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Masking on the high-frequency side of a masking tone : effects on masked threshold of selectively masking… Krammer, Franz Georg 1973

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MASKING ON THE HIGH-FREQUENCY SIDE OF A MASKING TONE: EFFECTS ON MASKED THRESHOLD OF SELECTIVELY MASKING LOW-FREQUENCY COMBINATION TONES OF DIFFERENT TYPES FORMED BY SIGNAL AND MASKER by F r a n z G, Kraimaer B.A., U n i v e r s i t y o f B r i t i s h Columbia, 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS i n the Department o f P sychology We ac c e p t t h i s t h e s i s as conforming t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA September, 1973 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 i t 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. Department of PSYCHOLOGY The University of British Columbia Vancouver 8 , Canada Date September 14. 1973 ABSTRACT When s i g n a l s a r e p r e s e n t e d on the immediate h i g h -fre q u e n c y s i d e o f a pure-tone masker, i t i s not the s i g n a l , but r a t h e r combination tones, t h a t a r e d e t e c -t e d a t masked t h r e s h o l d (Greenwood, 1971b)„ A t low to moderate masker i n t e n s i t i e s , the d e t e c t e d combination tone i s 2f^ minus f ^ , where f 1 i s the masker f r e q u e n c y and i s the s i g n a l f r e q u e n c y . A t h i g h e r masker l e v e l s , the combination tone f^f^ becomes l a r g e enough to p l a y a r o l e * To i n v e s t i g a t e t h i s r o l e , the combination tones f 2 ~ f 2 and 2 f ^ - f 2 were s e l e c t i v e l y masked i n d i f f e r e n t low-frequency r e g i o n s , t o i d e n t i f y t h e i r e f f e c t s . As has been known, the n o t c h r e g i o n on the h i g h s i d e o f the pure-tone masking p a t t e r n c r e a t e s a secondary peak, which s h i f t s upward i n f r e q u e n c y and h e i g h t as masker i n t e n s i t y i s raised,, R e s u l t s i n d i c a t e t h a t the f r e q u e n -cy l o c a t i o n o f t h i s secondary peak has an upper l i m i t a t 1.85f^, and t h a t the secondary peak marks the b e g i n n i n g o f s i g n a l - d e t e r m i n e d masked t h r e s h o l d s . A t masker l e v e l s s u f f i c i e n t t o s h i f t t h i s secondary peak upward, beyond 1.5 times the masker frequ e n c y ( f 1 ) , both 2f^-f^ (or C 1) and £2~^l ( o r D l ) a r e d e l e t e d w i t h about equal l i k e l i -hood when s i g n a l s a r e i n the h i g h e r - f r e q u e n c y p a r t o f the n o t c h r e g i o n , a t f r e q u e n c i e s between l 0 5 f . . and the i i i s econdary peak,, That i s , e i t h e r o r D^, or b o t h , may be d e t e c t e d s i n c e t h e i r i n t e n s i t i e s become q u i t e s i m i l a r a t h i g h e r l e v e l s , as supported by more d i r e c t e s t i m a t e s (Greenwood, 1971b, 1972a,b,c; H a l l , 1971, 1972a,b). i v TABLE OF CONTENTS Page. ABSTRACT . i i TABLE OF CONTENTS i v LIST OF FIGURES . .. v i ....ACKNOWLEDGMENT .,-0-»»*^ *»e..••.o..»».«»».•.««•»• .... •...» •«•..•..•-,•,•-.•>...•»..• • . i x INTRODUCTION . .. 1 D e f i n i t i o n s and E x p l a n a t i o n s o f Terms 1 H i s t o r i c a l Synopsis 5 Masking on the High-Frequency S i d e o f a Masking Tone 10 Purpose o f the Study and S t r a t e g i e s Used 12 METHODS AND APPARATUS , . . 17 Gene r a l Method 17 S t i m u l i and C o n t r o l s 18 Su b j e c t s 19 Apparatus 20 Ge n e r a t i o n o f n o i s e bands 22 C a l i b r a t i o n 23 C h r o n o l o g i c a l D e s c r i p t i o n o f an Ex p e r i m e n t a l S e s s i o n 25 RESULTS 28 Masking and D 1 S e l e c t i v e l y 30 Moderately i n t e n s e maskers (secondary peak near or below 1.5f.) .... 31 V More i n t e n s e maskers (secondary peak above 1.5f^) 34 Masked t h r e s h o l d s between f± and 1 . 5 ^ 35 Masked t h r e s h o l d s between 1.5f^ and the secondary peak 39 Masking and D.^  S i m u l t a n e o u s l y 47 Wide band of n o i s e «... 49 Medium-width bands o f n o i s e .... 50 E f f e c t o f n o i s e bandwidth 51 Two narrow bands of n o i s e 57 E f f e c t s o f Noise L e v e l 58 DISCUSSION 63 S i g n i f i c a n c e o f Masked Th r e s h o l d s i n the Notch 63 S i g n a l I n t e n s i t i e s a t which C 1 , D^, or the S i g n a l I t s e l f a r e D e t e c t e d a t T h r e s h o l d 66 Dips i n the Masking P a t t e r n 68 L i m i t i n g P o s i t i o n o f the Secondary Peak 70 G e n e r a l i t y and i m p l i c a t i o n s o f the R e s u l t s ... 71 •Masking by complex s t i m u l i 72 Masked t h r e s h o l d o f the s i g n a l i t s e l f ... 74 BIBLIOGRAPHY 76 v i LIST OF FIGURES F i g u r e Page 1 P o r t i o n s of masked audiograms produced by pure-tone maskers at 2000 Hz, p r e -sented a t ascending l e v e l s of the masker f o r one s u b j e c t . . . . . . . . . 8 2 P o r t i o n s of masked audiograms produced by pure-tone maskers a t 85 dB SPL p r e -sented a t 1.2 or 2.0 kHz accompanied by low-pass n o i s e s p r e s e n t e d a t ascend-i n g spectrum l e v e l s 9 3 B l o c k diagram of the apparatus 21 4 Masked t h r e s h o l d s above the pure-tone masker ( f 1 = 1.3 kHz or 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f a band o f n o i s e j u s t below the tone masker ( f ^ ) 32 5 Masked t h r e s h o l d s above the tone masker ( f 1 = 1.3 kHz or 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f a band o f n o i s e j u s t below a f r e q u e n c y h a l f t h a t o f the masker ( , 5 f ^ ) . . 33 6 Masked t h r e s h o l d s above the tone masker ( f 1 = 1.3 k H z ) — b e f o r e and a f t e r a d d i t i o n of a band o f n o i s e , e i t h e r j u s t below f_> P2L J U S T below . 5 ^ f o r s u b j e c t X T 36 7 Masked t h r e s h o l d s above the tone masker (f ^ = 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f a 600-Hz band o f n o i s e , e i t h e r j u s t below f't o r j u s t below .5f f o r sub-j e c t K L . 