standard speech discrimination tests. The widely employed CID Auditory Test W-22 (W-22), when presented in quiet, has been shown to be of l imited value in separating normal-hearing patients from those with auditory pathology (Carhart, 1965; Keith and Ta l i s , 1972; Sher and Owens, (1974). Data published by Carhart (1965) on the word recognition scores of 170 hearing-impaired veterans tested with W-22 recordings, revealed that 60% achieved scores of 90$ or better. In this case, the speech discrimination test had fa i led to identify either the presence, the type, or the extent of hearing loss in 60% of the patients. His findings were substantiated by Keith and Tal is (1972) with c l i n i c a l data from 170 of the ir patients with sensorineural hearing losses. Sher and Owens (1974), following subject c r i te r i a similar to that used in the present study, observed mean scores - 4 -of 94.6$ on the W-22 test for 35 l isteners with normal hearing to 2000 Hz and high-frequency cochlear hearing losses. In the l ight of studies such as these, speech discrimination testing in quiet appears to be of l i t t l e diagnostic value for many people. Several studies have examined the diagnostic value of speech discrimination testing in the presence of noise. Simonton and Hedgecock (1953) used a mixture of white noise and two pure tones, 60 Hz and 112 Hz, with the Harvard Phonetically Balanced word l i s t s (PB-50's), to study the effects of noise on normal and hearing-impaired subjects. They found no difference in the performance of l isteners with normal hearing or with conductive hearing losses. Subjects with sensorineural hearing losses, however, showed increased loss of discrimination when tested in noise. Similar results were reported by Palva (1955) using a continuous white noise masker and a +10 dB signal-to-noise (S/N) ra t io . Speech discrimination scores for his sensorineural-impaired l isteners suffered "d is t inct ly , and in some cases severely" (Palva, 1955 > p. 240) . Palva concluded that speech discrimination testing in noise might be useful in the diagnosis of perceptive (sensorineural) deafness. Ross et a l . (1965), as part of a larger study, examined whether speech discrimination testing in white noise had any c l i n i c a l u t i l i t y . The absolute dis-crimination scores achieved by their hearing-impaired subjects - 5 -were poorer than those of the normal-hearing subjects for both the quiet and noise conditions. However, the relative d is -crimination shi f t due to noise fa i led to show any significant differences between the two groups. Ross et a l . ( 1 9 6 5 ) , suggested that the use of different kinds and sensation levels of noise could result in the desired differentiat ion of the groups. Cooper and Cutts (1971) examined changes occurring in the slope of the art iculat ion function with the introduction of noise. Northwestern University Auditory Test Number 6 (NU-6) was presented monaurally with cafeteria noise at S/N ratios of +5, +3, and -+12 dB. Consistent with the previous studies, the mean performance of the sensorineural group was s igni f icant ly poorer than that of the normal group. But, as found in Ross et a l . ' s (1965) data, the slopes of the art iculat ion functions were not s igni f icant ly different for the two groups. In a study by Keith and Talis ( 1 9 7 2 ) , W-22 words were mixed with white noise in an attempt to provide a more def init ive d i f ferent ia l diagnosis of hearing impairments. Subjects with normal hearing, high-frequency cochlear losses, and f la t cochlear losses were tested in quiet and at three different S/N ratios (-1-8, 0, -8 dB). As the S/N ratio decreased from 8 to -8 dB, the discrimination scores - 6 -deteriorated and the difference in mean scores among the groups increased. The mean score of the normal-hearing group deteriorated approximately 52% from the quiet condition to -8 dB S/N ratio, the mean score of the group with high-frequency losses deteriorated approximately 57%, and the mean score of the group with f l a t losses deteriorated approximately 67%. These results indicated that speech discrimination testing in the presence of noise could help differentiate between patients with cochlear hearing impairments and normal listeners. One striking finding common to a l l of these studies of speech discrimination performance in the presence of noise was the extreme va r i a b i l i t y among subjects, both normal and hearing-impaired. In a study of normal-hearing individuals, Rupp and Phillips (1969) found marked var i a b i l i t y in individual performance, both in white noise and in speech-spectrum noise. One normal group tested in speech-spectrum noise at OdB S/N ratio actually demonstrated a range of 88% i n individual performance. Simonton and Hedgecock ( 1 9 5 3 ) , Palva (1955)> and Ross et a l . (1965) noted wider variations in the scores of their sensorineural-impaired subjects than in their normal-hearing groups. Discrimination scores of the impaired group in Cooper and Cutts' (1971) experiment increased i n variabil-i t y with lower S/N ratios. At two of the S/N ratios tested, - 7 -they demonstrated a range of scores twice that of the normal group. This led the authors to conclude that something more than a simple masking effect was operating to reduce the performance in noise of this impaired population. Keith and Tal i s (1972) found that the use of increasing levels of a white noise masker resulted in increasingly wider ranges of scores within a l l three subject groups. This finding made the diagnosis of a particular hearing impairment based on speech discrimination scores very d i f f i c u l t . Clearly, the var iab i l i ty introduced by the use of white noise with speech limited i t s diagnostic value. In an interesting approach to this problem, Olsen et a l . (1975) attempted to determine whether this var iab i l i ty of results, i t se l f , could be diagnostically useful in dif ferent-iat ing kinds of hearing impairments. They presented NU-6 word l i s t s in quiet and in white noise (OdB S/N ratio) to six groups of subjects including normal l isteners and subjects with various types of auditory pathologies. Differences of l+Ofo or more between scores in quiet and in noise were observed for fewer than 1% of the normal ears tested. Similar d i f f e r -ences were found, however, for 8% of the ears with noise trauma (high-frequency loss) and Lfifo of the ears with Meniere's disease (f lat loss). These findings concurred with those of Keith and Tal is (1972). Olsen et a l . (1975) concluded that a finding of a large difference in scores obtained under these - a -conditions in quiet and in noise for either a normal listener or a listener with a high-frequency sensorineural hearing loss could be interpreted as indicating neural involvement some-where in the auditory system. Therefore, results from speech-testing in noise could be useful in revealing abnormal auditory function " . . . but not in suggesting a particular site of involvement as being responsible for the dysfunction" (Olsen et a l . , 1975, p. 3^2). In their concluding remarks, Keith and Talis (1972) maintained that speech-in-noise testing could only become diagnostically feasible i f the va r i a b i l i t y of results could be reduced. They proposed altering the masking noise by using lowpass f i l t e r e d noise as suggested by Liden (196?). Liden had stated that simultaneous presentation of word l i s t s with a 500 Hz low-pass fi l t e r e d white noise at a S/N ratio of -3 dB, made discrimination test results more sensitive diagnostically. He noted that in such a noise, the scores of patients with high-frequency sensorineural hearing losses and normal discrimination in quiet could drop to as low as 10 to 20%, while a normal-hearing individual maintained 90% i n t e l l i g i b i l i t y . On the basis of Liden's suggestion, Cohen and Keith (1976) devised a study to determine whether normal and hear-ing-impaired subjects could be differentiated without increas-- 9 -ing the variability of their scores. A 500 Hz, low-pass fi l t e r e d white noise was mixed with taped W-22 word l i s t s and presented monaurally to subjects with 1) normal-hearing, 2) high-frequency hearing loss, and 3) f l a t hearing loss. Speech discrimination performance was then measured in quiet and at -4 and -8 dB S/N ratios. Test results confirmed not only Liden's (1967) findings, but also the suggestion of Ross et a l . (1965) that different types and sensation levels of noise could provide clearer differentiation of speech dis-crimination performance. While scores of the three groups were similar in quiet, the more negative the S/N ratio, the greater the separation of group scores. The low-pass noise provided a greater degree of separation of the groups than did the white noise employed in Keith and Talis's (1972) study. Type and level of continuous noise used as the speech masker, therefore, appears to be one of the c r i t i c a l factors in the differentiation of normal and hearing-impaired listeners. Several recent studies have investigated the effects of other types of noise on speech discrimination performance. Carhart and Tillman (1970) measured discrimination for monosyllables against a background of competing sentences. Four groups comprising subjects with 1) normal hearing, 2) conductive losses, and sensorineural impairments with 3) good