7 ; 37 8 Masked t h r e s h o l d s above the tone masker ( f _ = 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f a 600-Hz band o f noise> e i t h e r j u s t below f 1 , o r j u s t below .5f- f o r sub-j e c t FK. 7 38 v i i F i g u r e Page 9 Masked t h r e s h o l d s above a tone masker ( f 1 = 2 kHz) a t 80 dB S P L — b e f o r e and a f t e r a d d i t i o n o f a 600-Hz band o f n o i s e , e i t h e r j u s t below f ^ , or j u s t below ,5f ^  f o r s u b j e c t RB....... 41 10 Masked t h r e s h o l d s above a tone masker ( f 1 = 2 kHz) a t 80 dB S P L — b e f o r e and a f t e r a d d i t i o n o f a narrow, 50-Hz, -band o f - n o i s e , e i t h e r j u s t below f ^ , or j u s t below .5f^ f o r s u b j e c t RB 42 11 Masked t h r e s h o l d s above a tone masker (f-= 2 kHz) a t 85 dB S P L — b e f o r e and a f t e r a d d i t i o n o f a 600-Hz band o f n o i s e , e i t h e r j u s t below f - , o r j u s t below • 5 f 1 f o r s u b j e c t RB 43 12 Masked t h r e s h o l d s above a tone masker ( f 1 = 2 kHz) a t 85 dB S P L — b e f o r e and a f t e r a d d i t i o n o f a narrow, 50-Hz, band o f n o i s e , e i t h e r j u s t below f ^ , ...or j.ust Jb.elow ,..5f^ f o r subj,ect,RB................... . ....44 13 Masked t h r e s h o l d s above a tone masker (f^= 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f a wide (1400-Hz), e f f e c t i v e l y low-pass, band o f n o i s e below f ^ 48 14 Masked t h r e s h o l d s above a tone masker ( f 1 = 1.3 k H z ) — b e f o r e and a f t e r a d d i t i o n o f 350-Hz band o f n o i s e , c e n t e r e d a t . 5 f 1 , f o r s u b j e c t TT 52 15 Masked t h r e s h o l d s above a tone masker ( f 1 = 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f a 600-Hz band o f n o i s e , c e n t e r e d a t 1 kHz ( . 5 f 1 ) , f o r s u b j e c t TC 53 16 Masked t h r e s h o l d s above a tone masker (f^= 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f a 600-Hz band of n o i s e , c e n t e r e d a t 1 kHz ( . 5 ^ ) , f o r s u b j e c t RB...... 54 v i i i F i g u r e Page 17 Masked t h r e s h o l d s above a tone masker (f^= 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f n o i s e bands o f d i f f e r e n t w i d t h (1400 Hz, 600 Hz, or 50 Hz), f o r s u b j e c t RB 55 18 Masked t h r e s h o l d s above a tone masker (f^= 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f two narrow bands o f n o i s e below the tone masker 56 19 Masked t h r e s h o l d s above a tone masker (fj = 2 k H z ) — b e f o r e and a f t e r a d d i t i o n o f a 50=Hz band of n o i s e a t d i f f e r e n t l e v e l s 59 ix ACKNOWLEDGMENT I don't have s u f f i c i e n t space here t o express f u l l y my thanks t o D r e Donald Do Greenwood f o r h i s p a t i e n c e , encouragement, and a s s i s t a n c e d u r i n g a l l phases o f t h i s research,, S p e c i a l thanks must go t o Gordon D„ Young f o r nhrs*ready " a s s i s t a n c e whenever I r e q u i r e d i t , and espe-c i a l l y f o r h i s e s s e n t i a l a d v i c e on t e c h n i c a l matters„ I am a l s o t h a n k f u l t o D r 0 R. Lakowski f o r r e a d i n g the t h e s i s and p r o v i d i n g v a l u a b l e s u g g e s t i o n s on v e r y s h o r t n o t i c e . INTRODUCTION The f o l l o w i n g paper d e a l s w i t h two important con-cerns i n the s t u d y o f h e a r i n g : masking and combination tones. Between the s e , important i n t e r r e l a t i o n s h i p s e x i s t which e l u c i d a t e both c l a s s e s o f phenomena. Fo r example, c e r t a i n combination tones i n f l u e n c e the mask-i n g p a t t e r n which i s o b t a i n e d even i n the s i m p l e s t exper-i m e n t a l c o n d i t i o n s (Greenwood, 1971b; 1972a,b,c). D e f i n i t i o n s and E x p l a n a t i o n s o f Terms Masking i s q u a n t i t a t i v e l y d e f i n e d as "the amount by which the t h r e s h o l d o f a u d i b i l i t y o f a sound i s r a i s e d by the presence o f another (masking) sound"; and q u a l i -t a t i v e l y d e f i n e d as "the p r o c e s s by which the t h r e s h o l d o f a u d i b i l i t y o f a sound i s r a i s e d by the presence o f another (masking) sound" (USA Standard, 1960, S l . l ) . The sound whose t h r e s h o l d i s r a i s e d i s c a l l e d t h e s i g n a l and t h e masking sound t h a t produces the i n c r e a s e i n t h r e s h o l d i s c a l l e d the masker. A pure-tone masker, f o r example, has a peak masking e f f e c t on s i g n a l s c l o s e t o the f r e q u e n c y o f the masker ( f ^ ) . The masking e f f e c t on s i g n a l s below f ^ (the masker f r e -quency) extends over a narrow f r e q u e n c y range and d e c r e a s e s r a p i d l y as s i g n a l f r e q u e n c i e s d e c r e a s e immediately below f - . The masking e f f e c t on the h i g h - f r e q u e n c y s i d e o f the -2-masker, on the o t h e r hand, extends over a g r e a t e r range o f s i g n a l f r e q u e n c i e s , p a r t i c u l a r l y a t h i g h e r i n t e n s i t i e s o f the masker. T h i s asymmetry i n the masking e f f e c t i s c o n s i s t e n t w i t h the observed asymmetry o f the shape o f the t r a v e l i n g - w a v e envelope on the b a s i l a r membrane i n response t o a pure tone,, For t h i s , and o t h e r , reasons masking has o f t e n been used i n attempts t o i n f e r a u d i t o -r y mechanisms a t the p e r i p h e r y of the system. A combination tone i s a tone produced w i t h i n the a u d i t o r y system when two or more tones o f s u f f i c i e n t i n -t e n s i t y a r e p r e s e n t e d s i m u l t a n e o u s l y . There a r e grounds f o r e x p l a i n i n g the o r i g i n o f combination tones as a r e -s u l t o f n o n l i n e a r amplitude d i s t o r t i o n i n the i n n e r ear (Zwicker, 1955, 1968; Plomp, 1965; G o l d s t e i n , 1967; Green-wood, 1971b) r a t h e r than as a r e s u l t o f p u r e l y n e u r a l p r o -c e s s e s . A l t h o u g h , assuming n o n l i n e a r i t y , the f r e q u e n c y l o c a t i o n o f innumerable combination tones may be c a l c u -l a t e d , w i t h r e s p e c t t o the f r e q u e n c i e s o f p r i m a r y tones, evidence f o r the e x i s t e n c e o f o n l y a s m a l l number o f these combination tones has been produced (Plomp, 1965). E x i s t i n g combination tones a r e c l a s s i f i e d and d e s c r i b e d , and t h e i r f r e q u e n c i e s c a l c u l a t e d , a c c o r d i n g to d i f f e r - ences o f i n t e g r a l m u l t i p l e s o f the f r e q u e n c i e s o f each o f the p r i m a r y tones. No c o n v i n c i n g e v i d e n c e has y e t been p r e s e n t e d to suggest t h a t combination - 3 -tones d e s c r i b e d by the sum o f i n t e g r a l m u l t i p l e s a r e produced w i t h i n the a u d i t o r y system (Plomp, 1965). A combination band o f n o i s e i s the r e s u l t o f the same p r o c e s s as t h a t p r o d u c i n g combination tones, but the combination band i s produced by a tone and a band o f n o i s e or by two bands o f n o i s e . I t s fr e q u e n c y l o c a t i o n and band l i m i t s a r e c a l c u l a t e d i n a s i m i l a r way as the frequen c y l o c a t i o n o f combination tones i s c a l c u l a t e d . That i s , the fr e q u e n c y components o f one of the p r i m a r i e s i n t e r a c t w i t h those o f the o t h e r i n the same way as the p r i m a r i e s g e n e r a t i n g combination tones i n t e r a c t (Green-wood, 1971b; 1972a,b,c). In s h o r t , .combination tones a r e c l a s s i f i e d under the two main c a t e g o r i e s , summation tones and d i f f e r e n c e tones. Summation tones a r e d e s c r i b e d by the sum o f i n t e g r a l m u l t i -p l e s of the f r e q u e n c i e s o f each o f the primary tones; and d i f f e r e n c e tones a r e d e s c r i b e d by the d i f f e r e n c e o f i n t e g r a l m u l t i p l e s o f the f r e q u e n c i e s o f each o f the p r i -mary tones. Only the l a t t e r type o f combination tone i s o f i n t e r e s t h ere, f o r the reason g i v e n above. D i f f e r e n c e tones may be f u r t h e r s u b - c l a s s i f i e d i n t o those o f odd o r d e r and those o f even o r d e r . The more prominent odd-order combination tones ( C n) a r e those whose fr e q u e n c y i s g i v e n by the e x p r e s s i o n ( n + l ) f 1 ~ n f 2 , where ri i s an i n t e g e r (n = 1,2,3...) , f , i s the freque n c y - 4 -o f the lower p r i m a r y component (e.g. the masker), and f ^ i s the f r e q u e n c y o f the upper p r i m a r y component (e.g. the s i g n a l ) . The more prominent even-order d i f f e r e n c e tones (D^) a r e those whose f r e q u e n c y i s g i v e n by the e x p r e s s i o n n ( f 2 - f ^ ) , n = 1 or 2. The two most r e a d i l y a u d i b l e combi-n a t i o n tones a r e those d e s c r i b e d by the p r e c e d i n g formulas when n = l . They a r e the c u b i c d i f f e r e n c e tone, 2f^-f,>, or , so c a l l e d because i t s f r e q u e n c y i s p r e d i c t e d by t h i r d - o r d e r , or c u b i c , terms i n the power s e r i e s expansion o f the n o n l i n e a r i t y ; and the q u a d r a t i c , or second-order, d i f f e r e n c e tone, f ^ - f ^ , o r D^, p r e d i c t e d from q u a d r a t i c terms. The .behavior o f the frequency o f and d u r i n g the m a n i p u l a t i o n o f the f r e q u e n c i e s o f the p r i m a r i e s i s o f p a r t i c u l a r concern here. As the f r e q u e n c y s e p a r a t i o n between the p r i m a r i e s i s i n c r e a s e d , from n i l t o octave s e p a r a t i o n , the f r e q u e n c y o f D^ ^ i n c r e a s e s from z e r o t o t h a t o f the lower p r i m a r y , and the f r e q u e n c y o f de-c r e a s e s from t h a t o f the lower p r i m a r y t o z e r o . The two d i s t o r t i o n p r o d u c t s c o i n c i d e i n f r e q u e n c y a t «5f^ when the fr e q u e n c y o f the h i g h e r p r i m a r y ( f 2 ) ^ s times t h a t o f the lower (f^)» For s m a l l e r f r e q u e n c y s e p a r a t i o n s o f the p r i m a r i e s ( i . e . f^/f^ s m a l l e r than 1.5), the f r e q u e n c y o f i s , o b v i o u s l y , lower than t h a t o f C. , and f o r g r e a t e r f r e q u e n c y r a t i o s than 1.5, 5-the f r e q u e n c y o f i s lower - than t h a t o f D 1. F o r exam-p l e , i f the f r e q u e n c y o f the lower p r i m a r y or masker i s 2kHz ( f ^ = 2kHz), and the f r e q u e n c y o f the upper primary or s i g n a l i s 2.5kHz ( f 2 = 2.5kHz), the f r e q u e n c y of i s 1.5kHz and the f r e q u e n c y o f i s 0.5kHz (see F i g . 11, schematic no. 1). H i s t o r i c a l S y n o p s i s D i f f e r e n c e tones a r e a r e l a t i v e l y e a r l y concern i n t h e study o f h e a r i n g . Around the middle o f the 18th c e n t u r y , Georg Andreas Sorge, Guiseppe T a r t i n i , and Jean-B a p t i s t e Romieu i n d e p e n d e n t l y p u b l i s h e d f i n d i n g s r e l a t i n g t o the d i s c o v e r y o f d i f f e r e n c e t o nes. Each has been c r e d i t e d w i t h the d i s c o v e r y . T a r t i n i , however, i n a l a t e r p u b l i c a t i o n , c l a i m s t o have d i s c o v e r e d them-at the e a r l i e s t d a t e , 1714 (Jones, 1937). Much work on combi-n a t i o n tones had been done i n the 19th c e n t u r y , but i n t e r e s t i n t h i s phenomenon s u b s i d e d i n the e a r l y p a r t o f the 20th c e n t u r y , d e s p i t e the development o f more s o p h i s t i c a t e d t e c h n i q u e s which c o u l d have been u s e f u l i n s t u d y i n g them. L a t e l y , p a r t i c u l a r l y w i t h i n the l a s t decade, i n t e r e s t i n combination tones has been r e v i v e d , w i t h i n v e s t i g a t o r s such as Plomp, Zwicker, G o l d s t e i n , Greenwood, and Smooren-burg p u r s u i n g i n t e n s i v e s t u d i e s . Recent e v i d e n c e shows t h a t a t l e a s t one of the sources r e s p o n s i b l e f o r the -6-c r e a t i o n o f combination tones i s l o c a t e d i n the i n n e r e a r . One combination tone i n p a r t i c u l a r , C^, i s o f i n t e r e s t s i n c e i t s amplitude d e c r e a s e s s h a r p l y w i t h the f r e q u e n c y s e p a r a t i o n o f pr i m a r y tones ( G o l d s t e i n , 1967), T h i s amplitude decrease seems t o r e f l e c t c h a r a c t e r i s t i c s o f t h e freque n c y a n a l y s i s o f the i n n e r e a r . The study o f masking