discrimination, and with 4) f a i r discrimination, were tested - 10 -monaurally in a sound f i e l d . Northwestern University test 2 was presented i n quiet and at four S/N ratios (+12, -f6, 0, and -6 dB). Interference functions plotted from this data revealed that subjects with conductive losses performed as well as those with normal hearing when the sensation level of the signal was the same for both groups. Performances of both sensorineural groups, in contrast, were significantly dis-turbed by the competing sentences. Their interference functions were shifted 12 to 15 dB to the right of a reference function (plotted from previous normal data) as i f the masking efficiency of the sentences had increased by 12 to 15 dB compared to that exhibited for the normal and conductive subjects. Carhart and Tillman (1970) suggested, therefore, that the presence of sensorineural hearing loss reduced the subject's a b i l i t y to resist interference from the competing speech. They noted that a similar "overmasking" effect can occur when the competition is a spectrally complex steady-state noise but to a much lesser degree than that found with the competing sentences. Thus, traditionally used maskers such as white or speech spectrum noise might provide less satis-factory competition because they e l i c i t less "overmasking". The effects of modulated noise on the speech i n t e l l i g i -b i l i t y of hearing impaired listeners was examined by Shapiro et a l . (1972). They presented speech with continuous or modulated white noise monaurally under headphones at four - 11 -S/N ratios (-8, -12, -16, and -20 dB). The speech material was NU-6 monosyllabic words. Subjects with sensorineural losses performed poorly under a l l experimental noise condit-ions, especially in continuous noise. As in previous studies, the mean performance of this group was consistently lower than that of the normal-hearing group. Contrary to other findings, however, the slope for the normal-hearing subjects was con-siderably steeper with increasingly negative S/N ratios than that for the hearing-impaired subjects. The authors offered no explanation for this unusual finding. The difference in scores between their two subject groups was less than that found in Carhart and Tillman's (1970) study, a fact, they attributed to the different spectra of the interfering noises. Finally, two studies by Findlay (1976), and Findlay and Denenberg (1977), compared the a b i l i t y of normal and noise-exposed subjects to discriminate speech under d i f f i c u l t listening conditions. The noise-exposed group had normal thresholds to 2000 Hz with high-frequency sensorineural hearing losses. Findlay found significant differences between the groups on three speech discrimination tasks: PB-50 l i s t s presented at 40 dB sensation level (SL), and W-22 l i s t s pre-sented at 30 dB SL in the presence of either speech-spectrum noise or "cocktail party" noise. The use of W-22 l i s t s i n cocktail party noise provided the greatest and most consistent - 12 -dif ferentiat ion between the two groups, in agreement with previous findings using competing speech as maskers. In their continuation of this study, Findlay and Denen-berg (1977) compared the performances of a group with normal hearing and two groups with high-frequency hearing loss: a younger group with predominantly noise-induced hearing loss and an older group with presbycusis and some noise exposure. Again, ¥-22 words were presented with competing cocktail party noise. Two test conditions were applied: one with words and noise unfi ltered, the other with both words and noise low-pass f i l te red at 1800 Hz. This condition was to rule out any effects on speech discrimination performance resulting from differences in the high frequency sens i t iv i ty of the normals and the hypacusics. A -4 dB S/N ratio was employed. In the unfi ltered condition, the discrimination performance of the normal l isteners was s igni f icant ly better than either hearing-impaired group. In the low-pass f i l te red condition, however, the younger, noise-exposed subjects achieved a s ignif icantly higher leve l of performance than did the normal-hearing group. Findlay and Denenberg (1977) ventured a possible explanation for this tota l ly unexpected result: perhaps the normal l isteners re l ied heavily on high-frequency cues to discriminate speech in noise and therefore encountered considerable d i f f i cu l t y when presented with only - 13 -mid-frequency information. Listeners with limited high-frequency hearing, on the other hand, might have learned to make better use of their mid-frequency hearing and hence, adjusted more readily to the f i l te red condition. Bilger et a l . (1974, 1976) also found that sensorineural hearing loss does not always cause listeners to perform poor-l y in noise. Ten subjects with normal hearing and eighteen with moderate sensorineural hearing losses were tested in quiet and in noise for a consonant recognition task. This consisted of sixteen consonants paired with three different vowels in a consonant-vowel (GV) context. Ten l i s t s of 96 syllables were then presented monaurally via headphones at 100 dB SPL with competing broad-band noise at 95 dB SPL. Upon introduction of the noise, the mean per cent recognition scores of the normal group dropped from 75$ to 50%. Of the eighteen subjects with sensorineural hearing loss, however, nine demonstrated no adverse effect in noise and only four showed the drop seen in the normal group. Those with low performanc-es in quiet tended to perform as well in noise as in quiet. In their discussion of these findings Bilger et a l . (1976) suggested that the ab i l i t y to tolerate noise or to l i s ten in noise is distributed independently of sensorineural hearing loss. It i s only that people with hearing losses complain about i t , whereas normal l isteners do not. This - 14 -remark certainly merits serious consideration. However, their observation that sensorineural hearing loss does not always result in decreased performance was based on results from subjects described simply as having "appreciable sensorineural loss" (p. 393). Neither the configuration nor the extent of their losses were detailed. They did note, however, that the subjects selected had discrimination scores not exceeding 76% on testing with ¥ - 2 2 ' s , and had speech reception thresholds of 30 dB or better. Their subjects, therefore, probably represented a variety of degrees of sensorineural hearing loss including f la t losses, and not solely the high-frequency configuration usually cited as demonstrating the greatest d i f f i cu l t i e s hearing speech in noise. The study by Cohen and Keith (1976) discussed previously, concerning the effects of low-pass noise on speech discrimina-t ion testing, reported similar findings for subjects with f la t hearing losses. These subjects gave consistently higher performances in the presence of noise than those with high-frequency hearing losses, achieving scores of 85.4% (at - 4 dB S/N ratio) and 80.5% (at -12 dB S/N ratio) compared with the 70.5% and 55.4% scores of the lat ter group. Both levels of performance were lower than that of the normal-hearing group. Cohen and Keith (1976) then queried what effect the different overall sound pressure levels of speech and noise had on the - 15 -discrimination scores of the groups. Since speech was present-ed at 40 dB SL, the words were presented at average levels of 64.4 dB SPL for the normals, 75.5 dB SPL for those with high-frequency losses, and 96.2 dB SPL for those with f l a t losses. To examine this question, a second experiment was devised in which f ive normal-hearing subjects were tested at levels equal to the levels presented to the f la t - loss group in the f i r s t experiment. Word-recognition scores were obtained monaurally at -4 and -12 dB S/N ratios, with words presented at 96 dB SPL and noise at 100 and 108 dB SPL. Results showed that when speech and noise were presented at equal sound pressure levels, normal-hearing subjects had s igni f icant ly poorer word-recognition scores (70.4$ at -4 dB S/N ratio and 26.4$ at -12 dB S/N ratio) than subjects with f l a t cochlear hearing losses. The similar findings in these two studies suggest that the results of Bilger at a l . (1976) may have been due to the presence of persons with f l a t hearing losses in their subject group. However, as i l lustrated by Cohen and Keith's (1976) original experiment, subjects with high-frequency sensorineur-a l hearing losses consistently showed the largest deterioration in performance in noise of the three groups. This group, un-l ike the group with unspecified sensorineural losses of the study of Bilger et a l . (1976), did perform poorly in noise. - 16 -The results of these studies provide evidence that people with normal hearing and those with high-frequency sensorineur-a l hearing loss do perform dif ferently under speech discrimi-nation tests in noise. This dist inct ion is not a clear-cut one in view of the var iab i l i ty of results and the often confl ict ing findings documented. One fact that does emerge from these studies is that individual performance in noise cannot be predicted accurately on the basis of speech dis-crimination scores measured in quiet. 