has been o f more r e c e n t s c i e n -t i f i c i n t e r e s t than t h a t o f combination tones, i n p a r t s i n c e i t i s d i f f i c u l t t o study without the a i d o f e l e c -t r o n i c i n s t r u m e n t s . Mayer (1876) was p r o b a b l y the f i r s t t o make s y s t e m a t i c , though u n q u a n t i f i e d , o b s e r v a t i o n s c o n c e r n i n g masking. S p e c i f i c a l l y , he noted t h a t sounds «©£ ,-.eo3as-ide-rable i n t e n s i t y -could-be made i n a u d i b l e ( i . e . . masked) by sounds o f lower f r e q u e n c y but not by sounds o f h i g h e r f r e q u e n c y . The f i r s t e x t e n s i v e study o f masking was performed by Wegel and Lane (1924). They were the f i r s t t o p l o t masked audiograms. A masked audiogram i s a g r a p h i c a l r e p r e s e n t a t i o n o f the masked t h r e s h o l d o f a s i g n a l i n the presence o f one o r more maskers, c o n s i s t i n g o f one or more tones, n o i s e s , o r combinations o f them. Masked t h r e s h o l d i s u s u a l l y p l o t t e d i n d e c i b e l s sound p r e s s u r e l e v e l (dB SPL) 1 as a f u n c t i o n o f the fr e q u e n c y o f the .^.Decibels Sound Pre s s u r e L e v e l a r e measured w i t h r e s p e c t to a r e f e r e n c e l e v e l o f 0.0002 dyne per square c e n t i m e t e r , which i s r e a s o n a b l y c l o s e t o the average t h r e s h o l d o f h e a r i n g o f a 1000-cycle tone. -7-s i g n a l . Wegel and Lane kept the masking tone at a con-stant frequency and i n t e n s i t y . The i n t e n s i t y of the s i g n a l tone at which the l a t t e r could j u s t be heard was then p l o t t e d at s u i t a b l y chosen frequencies of the s i g n a l , both above and below the masking tone. This method of study-ing masking i s s t i l l appropriate to the study of many questions, and i s b a s i c a l l y the same method used i n the studies that axe the subject of t h i s paper. Wegel and Lane (1924) have pointed out that, at high i n t e n s i t i e s of the masking tone, and at s i g n a l frequencies j u s t above that of the masking tone, a d i f f e r e n c e .tone i s heard instead of the s i g n a l at masked threshold. These investigators a l s o reported the presence of a secondary peak i n such masked audiograms. They miscentered i t , however, at the second harmonic, probably because they assumed the secondary peak to r e s u l t from masking by the second harmonic. This error was pointed out by l a t e r s t udies, p a r t i c u l a r l y by Ehmer (1959) and more extensively by Greenwood (1971b). Wegel and Lane f a i l e d to observe a secondary peak at lower masker i n t e n s i t i e s f o r lack of a s u f f i c i e n t l y d e t a i l e d mapping of the frequency range below the second harmonic. This secondary peak s h i f t s upward i n frequency as masker i n t e n s i t y increases and becomes the peak observed by Wegel and Lane at high in t e n -s i t y (see F i g . 1). t o f a c e page 8 F i g . 1. (A) P o r t i o n s o f masked audiograms p r o d u c e d by p u r e - t o n e maskers a t 2000 Hz f o r one s u b j e c t . Each c u r v e i s d i s p l a c e d on t h e o r d i n a t e t o p r e v e n t o v e r l a p o f c u r v e s . (B) The same c u r v e s a r e p l o t t e d i n a t h r e e - d i m e n s i o n a l g r a p h , as smooth c u r v e s — t h i s t i m e on t h e same o r d i n a t e s c a l e b u t w i t h masker l e v e l i n d i c a t e d on a r e c e d i n g z - a x i s i n o r d e r t o r e d u c e o v e r l a p . The l o w p o i n t o f t h e n o t c h r e m a i n s a t n e a r l y t h e same f r e q u e n c y from low t o moderate l e v e l s , and a t h i g h e r masker l e v e l s , f o r t h i s and some o t h e r s u b j e c t s , a s m a l l s h e l f and t h e n an i n f l e c t i o n b e g i n s a t t h e same f r e q u e n c y . A t masker l e v e l s t o o low t o p r o d u c e a n o t c h , t h e h i g h - f r e q u e n c y s i d e o f t h e masking p a t t e r n i n t e r s e c t s t h e q u i e t t h r e s h o l d c u r v e a t v e r y n e a r l y t h e same s i g n a l f r e q u e n c y . The s e c o n d a r y peak i n c r e a s e s i n s i z e w i t h masker l e v e l and moves upxvard i n f r e q u e n c y , i n c r e a s i n g t h e s i z e o f t h e n o t c h m a r k e d l y ( f r o m Greenwood, 1971b, F i g . 6 ) . -8-i ' i 1 i 1 i FIGURE X t o f a c e page 9 F i g . 2. P o r t i o n s o f masked audiograms produced by pure-tone maskers a t 85 dB SPL p r e s e n t e d a t 1.2 or 2.0 kHz ( t o p and bottom, r e s p e c t i v e l y ) accompanied by low-pass n o i s e s p r e s e n t e d a t ascending spectrum l e v e l s . The i n -s e t graphs d i s p l a y the s t i m u l i used. The n o i s e accom-panying the tone o f 1.2 kHz s t a r t s t o c u t o f f a t 1050 Hz w i t h a s l o p e o f 110 dB/oct f o r the f i r s t 25 dB. The n o i s e w i t h the 2.0-kHz tone was the same as i n F i g . 10. In each graph, curves through s o l i d c i r c l e s d i s p l a y masked audiograms produced by the masking tone p r e s e n t e d a l o n e . The added low-pass n o i s e p r o g r e s s i v e l y e l i m i n a t e s the n o t c h . I n c l u d e d i n the upper graph i s the masked audiogram p r o --duced ,foy t he most .intense low-.pass. .noise..when .presented a l o n e . The n o i s e a l o n e produces l i t t l e masking i n the n o t c h r e g i o n . A t a spectrum l e v e l 20 dB lower, a t which i i t i s s t i l l c o m p l e t e l y e f f e c t i v e i n e l i m i n a t i n g the n o t c h , the n o i s e alone produces no masking i n the r e g i o n o f the notch (from Greenwood, 1971b, F i g . 11, p. 515). - 9 -FIGURE 2 -10-Masking on the High-Frequency S i d e o f a Masking Tone The trough between the masking peak and the second-a r y peak o f the masked audiogram has been r e f e r r e d t o as a " n o t c h " (Greenwood, 1971b). The low p o i n t o f the n o t c h i s l o c a t e d a t a p p r o x i m a t e l y one c r i t i c a l bandwidth above the f r e q u e n c y o f the masking tone (see F i g . 1, reproduced from Greenwood, 1971b, F i g . 6 ) . E v i d e n c e o f t h i s n o t c h i s a l s o p r e s e n t i n the d a t a o f C h i s t o v i c h (1957), Small (1959), Ehmer (1959), and Greenwood (1961). Audiograms o b t a i n e d by t h e s e i n v e s t i g a t o r s r e v e a l n o t c h - or s h e l f -l i k e shapes o f the masking p a t t e r n on the h i g h - f r e q u e n c y s i d e o f pure-tone masked audiograms when the masker i s a t "low-moderate" to h i g h l e v e l s . These f e a t u r e s o f shape have been i n v e s t i g a t e d i n g r e a t e r d e t a i l by Greenwood (1971b). By i n t r o d u c i n g a low-pass n o i s e w i t h an upper c u t - o f f f r e q u e n c y below t h a t of the pure-tone masker, he was a b l e t o e l i m i n a t e t h i s notch (see F i g . 2, reproduced from Greenwood, 1971b, F i g . 11, p. 515). Lower l e v e l s of t h i s n o i s e reduced t h e s i z e o f the n o t c h to a l e s s e r e x t e n t than d i d h i g h e r l e v e l s . In o t h e r words, the n o i s e , which was l o c a t e d below the f r e q u e n c y o f the pure-tone masker, i n c r e a s e d " s i g n a l " masked t h r e s h o l d above the f r e q u e n c y o f the masker, p a r t i c u l a r l y i n the r e g i o n i n which the n o t c h o c c u r s . In t h i s r e g i o n o f s i g n a l f r e q u e n c i e s , the n o i s e alone e i t h e r d i d not mask a t a l l , or i n s u f f i c i e n t l y -11-t o account f o r such an e f f e c t . From t h i s e v i d e n c e , Green-wood (1971b) i n f e r r e d t h a t i t i s the masking o f the combi-n a t i o n tones C n (with f r e q u e n c i e s ( n + l j f ^ n f g — p a r t i c u l a r l y C^) by the low-pass n o i s e t h a t i s r e s p o n s i b l e f o r the t h r e s h o l d e l e v a t i o n on the h i g h - f r e q u e n c y s i d e o f the masked audiograms. In other words, i t has been concluded t h a t i t i s ( a t 2 ^ . ^ , w i t h masker a t f 1 and s i g n a l a t f g ) , r a t h e r than the s i g n a l per se which i s d e t e c t e d a t masked t h r e s h o l d , as measured i n the f r e q u e n c y r e g i o n immediately above the masker, where a notch o c c u r s . That i t i s a c t u a l l y the c u b i c d i f f e r e n c e tone, 2 f ^ - f g ( o r C ^ ) , and not the q u a d r a t i c d i f f e r e n c e tone, f„-f^ (or D,), t h a t i s d e t e c t e d i n t h i s case i s supported by p r i o r f i n d i n g s o f Zwicker (1955) and G o l d s t e i n (1967). They demonstrate t h a t o n l y i s generated a t r e l a t i v e l y low l e v e l s o f the p r i m a r y tones ( p r o v i d i n g t h a t the l a t -t e r a r e c l o s e t o g e t h e r ) whereas D 1 i s not generated u n t i l much h i g h e r l e v e l s o f the p r i m a r i e s a r e reached. As the i n t e n s i t y o f the pure-tone masker i s i n c r e a s e d from moderate t o h i g h l e v e l s , the n o t c h ( i . e . the l o c a l minimum o f the masked audiogram between i t s main and s e c -ondary peaks) may move t o a h i g h e r f r e q u e n c y l o c u s as the l o c a t i o n o f the secondary peak a l s o moves t o a h i g h e r f r e -quency l o c u s (see F i g . l ) . A t these primary i n t e n s i t i e s , the p o s s i b i l i t y e x i s t s t h a t D- as w e l l as, or i n s t e a d o f , -12-may determine the masked t h r e s h o l d s t h a t a r e measured when the s i g n a l i s i n some p o r t i o n o f the f r e q u e n c y r e -g i o n between t h e main and secondary peaks o f the masked audiogram. S i n c e , a t low t o moderate l e v e l s o f the p r i -m a r i e s , the i n t e n s i t y o f drops q u i t e s h a r p l y as the fr e q u e n c y s e p a r a t i o n between masker and s i g n a l i s i n -c r e a s e d ( G o l d s t e i n , 1967; Greenwood, 1971b, 1972c), i t i s most p r o b a b l e t h a t , i f i s sometimes the combination tone t h a t i s d e t e c t e d a t masked t h r e s h o l d , then i t w i l l p l a y t h i s r o l e when the f r e q u e n c y o f the s i g n a l i s r e l a -t i v e l y removed from t h a t o f the masker—most p r o b a b l y i n the r e g i o n above the low p o i n t o f the not c h and below the ^secondary peak ,( see .Fig .1).. Purpose o f the Study and S t r a t e g i e s Used In order t o determine which combination tone, o r D^, i s a c t u a l l y d e t e c t e d when s i g n a l i n t e n s i t y i s a t masked t h r e s h o l d ( i n v a r i o u s f r e q u e n c y r e g i o n s on the hi g h - f r e q u e n c y s i d e o f pure-tone masked audiograms p r o -duced by h i g h - i n t e n s i t y maskers), the s t r a t e g y d e s c r i b e d i n t he f o l l o w i n g paragraph has been used. R e s u l t s o f t h i s k i n d a re i n t e n d e d t o a i d i n i n t e r p r e t i n g the shape o f the hi g h - f r e q u e n c y s i d e o f the pure-tone masked a u d i o -gram. A band-pass n o i s e , s t r a t e g i c a l l y p l a c e d below the masker, w i l l i t s e l f mask e i t h e r D- o r C., or b o t h , -13-depending on the f r e q u e n c y o f the s i g n a l . That i s t o say. the n o i s e w i l l do so when the s i g n a l f r e q u e n c y i s w i t h i n c e r t a i n l i m i t e d ranges o f f r e q u e n c i e s above the masking tone. I f o n l y the s t r o n g e r o f the two combina-t i o n tones i s masked by the n o i s e , when the s i g n a l i s near t h r e s h o l d , then the combination tone t h a t i s masked cannot p l a y the r o l e o f the d e t e c t e d tone a t masked t h r e s h -o l d . Consequently, i f the combination tone t h a t i s now masked by the band-pass n o i s e i s the tone t h a t i s n o r m a l l y d e t e c t e d a t masked t h r e s h o l d , i t i s e a s i l y seen t h a t the s i g n a l , i n order t o be d e t e c t a b l y p r e s e n t , w i l l have to be r a i s e d i n i n t e n s i t y u n t i l e i t h e r the ot h e r combination tone o r t h e s i g n a l i t s e l f becoses a u d i b l e , whichever oc-cu r s f i r s t . On the other hand, i f t h e masked t h r e s h o l d i s not r a i s e d by the band-pass masking n o i s e , then i t can be i n f e r r e d t h a t e i t h e r the ot h e r combination tone