2.2. FACTORS CONTRIBUTING TO REDUCED SPEECH DISCRIMINATION  IN NOISE Reduced speech discrimination performance with the introduction of noise has been attributed to a number of factors. The two most obvious are the degree of sensorineur-a l involvement of the l istener and the masking effect of the noise which shifts the art iculation function to the right of i t s position in quiet. Most theories proposed in the studies reviewed in the previous section revolve around some aspect of the interaction of these two variables. D i f f i cu l t ies in speech discrimination among listeners with high-frequency sensorineural hearing losses are most commonly ascribed to reduced high-frequency sensit iv i ty. As Bess and Townsend (1977) pointed out, the hearing loss effect-ively reduces or eliminates the audibi l i ty of the high-frequency - 17 -consonant sounds necessary for word i n t e l l i g i b i l i t y . The frequencies required for the optimum understanding of speech, therefore, merit consideration. French and Steinberg (1947) used various high- and low-pass f i l t e r conditions in a syl lable discrimination task to determine the relative importance of different frequencies to discrimination. With each successive cut-off of the high-frequency portion of the spectrum, they found a progressive deterioration of syl lable discrimination. Similar results were obtained upon rejection of the low frequencies. The range of frequencies either 1900 Hz and above or 1950 Hz and below each gave about a 69$ correct score. In other words, the elimination of a l l frequencies, for example, above 1900 Hz would s t i l l leave approximately 70$ of the syllables i n te l l i g i b le to a normal-hearing l i s tener. Decreased dis-crimination under low-pass f i l t e r i n g conditions was also noted by Giolas and Epstein (1963). Their study demonstrated that the influence of frequency on discrimination was further modified by such variables as the fami l iar i ty and type of speech materials employed, the art iculat ion characteristics of the speaker, and the quality of the recording. Researchers could not agree upon the importance of frequencies above 2000 Hz in the understanding of speech. Several studies on noise-induced hearing loss attempted to - 18 -relate auditory acuity at selected frequencies to speech discrimination ab i l i t y . Quiggle et a l . (1957), and later , Glorig et a l . (1961) reported that thresholds at 5 0 0 , 1000, and 2000 Hz were adequate for predicting the hearing and understanding of everyday speech. Although speech theoretic-a l l y contains frequencies from 300 to 4000 Hz, i t s redundancy, they f e l t , made the contribution of the higher frequency information unnecessary. The same three frequencies were recommended for evaluation of impairment for purposes of compensation by the Committee on the Conservation of Hearing of the American Academy of Opthalmology and Otolaryngology (AA0O) (Lierle, 1959). On the other hand, Mullins and Bangs (1957) found that speech discrimination scores correlated most highly with auditory thresholds obtained at 2000 and 3000 Hz. This led them to conclude "that these two frequencies are relat ively more important for speech discrimination than are the other frequencies" (Mullins and Bangs, 1957, p. 1 5 4 ) . In an excellent treatment of this issue, Kryter et a l . (1962) suggested that test measures, such as those employed by Quiggle et a l . (1957), and Glorig et a l . (1961), tended to underestimate the importance of acuity at frequencies above 2000 Hz for understanding speech. Their use of "thresholds of i n t e l l i g i b i l i t y " rather than word i n t e l l i g i b i l i t y scores, spondees rather than phonetically balanced monosyllabic words - 19 -(PB's), and quiet, distortion-free test conditions did not adequately assess the ab i l i t y to understand speech under actual.day-to-day l i stening conditions. Hence Kryter et a l . (1962) designed a study to determine the frequencies necessary for the understanding of speech under various conditions of noise and frequency distortion more closely resembling real everyday l istening conditions. Seven groups of subjects with normal hearing and different degrees of noise-induced hearing loss took part in the experiment. Recorded phonetically balanced words and Harvard sentences were presented monaurally via headphones, both in quiet and in speech-spectrum noise at selected S/N ratios. The speech materials were low-pass f i l te red at 7 00 0 Hz for some tests and at 2000 Hz for others. Test results showed that information in the speech frequencies above 2000 Hz made a significant contribution to the i n -t e l l i g i b i l i t y of the sentences in the presence of noise even for subjects with very large losses in those frequencies. On the basis of their data, Kryter et a l . (1962) found 2000, 3000, and 4000 Hz to be the most important frequencies for predicting the i n t e l l i g i b i l i t y of speech. In l ight of the AA00 recommend-ations and various other studies, however, they concluded that an average of the losses at 1000, 2000, and 3000 Hz for pre-dicting the ab i l i t y of hearing-impaired persons to understand everyday speech, would be a reasonably val id compromise. - 20 -Harris (1965), in his exploration of the effects of hearing loss upon the discrimination of mildly and severely distorted speech, also concluded that 1000, 2000, and 3000 Hz were the most important frequencies for understanding speech. Like Kryter et a l . (1962), he argued that one rarely has the opportunity under normal l i s tening conditions, to hear clearly articulated speech in quiet. He observed that . . . ordinari ly . . . there is considerable masking noise, often the acoustics of the l istening space are poor, the peaks of conver-sation occur during meal time, often the talker is smoking, chewing gum, or at least talking with slovenly vocal gesture . . . (Harris, 1965, p. 830). In an ear l ier paper, Harris (I960) demonstrated how mild sources of distort ion, taken individually, may reduce i n -t e l l i g i b i l i t y only s l ight ly, but, when combined may result in a drastic deterioration of i n t e l l i g i b i l i t y . In the 1965 study, he reasoned that i f a patient's audiogram could be considered a type of distort ion, then an audiometric defect, irrelevant to the understanding of normal speech, may exert a pronounced effect on even mildly distorted speech. Accordingly, he conducted discrimination tests on 52 subjects with sensorineur-a l hearing loss, using sentences distorted by 1) speakers wearing nose clamps, 2) speed, 3) interruptions, and 4) reverberations (Harris, 1965). The mean score from these four conditions was averaged with the score for undistorted - 21 -speech to create "50$ distorted speech," a condition assumed to approximate everyday listening. As previously stated, the three frequencies found to correlate most highly with this condition were 1000, 2000, and 3000 Hz. That frequencies above 2000 Hz do play an important role i n the perceptibility of speech was given further support i n a paper by Sher and Owens ( 1 9 7 4 ) . They discovered that individuals with normal hearing to 2000 Hz accompanied by a high-frequency loss do have d i f f i c u l t y identifying a substant-i a l number of phonemes as compared to normal listeners. Con-sideration of these findings along with the reports of d i f f i c u l t i e s experienced in hearing distorted speech (Harris, 1 9 6 5 ) , and speech in noise (Kryter et a l . , 1962) indicates, in the words of Sher and Owens ( 1 9 7 4 ) , "that the problems these people often complain of are real" (p. 678). Findlay ( 1 9 7 6 ) and Findlay and Denenberg ( 1 9 7 7 ) advanced the theory that decreased speech discrimination among subjects with noise-induced hearing loss above 2000 Hz may be partly due to undetected midfrequency auditory dysfunction. Using fixed-frequency Bekesy audiometry, Findlay found that this group consistently demonstrated separation of the continuous and pulsed-tone tracings at 2000 Hz, indicating the presence of cochlear dysfunction. No such separation was found in the normal group. Findlay and Denenberg (1977) then evaluated - 22 -speech discrimination performance in noise with the signal low-pass f i l t e r e d at 200G Hz, to determine i f auditory-dysfunction at frequencies less than 2000 Hz was contribut-ing, in fact, to the discrimination d i f f i c u l t i e s . The results, reviewed in the previous section, revealed that the noise-exposed listeners performed at a higher level than did the normal listeners. There was no evidence that subtle mid-frequency cochlear dysfunction was hampering the dis-crimination performance of this group. Findlay and Denenberg (1977) concluded that: . . . the complaint of noise-exposed listeners that they experience undue d i f f i c u l t y discriminat-ing speech in the presence of competing noise appears to be wholly attributable to the loss of high-frequency sensitivity (p. 257). Some researchers believe that a possible secondary factor i n the decreased discrimination performance of listeners with high-frequency sensorineural hearing loss, i s the occurrence of an upward spread of masking. Bess and Townsend (1977) suggest that " . . . the better hearing in the lower frequencies causes a masking effect on important high-frequency cues" (p. 232). Danaher and Pickett (1975) noted that i f a low-frequency sound is presented at a high intensity level, i t w i l l produce masking that reduces the