or the s i g n a l i t s e l f i s d e t e c t e d a t masked t h r e s h o l d i n the s i g n a l f r e q u e n c y range i n q u e s t i o n . To approach t h i s l a t t e r q u e s t i o n , the band o f n o i s e below the masker can then be moved (without changing s i g n a l f r equency) t o a p o s i t i o n a p p r o p r i a t e f o r masking the o t h e r combination component, i n o r d e r t o seek f u r t h e r i n f o r m a t i o n as t o i t s r o l e and t o a n a l y z e the t o t a l range o f p o s s i b i l i t i e s . I f n o i s e and s i g n a l f r e q u e n c i e s a r e such t h a t both C 3 and D- a r e masked a t the same time by the i n t r o d u c e d - 1 4 -n o i s e , i . e . when the s i g n a l i s w i t h i n a p a r t i c u l a r f r e q u e n cy range, an i n c r e a s e i n measured " s i g n a l " t h r e s h o l d w i l l i n d i c a t e t h a t e i t h e r o r D^, o r b o t h (with no d e c i s i o n between them p o s s i b l e ) a r e n o r m a l l y heard a t masked t h r e s h o l d . The i n c r e a s e d masked t h r e s h o l d t h a t r e s u l t s when bo t h and a r e masked s i m u l t a n e o u s l y w i l l c o r r e s p o n d t o a l e v e l a t which the s i g n a l p e r se i s d e t e c t e d i n s t e a d . T h e r e f o r e , when s i g n a l f r e q u e n c i e s a r e near 1.5 times f ^ , and and D 1 a r e cons e q u e n t l y too c l o s e t o g e t h e r t o be masked s e p a r a t e l y , i t i s i m p o s s i b l e t o d e c i d e whether i t i s o r D^, o r both, t h a t a r e b e i n g d e t e c t e d a t measured masked t h r e s h o l d . I t jLss, however, p o s s i b l e t o determine whether or not i t i s the s i g n a l i t s e l f t h a t i s d e t e c t e d i n t h i s c a se. When s i g n a l s a r e i n o t h e r f r e q u e n c y r e g i o n s on the hi g h - f r e q u e n c y s i d e o f the pure-tone masked a u d i o -gram, thus r e s u l t i n g i n more w i d e l y spaced and f r e -q u e n c i e s , both combination tones can, o f co u r s e , s t i l l be masked s i m u l t a n e o u s l y d e s p i t e t h e i r s e p a r a t i o n — e i t h e r (a) by u s i n g a wider band o f n o i s e or (b) by u s i n g two  narrow bands o f n o i s e a p p r o p r i a t e l y s i t u a t e d . The p l a n o f t h i s r e s e a r c h , then, begins w i t h one o f the l a t t e r two s i t u a t i o n s — t h e one i n which a wide ( e f f e c -t i v e l y low-pass) n o i s e i s s i t u a t e d below an i n t e n s e pure-tone masker. T h i s i s the e x p e r i m e n t a l c o n d i t i o n i n Greenwood's (1971b, F i g . 11, p„ 515) r e p o r t , t h a t -15-demonstrated t h a t the s i g n a l i t s e l f was not n o r m a l l y de-t e c t e d i n the e n t i r e n o t c h r e g i o n , but l e f t i t as an open q u e s t i o n , a t h i g h masker l e v e l s , as t o which combination t o n e — o r D 1 — - i s a c t u a l l y d e t e c t e d when the s i g n a l f r e -quency i s i n the not c h r e g i o n , or i n d i f f e r e n t r e g i o n s o f the n o t c h . The s i t u a t i o n i n which two narrow bands o f n o i s e a r e a p p r o p r i a t e l y p l a c e d below the masker t o mask and s i m u l t a n e o u s l y , p r o v i d e s an e s p e c i a l l y p o w e r f u l b a s i s f o r i n f e r e n c e , as the experiments o f t h i s paper w i l l d e s c r i b e and as can be o u t l i n e d b r i e f l y below. In t h i s case a l s o , an i n c r e a s e i n masked t h r e s h o l d , a f t e r mask-i n g and s i m u l t a n e o u s l y , w i l l c o rrespond t o a l e v e l a t which the s i g n a l i t s e l f i s heard. But i f t h i s l e v e l i s h i g h e r than the s i g n a l t h r e s h o l d measured when o n l y  one o f the combination tones i s masked by one o f the two n o i s e s , then i t i s the unmasked combination tone t h a t i s b e i n g d e t e c t e d a f t e r the s e l e c t i v e masking, and the masked t h r e s h o l d o b t a i n e d i n t h i s case w i l l c o rrespond t o the l e v e l o f the s i g n a l r e q u i r e d t o make the unmasked combi-n a t i o n tone a u d i b l e . F o r example, i f masking e i t h e r o r s e l e c t i v e l y causes no i n c r e a s e i n masked t h r e s h o l d i n t h e s p e c i f i c s i g n a l f r e q u e n c y r e g i o n under c o n s i d e r a -t i o n , whereas masking bo t h causes a marked i n c r e a s e i n t h r e s h o l d i n the same r e g i o n , then i t f o l l o w s t h a t both -16-and are of about the same i n t e n s i t y at the masked threshold as i t i s normally measured. With the exception of those s i g n a l frequencies that are approximately 1.5 times f ^ (the masker frequency), i n which case masking s e l e c t i v e l y i s precluded, a great deal of information may be obtained by s e l e c t i v e l y masking and D^. The data w i l l reveal not only whether the D l ' ^1* o r k ° t h a r e detected at masked threshold when the s i g n a l l i e s on the high-frequency side of a high-i n t e n s i t y pure-tone masker, but they w i l l also i n d i c a t e the i n d i v i d u a l s i g n a l l e v e l s f o r which C^ > D^, and the s i g n a l i t s e l f become audible. B a s i c a l l y , then, the f o l -lowing experiments c o n s t i t u t e a f u r t h e r e x t e n s i o n o f the attempts by Greenwood (1971b) to determine the "events" that are a c t u a l l y detected at masked thresholds i n the pure-tone masked audiogram, and the l e v e l s at which the s i g n a l i t s e l f , and D-, become audible. - 1 7 ; MBTHCOS AND APPARATUS General Method A modified von Bekesy tracking method was used. It was modified i n that thresholds were obtained at f i x e d and c l o s e l y spaced frequencies of the pure-tone s i g n a l . This method has advantages over the employment of a continuous sweep of the frequency range. Thresh-olds may be obtained both r a p i d l y and with reduced v a r i -a b i l i t y , since the subject i s not required to contend with rapid changes of threshold that accompany changes of frequency. For only one subject and one aspect of the experiment was there a departure from