audibility of sounds in the higher frequency regions. Interest in this concept - 23 -increased when research by Jerger, Tillman, and Peterson (I960), and Rittmanic (1962) suggested that l isteners with sensorineural impairment exhibit a greater spread of masking than do normal l isteners. Perhaps then, this could help explain the d i f ferent ia l discrimination performance of normal and hearing-impaired l isteners in noise. A recent study by Leshowitz (1977) attempted to relate tonal masking to this same problem. He found that masked speech i n t e l l i g i b i l i t y thresholds for subjects with noise-induced hearing loss or presbycusis, were approximately 10 dB higher than those found for normal l isteners. Pure-tone masking patterns were then measured. As much as 30 dB more upward spread of masking was revealed for the group with high-frequency hearing loss than for the normal group. In l ight of the strong positive relationship between the masked speech i n t e l l i g i b i l i t y threshold and the upward spread of masking, Leshowitz suggested that the masked threshold could be used to predict speech perception handicap in noise. Results of a study by Martin and Pickett (1970), however, fa i led to support the idea of increased upward spread of masking in sensorineural l isteners suggested by previous studies. Instead they found similar amounts of masking spread in both their normal-hearing subjects and in those with various degrees of hearing impairment. Within the sensorineural group - 24 -they noted marked differences in masking spread, leading them to conclude: . . . Sensorineural subjects, as a group, cannot be described as characterist ical ly showing either greater than normal or less than normal upward spread of masking (Martin and Pickett, 1970, p. 436). Consideration of upward spread of masking, therefore, as a possible contributing factor to decreased discrimination in noise, should be viewed with caution. These factors can supply at best a part ia l explanation of why l isteners with high-frequency sensorineural hearing loss experience such d i f f i cu l t ie s understanding speech in noise. 2.3. PHYSIOLOGICAL BASIS One important physiological study (Kiang and Moxon, 1974) does support the complaints of people with high-frequency hearing loss. In detailed studies of cats, they discovered that neurons with high characteristic frequencies (CF) also carry considerable information concerning stimuli in the speech-frequency region. Previous research (Bekesy, I960) had suggested that the entire cochlear partit ion including the high-frequency basal region, may respond to low-frequency stimuli i f presented at suff ic ient ly high intensity. Support for Bekesy's suggestion may be found by examination of the \ - 25 -tuning curve of an auditory nerve fibre. The typical tuning curve of a neuron with a high CF consists of a sharp t i p in the low threshold region at the CF and a long broad t a i l in the high threshold region extending into the low frequencies. This neuron w i l l be most sensitive to stimuli whose frequencies f a l l near i t s CF. However, presentation of a low-frequency stimulus at a suitably high intensity level may also activate the neuron via i t s low-frequency t a i l . Kiang and Moxon (1974) hypothesized that as these high CF neurons are broadly tuned throughout the speech frequencies, they could make a valuable contribution to the understanding of speech. In a quiet situation, most of the information in speech is carried by neurons with CF. in the speech region. This information is expressed in terms of the discharge rates associated with the phonetic elements of the speech stimulus, and in terms of the synchrony of f i r i n g of these discharges with the acoustic waveform of the stimulus. If a listener has normal hearing in the speech frequencies, he should have no d i f f i c u l t i e s hearing speech clearly in quiet. With the introduction of noise, however, the information on discharge rates carried by the speech frequency neurons, is eliminated. This leaves only the information regarding the synchronous f i r i n g of these neurons. In individuals with normal hearing, extra cues are s t i l l available in information carried by the - 26 -high CF neurons. Provided the speech signal is sufficiently-intense to activate these neurons, they w i l l carry the needed information on the rate of discharge and make the speech signal more i n t e l l i g i b l e . This additional information is not available to listeners with high-frequency hearing losses. Hence, while speech i n t e l l i g i b i l i t y i n noise remains high for normal-hearing individuals, i t deteriorates for those with high-frequency losses due to the absence of this extra cue. The results of Kiang and Moxon's (1974) study have implications for the examination of noise effects on the speech discrimination performance of normal-hearing and high-frequency impaired subjects. They suggest that as the intensity of a speech stimulus is increased beyond that level at which responses from the high CF neurons are elicited, the difference between the performance of the two groups should become more marked. This would be noted particularly i f the discharge rate information of the low CF neurons was abolished through the introduction of noise. Kiang and Moxon (1974) also demonstrated that with presentation of either wide-band noise or narrow-band noise centered around the CF, the entire tuning curve of a high CF neuron is elevated in threshold. This interference with the threshold of excitation of the neuron would result, therefore, in less differentiation in the performance of the two groups. On the other hand, with - 27 -presentation of a low-frequency noise, only the t i p of the tuning curve is elevated, leaving the threshold of the broad t a i l unaffected. These findings are consistent with the results of Keith and Talis ( 1 9 7 2 ) , Gohen and Keith ( 1 9 7 6 ) , and Liden (1967) which found that the use of low-pass noise improves the diagnostic effectiveness of speech discrimination measures in separating normal listeners from those with high-frequency sensorineural hearing loss. - 26" -CHAPTER 3 3. OBJECTIVES The masking effect of noise on the i n t e l l i g i b i l i t y of speech in individuals with high-frequency sensorineural hearing loss remains a controversial issue. Information gained from past studies has been incomplete, sometimes contradictory, and d i f f i cu l t to integrate into a clear picture of the problem. Moreover, no adequate explanation has yet evolved for the d i f ferent ia l effect of noise on speech i n -t e l l i g i b i l i t y i f indeed such an effect exists. The findings of Kiang and Moxon (1974) provide a promising physiological framework on which to bfise a systematic study of the issue. Their data, while compatible with c l i n i c a l observations, were obtained from cats. The present study attempts to determine whether their findings hold for human subjects as well. The objectives of this experiment are as follows: 1) To determine whether masking noise does d i f ferent ia l ly affect the speech i n t e l l i g i b i l i t y of subjects with normal hearing and subjects with high-frequency sensorineural hearing losses. 2) If i t does, what is the explanation? 3) To test whether Kiang and Moxon's findings can be ver i f ied with human data. - 29 -CHAPTER 4 4. METHOD 4.1. DESIGN The study has a two by three by three factor ia l design. The independent variables consist of: 1) two subject groups: Normals and Patients with high-frequency, noise-induced hearing loss, 2) three stimulus levels: 60, 70, and 80 dB SPL, and 3) three S/N ratios: +-5, +12, and+-19 dB. The dependent variable is the word discrimination score (WDS). 4.2. SUBJECTS Eighteen men with normal hearing and eighteen men with high-frequency sensorineural hearing losses served as subjects. The groups were designated N and P respectively. The N group consisted of twelve employees from the Workers' Compensation Board of Brit ish Columbia (W.C.B.), and six students from the University of Br i t i sh Columbia. A l l were in good health with no known histories of either ear pathology or prolonged noise exposure. Auditory thresholds - 30 -for this group were better than 25 dB HL (ANSI 1969) at the octave frequencies from 250 to 8000 Hz in the better ear. Their ages ranged from 22 to 37 years (mean age — 28 . 6 years). The P group was composed of eighteen patients from the Hearing Branch Clinic of the W.C.B. As i n the N group, subjects were healthy with no known history of middle ear pathology. A l l P group subjects, however, had extensive histories of prolonged noise exposure of industrial origin. Case histories and audiological evaluations conducted by W.C.B. audiologists confirmed that each patient had a high-frequency cochlear hearing loss with no evidence of retro-cochlear involvement. The original selection c r i t e r i a called for auditory thresholds no greater than 25 dB HL from 250 to 2000 Hz, and no less than 60 dB HL at 4000 Hz in the better ear. Time restrictions and the shortage of suitable subjects, however, necessitated a modification of the criterion regard-ing 2000 Hz to allow thresholds up to 35 dB HL. A total of eight subjects were included on the basis of the modified criterion. The effect of this change on the results of the study w i l l be considered in a later section. Patients were also to be further categorized on the basis of the severity of their hearing impairments at 4000 Hz, into mild, moderate, and severe classifications. As a l l but one subject (moderate) f e l l into the mild class (60 to 75 dB HL threshold at 4000 Hz), - 31 -this further breakdown scheme was abandoned. Ages of the P group subjects ranged from 37 to 84 years (mean age — 57.2 years). The mean pure-tone thresholds of the two subject groups are presented in Table 1, and diagramatically in Figure 1. 