t h i s method. A very slow ( 2 0 Hs/iain.) l i n e a r sweep was used f o r one e s p e c i a l l y r e l i a b l e subject (the experimenter, FK) i n the preliminary mapping of masked thresholds i n the region of the s e l e c t i v e masking noise which was l a t e r used to mask combination tones i n the experiments proper (see discussion of strategy i n the Introduction). Sweeps i n the opposite d i r e c t i o n , from high to low frequencies, and comparison of thresholds with those obtained from fixed-frequency measurements confirmed the r e l i a b i l i t y of the measurements obtained i n t h i s s i n g l e case i n which t h i s method was used. In the experiment proper, the method of fixed-frequency signals was used exclusive-l y . -18 Stimuli and Controls The s i g n a l was a s e r i e s of tone bursts repeated at a rate of 3 times per second. Signal duration was 165 msec, with onset and decay periods of 30 msec, to e l i m i -nate detectable t r a n s i e n t s . Signal pulses began at le a s t 300 msec, a f t e r the onset of the continuous masker. The s i g n a l was continuously monitored by an o s c i l l o s c o p e . The maskers consisted of a band of noise, a pure tone, or a pure tone presented simultaneously with one or two bands of noise. The maskers remained at the same f r e -quency and i n t e n s i t y throughout the experiment. Signal l e v e l was varied by the subject i n d i s c r e t e 2-dB steps. Signal frequencies were separated by small d i s c r e t e i n -t e r v a l s , and presented consecutively from low to high frequencies. Both masker and si g n a l were presented mon-aural ly« A l l s t i m u l i were presented i n s i d e an IAC double-walled sound-proof room. To map the high - i n t e n s i t y , pure-tone masked audio-grams of t h i s study, i t i s p a r t i c u l a r l y important to determine thresholds at successively adjacent frequen-c i e s i n order to minimize the lapse of time between determinations of threshold i n the same part of the masked audiogram. Minimizing the time i n t e r v a l between adjacent frequencies i s advisable i n order to minimize 2 the l i k e l i h o o d of any small changes i n sensation l e v e l , -19-2 since small changes i n the sensation l e v e l (dB SL) of the masker r e s u l t i n large changes i n the masked thresh-o l d on the high-frequency side of the masking pattern* 2 To c o n t r o l f o r any small changes i n masker SL > "tone alone" and "tone plus noise" conditions were alternated at each s i g n a l frequency used during the experimental session. This a l t e r n a t i o n of conditions was intended to prevent any change i n threshold, over time, from con-founding the change i n threshold due to the introduction of the noise, which i s the c r i t i c a l v ariable i n the following experiments (see discussion of experimental strategy i n the Introduction). R e l i a b i l i t y of thresholds was ascertained by i n t r o -ducing, without the subject's knowledge, checks of his performance. Such checks consisted of r a i s i n g or lower-ing the i n t e n s i t y of the s i g n a l during a t r i a l , removing the s i g n a l , or repeating the t r i a l ; and observing the subject's corresponding compensatory response. Subjects Five subjects, including the author (FK), p a r t i c i -pated i n the study. A l l were i n t h e i r early twenties. 2 Sensation Lev e l , abbreviated SL, indicates the number of decibels (dB SL) a sound i s above the threshold of hearing at that frequency (Stevens and Davis, 1938). -20-Subjects were selected f o r normal quiet threshold curves without marked i r r e g u l a r i t i e s i n the frequency range i n which thresholds were obtained. They were also chosen f o r a b i l i t y to perform stably and r e l i a b l y throughout an experimental s e s s i o n c Subjects were f a m i l i a r i z e d with the task of tracking thresholds during at l e a s t f i v e , one-hour sessions. At the end of t h i s period of fam-i l i a r i z a t i o n and assessment of the subject's r e l i a b i l i t y , e i t h e r data-gathering began, or the subject was dismissed. Subject RB, from whom the most complete data was obtained, has had about one year's experience i n the threshold-tracking task before serving i n the present study. Apparatus The masking tone was generated by a Hewlett-Packard (Model 203-A) o s c i l l a t o r . A General Radio o s c i l l a t o r (Model 1309-A) generated the s i g n a l . Signal pulses were produced by an e l e c t r o n i c switch and gate. Another switch, c o n t r o l l e d by the subject during threshold determinations, was located inside the IAC double-walled sound-proof room. This switch was used to c o n t r o l an attenuator and a Hewlett-Packard (Moseley 7035AM) X-Y recorder (see F i g . 3 ) . Pushing the lever on the switch i n one d i r e c t i o n lowered the si g n a l l e v e l by 2 dB, and pushing the lever i n the opposite d i r e c t i o n r a i s e d the s i g n a l l e v e l by 2 dB. Simultaneously, when the subject -21-SIGNAL OSC. AMPLI-FIER SIGNAL ATTEN. BAND-PASS FILTER ~*° "B — 1 CARRIER BALANCED LOW-PASS NOISE OSCILLATOR MODULATOR FILTER©) GEN. F i g . 3. Block Diagram of the apparatus -22-1owered the s i g n a l l e v e l by 2 dB, the pen on the X-Y r e c o r d e r moved downward and then t o t h e r i g h t (about 1 mm.). S i m i l a r l y , each time the s u b j e c t r a i s e d the s i g n a l l e v e l 2 dB, the pen moved upward 4 mm. and then a g a i n f o r w a r d . G e n e r a t i o n o f n o i s e bands N o i s e bands were generated so as t o produce masking e f f e c t s t h a t a r e as l o c a l i z e d as p o s s i b l e . S p e c i f i c a l l y , they were produced w i t h s t e e p s k i r t s and a t the minimum l e v e l s u f f i c i e n t t o the purpose. The l e v e l s o f the n o i s e s were chosen so as t o mask tones a t l e v e l s below 40 dB SPL. T h i s l e v e l was a r r i v e d a t as being t h e lowest .which .would.ensure masking, of .any .combination tone generated by masker and s i g n a l , f a l l i n g w i t h i n the n o i s e . S i n g l e narrow bands o f n o i s e , 50 Hz i n w i d