4.3. STIMULI 4.3.1. Description Taped recordings of CID Auditory Test ¥-22 were presented simultaneously with pink noise to compare the WDS's of the two groups. Subjects were assigned to one of three stimulus level groups, each consisting of s ix N's and six P's. For each subject, the speech stimuli then were presented at a constant intensity level while the noise level was varied to produce the required S/N ratios of +19, +12, and +5 dB. A preliminary sample of subjects had led to the establishment of this range of S/N ratios to allow a broad spread of performance. Inasmuch as 500 Hz is important in speech and is less affected by noise exposure and presbycusis than other speech frequencies, the pure-tone threshold at 500 Hz was selected as the reference for the stimulus leve l . Within each of the stimulus-level conditions a certain amount of allowance was given to the subjects depending on their 500 Hz thresholds. Table 1. Means and ranges of pure-tone thresholds of the Normal group and the Patient group at the test frequencies. HEARING THRESHOLD LEVELS (ANSI 1969) SUBJECT GROUP 25O Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz Normals: Mean 3.05 2.78 1.39 .28 -I.67 5.56 Range -5 - 10 -10 - 10 -10 - 10 -10 - 10 -10 - 20 -10 - 25 Patients: Mean 10.27 13.33 16.11 22.5 65 62.77 Range 0 - 2 0 0 - 30 5 - 3 0 5 - 3 5 5 0 - 8 5 35 - 100 - 33 -Figure 1. Mean pure-tone thresholds in dB HL (ANSI 1969) of the Normal group (A) and the Patient group (J_) at the test frequencies. Also shown is the slope of the skirt of the low-pass f i l t e r used to process the speech stimuli and noise ( . .) . HEARING THRESHOLD LEVEL IN dB 3 ) m o c m z o -< N •F-- 35 -Subjects with a threshold of 10 dB HL or more were given 10 dB more in stimulus leve l ; e.g., a subject in the 60 dB group with a threshold of 15 dB at 500 Hz was actually present-ed a stimulus level of 70 dB SPL. Subjects with a threshold of 5 dB HL at 500 Hz were given a 5 dB allowance. Subjects with 0 dB HL or less at 500 Hz were given no allowance. While efforts were made, therefore, to accommodate individual variations in threshold, i t was necessary to establish a ce i l ing level to avoid overlap with the succeeding stimulus level group. Any subject with a 500 Hz threshold exceeding this l imit was assigned a compromise threshold level of 10 dB HL for the purposes of the experiment. The three stimulus levels, 60, 70, and 80 dB SPL were chosen in an attempt to traverse the approximate threshold region of excitation of the high CF neurons. In this region, a marked differentiat ion in the performance of the N and P groups should appear. Data from Kiang and Moxon's (1974) study, obtained from cats, indicated that activation of these high CF neurons by low frequency stimuli occurred at intensity levels of 50 to 80 dB SPL. In the present study, the lowest leve l , 60 dB SPL, also corresponds to the region of quiet conversational speech. The highest leve l , 80 dB SPL, corresponds to very loud speech such as that found when conversing in a noisy environment. If the excitation of the - 36 -high CF neurons does play an important role in assisting speech i n t e l l i g i b i l i t y in noise, i t log ica l ly should occur within this stimulus intensity range. A different 50-word l i s t was presented at each S/N rat io. The same three l i s t s , 2-E, 3-A, and 4-A were used for a l l subjects, however, the particular combination of l i s t and S/N rat io was varied systematically from subject to subject, to avoid bias due to any possible variations in the d i f f i cu l t y of the tests. Fifty-word l i s t s rather than the more commonly employed ha l f - l i s t s of 25-words were used on the basis of findings by Chaiklin (1968), and Keith and Talis (1972). According to Keith and Tal is (1972), unpublished data reported by Chaiklin at the 1968 A.S.H.A. convention, indicated that ha l f - l i s t scores, although rel iable for normal l isteners, were unreliable with hearing-impaired l isteners due to the var iab i l i ty of their responses. Keith and Tal is (1972) also found poor correlation between ha l f - l i s t scores for their patients with sensorineural hearing losses. Moreover, as the noise level of the masker was increased, this correlation grew even poorer. They concluded, therefore, that use of half-l i s t s with a sensorineural population might result in a spurious score that could not be reproduced in a retest situation. - 37 -The word-lists were passed through a low-pass f i l t e r with a cut-off at 2000 Hz. This was to ensure that any real differences noted in the speech discrimination performances of the two groups were not due to their inequality in thresholds in the high frequencies. Figure 1 shows the s imi lar i ty between the slopes of the f i l t e r dB/octave) and the P subjects' audiograms between 2000 and 4000 Hz. Similar use of f i l te red speech tests to compare the performance of normal hearers and subjects with sensorineural hearing loss were reported by Sher and Owens (1974), Cohen and Keith (1976), and Findlay and Denenberg (1977). The pink noise used as the masker in the study was also low-pass f i l t e red at 2000 Hz. As noted ear l ier, Kiang and Moxon (1974) found that low-pass noise kept the excitation threshold of the t a i l of high CF neurons low so as to give a greater differentiat ion of scores between the two groups. 4.3.2. Preparation of Stimulus Tapes One master tape was prepared with the f i l tered word-lists on track 1, and f i l te red pink noise on track 2. From this master, three cassette tapes were made, each with a different ordering of the l i s t s , for use in the actual test situation. Records containing l i s t s 2-E, 3-A, and 4-A of CID Auditory Test W-22 were played on a BSR 710 turntable. From - 38 -there the signal was passed through a Marantz 2215 amplifier, and then through a Rockland Programmable Dual Hi/Lo F i l t e r with a low-pass cut-off frequency of 2000 Hz. Two f i l t e r s were connected in cascade to give a f i l t e r slope, above the cut-off frequency, of 48 dB per octave. The f i l tered output was then taped by means of a Revox A 77 tape recorder, d irect ly onto track 1 of the master tape. The test words were recorded so that their intensity peaked at 0 dB on the VU meter. Pink noise,' generated by a random noise generator (General Radio Corporation, Model 1382), was s imi lar ly processed through the low-pass f i l t e r i n g system and recorded on track 2 of the master tape. Three cassette tapes were produced from the original master. A Sony Stereo Cassette-Corder (model TC-158 SD) incorporating a Dolby B noise reduction system, was used to record the taped speech stimuli and noise onto Memorex ATC cassettes. A 30-second si lent interval was inserted between each l i s t on the cassettes. 4.4. EQUIPMENT A l l experimental testing was conducted in a sound-treated test suite (Tracoustics, Model RS Z52) at the Hearing Branch of the W.C.B. in Richmond, B.C. The physical arrangement of the test room positioned the subject's head - 39 -equidistant between two side speakers (Madsen FF72) placed i n opposite corners of the room. Directly in front of the subject, at a distance of 1 meter, was a centre loudspeaker (Madsen FF74). The test tape, with i t s separate tracks for word l i s t s and noise, was introduced into a Madsen audiometer, Model 0B70, by means of an Akai dual-channel tape recorder, Model GXC-740D. The word l i s t s were routed through channel 1 of the audiometer and presented to the subject via the centre loudspeaker. Similarly, the noise was routed through channel 2 of the audiometer and presented via the two side speakers. Equipment calibration was checked periodically with a Bruel and Kjaer (B and K) sound level meter, Model 2204. A half-inch microphone equipped with nose cone (B and K Model 4165) on a 1 meter tripod, was placed at the position to be occupied by the subject's head, but with the subject absent from the f i e l d . It was connected to the sound level meter, located outside the test room. Speech and noise signals were calibrated separately. The intensity level of the incoming taped signal was adjusted to peak at zero on the VU meter of the audiometer. Then the output signal, either speech or noise from their respective speakers, was picked up by the microphone and measured by the sound level meter. The audiometer settings required to produce the desired S/N ratios were then recorded. - 40 -The acoustic spectrum of the pink noise was measured by a th ird octave spectrometer (B and K, Model 2114), in con-junction with a graphic level recorder (B and K, Model 2307). The third octave spectrum of this noise is presented in Figure 2. 4.5. PROCEDURE Subjects were seated on a chair maintained in a fixed position in the sound-treated chamber. Both the noise and the word l i s t s were presented at the required intensity levels to produce S/N ratios of 4-19, +12, and +5 dB in that order. The subjects were familiarized with the task and instructed to repeat the word heard. They were encouraged to guess i f they were not sure of the correct response. Responses were scored by the examiner in the standard manner: the entire word had to be accurately identif ied to be marked as correct. The administration of the entire test took approximately thirteen minutes. - 41 -Figure 2. The one-third octave band analysis of the pink noise masker measured at the ear-level of the subject in position in the test booth. 1/3 OCTAVE BAND LEVEL O <J1 o CD O O 3J m o c m z o -< X N - tn -- 43 -CHAPTER 5 5. RESULTS The results of the experiment are summarized in Tables 2 and 3 for the N and P groups respectively. These tables report the means, ranges, and standard deviations of scores obtained by the two groups under the different experimental conditions. In addition, Figures 3 and 4 show the WDS's plotted as a function of the experimental conditions. Figure 3a shows the WDS's obtained by the N group, and Figure 3b shows the WDS's obtained by the P group, under the nine possible combinations of stimulus leve l and S/N ra t io . Figure 4 presents another view of the same data, with the WDS's obtained by both groups at the three S/N ratios at stimulus levels of 60 dB ( 4 a ) , 70 dB ( 4 b ) , and 80 dB SPL ( 4 c ) . In these figures, the data points represent the mean WDS's under each condition while the l ines indicate the regression functions for each stimulus leve l . Examination of the data for both subject groups reveals a decrease in performance with increasing noise. The highest mean scores of the N group were obtained at a stimulus level of 60 dB, with successive decreases in mean scores at the 70 and 80 dB stimulus levels. The P group, on the other hand, - 44 -Table 2. Means, ranges, and standard deviations of Word Discrimination Scores for 18 Normal subjects. S/N STANDARD-STIMULUS LEVEL RATIO MEAN RANGE DEVIATION + 5 57.67 42 - 76 12.98 60 dB * H 2 79.00 72 - 86 6 .29 +19 83.33 74 - 88 5.47 + 5 55.00 50 - 66 6 . 4 2 70 dB +12 67.67 48 - 82 1 2 . 4 8 +19 79.67 74 - 90 5 . 9 9 + 5 48.00 44 - 52 2.83 80 dB +12 64.00 46 - 76 12.07 +19 75.00 66 - 84 6.90 - 45 -Table 3. Means, ranges, and standard deviations of Word Discrimination Scores for 18 Patients with high-frequency hearing loss. STIMULUS LEVEL S/N RATIO MEAN RANGE STANDARD DEVIATION + 5 41.67 34 - 46 4.27 60 dB +12 62.67 50 - 74 9.69 +19 76.00 64 - 90 11.38 + 5 45.00 28 - 64 14.35 70 d l +12 58.67 44 - 82 13.59 +19 61.67 76 - 90 6.12 + 5 37.67 30 - 50 7.42 80 dB +12 52.67 23 - 62 12.75 +19 67.67 5a - 76 7.94 - 46 -Figure 3. Mean WDS's of the a) Normal group (open symbols), and b) Patient group (closed symbols) at 3 S/N ratios for stimulus levels of 60 dB (triangles), 70 dB (circles} and 80 dB (squares). The regression lines of WDS's versus S/N ratios for each stimulus level are represented by for the 60 dB group, for the 70 dB group, and for the 80 dB group. W O R D D I S C R I M I N A T I O N S C O R E - 48 -Figure 4. Mean WDS's of the Normal and Patient groups at 3 S/N ratios for stimulus levels of a) 60 dB, b) 70 dB, and c) 80 dB. Symbols and regression l ines as defined in Figure 3. W O R D D I S C R I M I N A T I O N S C O R E - 50 -obtained their highest mean scores at the 60 and 70 dB stimulus levels with a decrease in performance at the 80 dB stimulus leve l . The overal l mean WDS's of the P group were consistently infer ior to those of the N group with one notable exception at a stimulus level of 70 dB with a +19 dB S/N rat io . In this condition, the P group achieved a mean WDS of 81.67$, 2$ higher than the 79.67$ score achieved by the N group. The slopes of the mean regression functions plotted in Figures 3 and 4 are summarized in Table 4. These slopes represent the regression of the mean WDS's achieved by the N and P groups at the three S/N ratios for stimulus levels of 60, 70, and 80 dB SPL. Again, the slopes of the P group were consistently steeper than those of the N group at a l l three stimulus levels. The raw WDS's of the two groups were subjected to an analysis of variance appropriate for a three-factor experiment (subject group, S/N rat io, and stimulus leve l ) . A summary of this three-way analysis is presented in Table 5« Results indicated that signif icant differences existed between the discrimination performances of the N and P groups (p z. .01). Significant differences were also noted among WDS's achieved at the three S/N ratios (p ^-.01), and at the three stimulus levels (p .01). A borderline case of interaction was noted - 51 -Table 4« Slopes representing the regression functions of the mean Word Discrimination Scores achieved by-Normal and Patient groups versus the 3 S/N ratios at the 3 stimulus levels. STIMULUS LEVELS SUBJECT GROUP 60 dB 70 dB 80 dB Normals 1.79 1.79 1.93 Patients 2.43 2.64 2.14 Table 5. Summary of 3-way analysis of variance of the results of the present study. SUMMARY OF ANALYSIS OF VARIANCE SUM OF DEGREES OF MEAN SOURCE OF VARIANCE SQUARES FREEDOM SQUARES F pZ.01 A Subjects 2,446•26 1 2,446.23 26.94 Yes B S/N Ratios 15,974.39 2 7,937.45 37.96 Yes C Stimulus Levels 1,630.39 2 340.45 9.26 Yes AB 373.74 2 139.37 2.09 No AC 257.13 2 123.59 1.42 No BC 295.55 4 73.39 .31 No ABC 69.49 4 17.37 .19 No Within Cell 3,172.67 90 90.31 TOTAL 29,275.67 107 - 53 -between the factors of subject group and S/N ratio. This was investigated further by a second s t a t i s t i c a l treatment of the data. A two-way analysis of variance was performed using the slope of the regression function of the raw WDS's versus the three S/N ratios as the dependent variable. The two independ-ent variables were the subject groups and the stimulus levels. The results of this analysis, summarized in Table 6, revealed a significant difference between the slopes of the regression functions of the two groups (p £-.05). This suggests that the speech discrimination of the two groups was differentially affected by noise level. Table 6. Summary of 2-way analysis of variance of the results of the present study. SOURCE OF VARIANCE SUMMARY OF ANALYSIS OF VARIANCE SUM OF SQUARES: DEGREES OF FREEDOM MEAN SQUARES P .05 A Subjects B Stimulus Levels AB Within Ce l l TOTAL 2.861 .147 .637 14.139 17.784 1 2 2 30 35 2.861 .0735 .3185 .4713 6.07 .16 .68 Yes No No V J 1 - 55 -CHAPTER 6 6. DISCUSSION AND CONCLUSIONS The results of the experiment support the findings of previous studies that noise does differentially affect the WDS's of people with normal hearing and those with high-frequency, sensorineural hearing loss. The mean WDS's of the P group in noise consistently f e l l below those of the N group with the exception of a high P score at 70 dB with a S/N ratio of -KL9 dB. The steeper slopes of the regression functions of the P group were shown to be significantly different from those of the N group. Noise, therefore, did have a more devastating effect on the discrimination performance of the hearing-impaired group. The differential performance of the two subject groups was not, however, as marked as that found in earlier studies, notably that of Cohen and Keith (1976). They achieved a separation of mean WDS's between their normal-hearing and high-frequency loss groups of 24.3$ at -4 dB S/N ratio and 37$ at -12 dB S/N ratio. The maximum separation achieved in the present case was approximately 16$ at S/N ratios of both +12 and +5 dB at the 60 dB stimulus level. Aside from differences in S/N ratio, one of the reasons for this reduced - 56 -separation could be the different stimuli used in the two studies. In Cohen and Keith's study, the stimuli were unfi ltered CID ¥-22 word l i s t s , whereas in the present study, f i l te red W-22 word l i s t s were employed. The normal-hearing subjects of the former study had, therefore, a l l the high-frequency consonantal cues which contribute s ignif icant ly to word i n t e l l i g i b i l i t y . As the hearing-impaired subjects lacked these cues due to their high-frequency hearing losses, the differentiat ion of speech discrimination performance between the groups with the introduction of noise was exaggerated. F i l ter ing of the stimuli in the present study effect ively eliminated these high-frequency cues for the normal subjects thereby reducing the difference between the mean ¥DS 's of the two subject groups. A similar reduction in the discrimination ab i l i t y of normal l isteners with f i l t e red stimuli was reported by Sher and Owens (1974). They presented a phoneme ident i f icat ion task, in quiet, to two groups of l i steners. One group had normal hearing to 2000 Hz with a high-frequency hearing loss, similar to the P group of the present study. The other group had normal hearing and received the speech stimuli through a low-pass f i l t e r with a cut-off frequency of 2040 Hz. Whereas the normal subjects generally scored 100% on this test in the unfi ltered condition, with f i l te red st imuli, there was no - 57 -significant difference between the mean scores (approximately 75 to 7 6 $ mean scores) of the two groups. When the f i l t e r skirt and the slope of the hearing loss were closely matched, the test behavior of the two groups was vir t u a l l y the same. Due to the f i l t e r e d speech stimuli, higher S/N ratios were employed in the present study than in previous experiments. As expected, the range of '•19 to +5 dB S/N ratio resulted in a broad range of scores. The combination of distortions introduced by f i l t e r i n g of the stimuli and the addition of noise made these S/N ratios sufficiently d i f f i c u l t for the purposes of the study. Lower S/N ratios might be considered for future research in this area to assess the course of the observed trends under increasingly d i f f i c u l t listening situations. As noted earlier, an exception to the generally poorer speech discrimination performance of the P group was found at the highest S/N ratio at the 70 dB stimulus level (Figure 4b). Here no significant difference between the mean scores of the N and P groups was found. Examination of the data presented in Figure 3a for the N group reveals decreasing speech discrimination performances with increasing stimulus levels. Examination of the data presented in Figure 3b for the P group, however, reveals - 58 -l i t t l e change in the performances at the 60 and 70 dB stimulus levels although a decrease in performance does occur at the 80 dB level. A l i k e l y explanation for these findings rests in the pure-tone thresholds of the two subject groups. As discussed in the "Subjects" section (4»2), the original subject c r i t e r i a for both groups had called for normal hearing (thresholds no greater than 25 dB HL) up to 2000 Hz. Lack of suitable subjects, however, necessitated a modification of these c r i t e r i a to allow the inclusion of several P subjects with thresholds of up to 35 dB HL at 2000 Hz. Whereas, ideally the mean pure-tone thresholds of the two groups would have been matched in this frequency range, the actual mean thresholds differed by from 7 dB at 250 Hz to 22 dB at 2000 Hz (see Table 1 ) . Efforts were made to accommodate individual variations in auditory function up to a li m i t of 10 dB when assigning stimulus presentation levels. However, due to this inequality of thresholds, some P subjects were presented stimuli at the 60 dB level, which were, in actuality, 10 to 15 dB softer than those stimuli presented to their N counterparts. This partially accounts for the large differentiation of scores seen at the 60 dB level for the N and P groups (Figure 4 a ) . The addition of a further 10 dB of stimulus intensity at the 70 dB level then probably brought the P group up to a more optimum level for word discrimination performance, already reached by the N group at the 60 dB level. - 59 -Increasing the stimulus level from 60 dB to 70 dB SPL for the P group therefore helped counteract the difference in thresholds between the two groups. However, the decrease in discrimination performance with increasing stimulus levels noted for the N group, was a trend in the opposite direction. Perhaps the opposition of these two effects in the P group therefore resulted in the s imi lar i ty of performance noted at the 60 and 70 dB levels. On the basis of Kiang and Moxon's (1974) study, the results of the present experiment were expected to demonstrate a marked difference in the discrimination performance of the two groups when the high-frequency neurons of the N group were called into play. It was anticipated that this would be revealed by a change in the slopes of the regression functions of the groups relat ive to each other when stimulus intensity was increased. The present study, however, fa i led to support this (Figure 5 ) . There are a number of possible reasons. The f i r s t is the confoundment of results aris ing from the inequality of thresholds of the two groups. The application of more stringent subject c r i te r i a , ideal ly specifying matched group thresholds no greater than 15 dB HL through to 2000 Hz, would c l a r i f y the actual affects of stimulus and noise levels on the WDS's of the two groups. - 60 -Figure 5 . Mean WDS's of the Normal group (open symbols) and Patient group (closed symbols) at the 3 stimulus levels for S/N ratios of +19 dB (triangles), +12 dB (circles), and -/-5 dB (squares). - 61 -STIMULUS PRESENTATION LEVEL IN dB SPL - 62 -A second, more fundamental reason is inherent in the experimental design. In an attempt to simulate more closely-actual everyday l istening conditions, the experiment was conducted in a sound f i e l d situation, using loudspeakers, rather than under headphones. This resulted in a number of unforeseen complications, the most notable of which was the aforementioned drop in the level of discrimination performance with increasing stimulus l eve l . Ordinarily, for normal l isteners under headphones in quiet, articulation curves for PB W-22 word l i s t s reach their maximum (PB max) at approximately 50 dB SPL and remain at a constant plateau to stimulus levels of 90 dB SPL or more (Davis and Silverman, 1970, p. 212). With a f i l t e red word l i s t , the maximum height of the plateau might be reduced (French and Steinberg, 1947), but the PB max should s t i l l be maintained at a constant leve l . In the case of the present study, however, the sound f i e l d WDS did not plateau but demonstrated a " ro l l -over" effect, where a speech intensity of 80 dB SPL was less i n te l l i g i b le than speech of 70 or 60 dB SPL. (This can be seen in Figure 5 which plots the mean WDS's of the two groups as a function of stimulus level with S/N ratio as the parameter.) One possible reason for this " ro l l -over" of WDS's with increasing stimulus levels could be the high overall noise levels used (Pollack and Pickett, 1958). Or, i t may have resulted from the use of the sound f i e l d test condition rather than the - 63 -headphones. The sound-treated test suite used in the experiment was a semi-reverberant room, not an anechoic chamber. Reverberation effects could have caused the observed deterioration of scores. Pollack and Pickett (1958) examined the deterioration of word i n t e l l i g i b i l i t y at high noise levels under headphones. With S/N ratio held constant, they observed a decrease in the i n t e l l i g i b i l i t y of monosyllabic words with increasing overall sound levels. The bend-down of the art iculat ion curves occurred at levels of 80 dB SPL and higher and was accentuated by decreasing the S/N rat io. A stringent series of controls was carried out to ensure that the effect was not a result of equipment distortion. Pollack and Pickett (1958), therefore, concluded that the decrease in i n t e l l i g i b i l i t y was most l i ke ly a result of "overloading" (p. 130) of the auditory system of the l i stener. Support for the possible deletrious effects aris ing from the sound f ie ld test condition may be found in several studies, two of which deal with the effect of hearing protection on speech i n t e l l i g i b i l i t y . Kryter (1946) assessed the effect of wearing earplugs on art iculation scores obtained in reverberant and anechoic environments. In the reverberant room, he discovered that for normal l isteners in the presence of 80 dB or more of noise, earplugs improved speech i n t e l -- 64 -l i g i b i l i t y for a l l speech intensities, while in less noise, they resulted in decreased i n t e l l i g i b i l i t y . Earplugs produced this improved articulation by effectively reducing the overall intensity level of speech and noise from high to medium levels while keeping S/N ratio constant. Similarly in the present experiment, WDS's generally improved with decreasing overall levels of speech and noise. Kryter found that for subjects not wearing earplugs, increasing the speech level with S/N ratio held constant resulted in a decrease of articulation scores, the same roll-over effect demonstrated in the present study. As the S/N ratio was decreased, the roll-over of the articulation curves was accentuated and occurred at success-ively lower speech intensity levels. Tests in the anechoic chamber, on the other hand, produced maximum articulation scores for both groups which did not decrease with higher speech levels but stayed at a constant plateau. Speech at 80 dB or more above threshold was heard equally well with plugs of without. Kryter (1946) attributed the divergence of articulation performance (approximately 10%) for the two test rooms at high speech levels, to reverberation effects which are present i n the reverberant room and absent in the anechoic one. Earplugs attenuated these effects below the listener's threshold. For listeners without earplugs, however, as the speech intensity - 65 -w a s raised, the masking effects of reverberation on i n t e l -l i g i b i l i t y increased. This resulted in the rol l -over of art iculat ion scores noted in the reverberant room. Kryter (1946) concluded that these reverberation effects "constitute unintel l ig ib le 'noise' that interferes with speech reception" (p. 416), and that the masking effect of this reverberant speech increased \\rith higher overall levels. Similar findings were reported in a recent study by Martin et a l . (1976) investigating the influence of earplugs and earmuffs on communication in noise. In agreement with Kryter (1946) , they found that speech discrimination improved with the wearing of ear protection in high noise levels (above 85 dBA) but was degraded i f protectors were worn in noise levels less than 65 dBA. Examination of their data obtained in a semi-reverberant room again reveals the rol l -over of discrimination scores with high speech intensit ies for unoccluded ears. They, however, did not attribute this decreased speech discrimination to reverberation effects. They referred to the possible occurrence of distort ion in the cochlea with increasing noise levels above 65 dB. This would appear to be akin to the auditory "overloading" of Pollack and Pickett (195$). As the wearing of ear protectors in high noise levels would reduce this distortion without affecting the S/N rat io, improved art iculat ion could result . - 66 -j n a study cited earlier, Kryter et a l . (1962) presented monosyllabic word l i s t s in noise monaurally to subjects with normal hearing and various degrees of sensorineural impairment. The speech stimuli, low-pass f i l t e r e d at 7000 Hz, were presented at overall levels of either 65 dB or 95 dB SPL. For the majority of their subjects, Kryter et a l . , found the speech presented at 65 dB was more i n t e l l i g i b l e than that presented at 95 dB. As in the present study, i n t e l l i g i b i l i t y decreased with higher stimulus intensity. A third possible reason why the anticipated change i n the slopes of the regression functions of the N and P groups failed to occur with increasing stimulus intensity, l i e s in the particular stimulus levels used. These levels, 60, 70, and 80 dB SPL may have been so high in intensity that most of the high CF neurons were already being activated at the 60 dB stimulus level. In such a case, l i t t l e change in the slopes would occur as a l l three levels would be representing the similar condition of high CF neuron excitation. To validate Kiang and Moxon's (1974) findings, the range of stimulus levels employed must span the threshold region of excitation of these neurons in order to demonstrate any change their excitation may bring about in word discrimination a b i l i t y . Stimulus levels, therefore, lower than 60 dB SPL may be necessary to produce the anticipated results in future studies in this area. - 67 -In summary, the results of the present study support the complaints of individuals with high-frequency sensorineural hearing losses that they experience d i f f i cu l t i e s hearing speech in noise. Presentation of a f i l te red word discrimination task in the presence of masking noise demonstrated that noise does have a significant d i f ferent ia l effect on the WDS's of the subject groups, N and P. A satisfactory explanation of these results based on the physiological findings of Kiang and Moxon (1974) was not accomplished, however, due to at least three possible complicating factors. These included the fa i lure to adequately match the pure-tone thresholds of the two groups up to 2000 Hz, the use of the sound f i e ld test condition rather than headphones, and the employment of too high stimulus levels. Kiang and Moxonfs (1974) study s t i l l provides a promising framework for the investigation of speech discrimination in noise. Future use of a similar f i l te red speech task under headphones (to eliminate reverberation effects), using carefully matched subjects and a lower range of stimulus levels, may provide a better experimental design for testing the application of Kiang and Moxon's data to humans. The more r e a l - l i f e situation tested in this study demonstrates that the deteriorating effect of noise on hard-of-hearing people is a complex problem that involves the interaction of many factors. - 68 -REFERENCES Bekesy, G. Von. Experiments in Hearing. New York: McGraw-Hill Book Company, I960. Bess, F. H., and Townsend, T. H. Word discrimination for l isteners with f l a t sensorineural hearing losses. Journal of Speech and Hearing Disorders, 1977, 42, 232-237. Bilger, R .C , Stiegel, M. S., and Rose, N. R. Consonant recognition in noise: sensorineural loss. Journal of  the Acoustical Society of America, 1974, 55» S87. (Abstract). Bilger, R. C , Stiegel, M. S., and Stenson, N. Effects of sensorineural hearing loss on hearing speech in noise. Transactions of the American Academy of Ophthalmology  and Otolaryngology., 1976.' 82, 363-365. Carhart, R. Problems in the measurement of speech discrimination. Archives of Otolaryngology, 1965, 82, 253-260. Carhart, R., and Tillman, T. W. Interaction of competing speech signals with hearing losses. Archives of  Otolaryngology, 1970, 91, 273-279. Cohen, R. L., and Keith, R. W. Use of low-pass noise in word-recognition testing. Journal of Speech and Hearing  Research, 1976, l g , 48-54. Cooper, J . C , and Cutts, B; P. Speech discrimination in noise, journal of Speech and Hearing Research, 1971, lit. Danaher, E. M., and Pickett, J . M. Some masking effects produced by low frequency vowel formants in persons with sensorineural hearing loss. Journal of Speech  and Hearing Research, 1975, 18> 261-271. Davis, H., and Silverman, S. R. Hearing and Deafness. New York: Holt, Rinehart, and Winston, 1970. - 69 -Findlay, R. C. Auditory dysfunction accompanying noise-induced hearing loss. Journal of Speech and Hearing  Disorders, 1976, _1, 374-384. Findlay, R. C , and Denenberg, L. J . Effects of subtle mid-frequency auditory dysfunction upon speech discrimination in noise. Audiology, 1977, 16, 252-259. French, N. R., and Steinberg, J . C. Factors governing the i n t e l l i g i b i l i t y of speech sounds. Journal of the  Acoustical Society of America, 1947, 12, 90-119. Giolas, T. G., and Epstein A. Comparative i n t e l l i g i b i l i t y of word l i s t s and continuous discourse. Journal of  Speech and Hearing Research, 1963, 6, 349-358. Glorig, A., Ward, W. D., and Nixon, J . Damage r isk c r i te r i a and noise-induced hearing loss. Archives of  Otolaryngology, 1961, 7_, 413-423. Harris, J . D. Combinations of distort ion in speech. Archives of Otolaryngology, I960, J7_2, 227-232. Harris, J . D. Pure-tone acuity and the i n t e l l i g i b i l i t y of everyday speech. Journal of the Acoustical Society of  America, 1965, 32, 824-830. Jerger, J . F., Tillman, T. W., and Peterson, J . L. Masking by octave bands of noise in normal and impaired ears. Journal of the Acoustical Society of America, I960, 32, 3 8 5 - 3 9 0 . 1 — Keith, R. W., and Ta l i s , H. P. The effects of white noise on PB scores of normal and hearing-impaired l isteners. Audiology, 1972, 11, 177-186. Kiang, N. Y. S., and Moxon, E. C. Tai ls of tuning curves of auditory nerve f ibres. Journal of the Acoustical  Society of America, 1974, J_, 620-630. Kryter, K. D. Effects of ear protective devices on the i n t e l l i g i b i l i t y of speech in noise. Journal of the  Acoustical Society of America, 1946, 18, 413-4T7T Kryter, K. D., Williams, C , and Green, D. M. Auditory acuity and the perception of speech. Journal of the  Acoustical Society of America, 1962, ^ 4., 1217-1223. - 70 -Leshowitz, B. Relationship of tonal masking to speech i n t e l l i g i b i l i t y in noise for l isteners with sensorineural hearing damage. Journal of the Acoustical Society of America, 1977, 62, S93T (Abstract). : Linden, G. Undistorted speech audiometry. In B. Graham (Ed.), Sensorineural Hearing Processes and Disorders. Boston: L i t t l e , Brown, 1967. L ie r le , D. M. Guide for the evaluation of hearing impairment: a report of the committee on conservation of hearing. Transactions of the American Academy of Ophthalmology  ancf Otolaryngology, 1959. 63, 236-238. Martin, A. M., Howell, K., and Lower, M. C. Hearing protection and communication in noise. In S. D. G. Stephens (Ed.), Disorders of Auditory Function (Vol. 2). London: Academic Press, 1976. Martin, E. S., and Pickett, J . M. Sensorineural hearing loss and upward spread of masking. Journal of Speech  and Hearing Research, 1970, 13, 426-437. Mullins, C. J . , and Bangs, J . L. Relationships between speech discrimination and other audiometric data. Acta Otolaryngologica, 1957, JtZ, 149-157. Olsen, W. 0., Noffsinger, D., and Kurdziel, S. Speech discrimination in quiet and in white noise by patients with peripheral and central lesions. Acta Otolaryngologica, 1975, 80, 375-382. Palva, T. Studies of hearing for pure-tones and speech in noise. Acta Otolaryngologica, 1955, lt£, 231-243. Pollack, I., and Pickett, J . M. Masking of speech by noise at high sound levels. Journal of the Acoustical Society  of America, 195^, 20, 127-130. Quiggle, R. R., Glorig, A., Delk, J . H., and Summerfield, A. Predicting hearing loss for speech from pure-tone audiograms. Laryngoscope, 1957, 62,, 1-15. Rittmanic, P. A. Pure-tone masking by narrow-noise bands in normal and impaired ears. Journal of Auditory  Research, 1962, 2, 287-304. - 71 -Ross, M., Huntington, D. A., Newby, H. A., and Dixon, R. F. Speech discrimination of hearing-impaired individuals in noise. Journal of Auditory Research. 1965, j>, 47-72. Rupp, R. R., and Phi l l ips , D. The effect of noise background on speech discrimination function in normal hearing individuals. Journal of Auditory  Research, 1969, 9, 60-63. Shapiro, M. T., Melnick, W., and VerMeulen V. Effects of modulated noise on speech i n t e l l i g i b i l i t y of people with sensorineural hearing loss. Annals of Otology,  Rhinology, and Laryngology, 1972, 81, 241-248. Sher, A. E., and Owens, E. Consonant confusions associated with hearing loss above 2000 Hz. . Journal of Speech  and Hearing Research, 1974, 12, 669-681. Simonton, K. M., and Hedgecock, L. D. A laboratory assessment of hearing acuity for voice signals against a background of noise. Annals of Otology,  Rhinology, and Laryngology, 1953, 62, 735-747. 

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