t h , were o b t a i n e d from a G e n e r a l Radio wave a n a l y z e r (1900-A), used as a f i l t e r . Where two 50-Hz bands o f n o i s e were used s i m u l t a n e o u s l y , the second was the f i l t e r e d output o f a b a l a n c e d modulator w i t h a c a r r i e r i n p u t a t the de-s i r e d c e n t e r f r e q u e n c y o f the output band and a modula-t i n g i n p u t c o n s i s t i n g o f a 25-Hz low-pass n o i s e from an e l e c t r o n i c f i l t e r . S i n c e steep s l o p e s were n e c e s s a r y f o r the wider (than 50-Hz bandwidth), band-pass n o i s e s , the n o i s e f e d t o the modulating i n p u t of the b a l a n c e d modulator was put through two low-pass f i l t e r s , j o i n t l y -23-p r o d u c i n g a s t e e p , 54 dB/octave s l o p e o f the low-pass n o i s e a t the i n p u t s t a g e . The output was s t e e p e r t o an e x t e n t dependent on f r e q u e n c y l o c a t i o n , and was a l s o f i l t e r e d t o remove c a r r i e r harmonics. The bandwidths o f the wider (than 50 Hz) n o i s e s , as g i v e n i n t h i s paper, a r e s p e c i f i e d a t n i n e , r a t h e r than the customary t h r e e , d e c i b e l s down. C u s t o m a r i l y , o n l y one f i l t e r i s used t o generate a band o f n o i s e . In t h i s case, the c u t - o f f f r e q u e n c y o f the f i l t e r s p e c i f i e s the edge o f the n o i s e a t 3 dB down. S i n c e , however, t h r e e f i l t e r s , s e t a t the same c u t - o f f f r e q u e n c i e s , were used, and each f i l t e r low-e r e d the edge by t h r e e d e c i b e l s , the edges o f the wider n o i s e s , a s s p e c i f i e d .by the cut o f f f r e q u e n c i e s of the f i l t e r s , a r e n i n e d e c i b e l s down. For example, the n o i s e o f 350-Hz bandwidth, as s p e c i f i e d by the c u t - o f f s e t -t i n g s o f the t h r e e f i l t e r , has a bandwidth of 300 Hz a c c o r d i n g t o the c o n v e n t i o n a l 3-dB down c u t - o f f p o i n t s ; the 600-Hz wide band, used here, has a c o n v e n t i o n a l w i d t h of 500-550 Hz (becoming narrower a t h i g h e r f r e q u e n c y ) ; and the 1400-Hz wide band o f n o i s e used i n t h i s study i s c o n v e n t i o n a l l y o n l y 1100 Hz wide. C a l i b r a t i o n Response o f the earphones (TDH-49) was f l a t up t o 3600 Hz. Measurements o f t h r e s h o l d s o b t a i n e d above 3600 Hz were c o r r e c t e d f o r the reduced response of the earphones. -24-S t i m u l i were c a l i b r a t e d both e l e c t r i c a l l y , by measuring the v o l t a g e a c r o s s the earphones w i t h a B a l l e n t i n e t r u e -r.m.s, v o l t m e t e r (Model 320), and a c o u s t i c a l l y , i n a 3 6-cm c o u p l e r w i t h a G e r e r a l Radio 1590 P5 microphone. N o i s e s produced w i t h the ba l a n c e d modulator were measured a c o u s t i c a l l y by s u b s t i t u t i n g an o s c i l l a t o r f o r the n o i s e g e n e r a t o r and measuring the output o f the b a l a n c e d modu-l a t o r w i t h the wave a n a l y z e r . C e r t a i n i n t e r m o d u l a t i o n d i s t o r t i o n p r o d u c t s have the same f r e q u e n c i e s as the combination tones s t u d i e d i n t h i s paper. When two tones a t l e v e l s o f 80 t o 90 dB SPL were measured a c o u s t i c a l l y , a l l i n t e r m o d u l a t i o n d i s t o r t i o n p r o d u c t s were more than 70 dB down. When one o f the tones i s lowered by as l i t t l e as 10 dB (as i n a masking e x p e r i -ment) , the d i s t o r t i o n components become unmeasurable. -25-C h r o n o l o q i c a l D e s c r i p t i o n o f an E x p e r i m e n t a l S e s s i o n The experimental s e s s i o n s were conducted d a i l y and l a s t e d about one hour, one to t h r e e minutes b e i n g r e q u i r e d t o measure each t h r e s h o l d . During each s e s s i o n , the s i g -n a l and maskers were s e t and monitored, u s i n g a true-r.m.s. v o l t m e t e r t o measure i n t e n s i t i e s and an e l e c t r o n i c coun-t e r t o measure and a d j u s t the f r e q u e n c i e s o f the s i g n a l , masker, and the c a r r i e r or c e n t e r f r e q u e n c y o f the n o i s e s . A f t e r c a l i b r a t i o n , the s t i m u l i were a d j u s t e d t o the d e s i r e d l e v e l s by means o f a t t e n u a t o r s . The s u b j e c t was s e a t e d i n a c o m f o r t a b l e , but unpadded, wooden c h a i r i n s i d e an IAC d o u b l e - w a l l e d soundproof room.. The sub-j e c t h e l d a ^switch - c o n t r o l l i n g the r e c o r d i n g •attenuator. T h i s s w i t c h was used t o v a r y the l e v e l o f the s i g n a l i n d i s c r e t e 2-dB s t e p s . The c i r c u m a u r a l CZW-6 socket (Zwis-l o c k i , 1955) o f the earphone was p o s i t i o n e d e v e n l y and t i g h t l y around the s u b j e c t ' s e a r . (Only one phone d e l i -v e r e d s t i m u l i . ) Wires l e a d i n g from the earphone were suspended behind the s u b j e c t ' s head i n order t o a v o i d n o i s e caused by t h e i r r u b b i n g a g a i n s t the s u b j e c t ' s c l o t h i n g . The s u b j e c t was i n s t r u c t e d t o respond to any sound or event ( i . e . to d e t e c t "anything") t h a t o c c u r s w i t h the r e p e t i t i o n r a t e o f the s i g n a l (3 times per s e c o n d ) . The s u b j e c t ' s response c o n s i s t e d o f l o w e r i n g the l e v e l of 26 the s i g n a l when he c o u l d hear a r e p e a t i n g sound, and r a i -s i n g the l e v e l o f the s i g n a l when he c o u l d n o t , 2 dB per d e c i s i o n . The s u b j e c t ' s cue t o b e g i n responding was the onset o f a sound r e p e a t i n g a t the r a t e o f 3 times per second. H i s cue t o s t o p responding was e i t h e r the t e r m i -n a t i o n o f the masker, o r , when q u i e t t h r e s h o l d s were mea-su r e d , an i n c r e a s e i n the i n t e n s i t y o f the s i g n a l f o l l o w e d by i t s t e r m i n a t i o n . The l e v e l o f the s i g n a l was f i r s t a d j u s t e d t o about 10 dB above q u i e t t h r e s h o l d and the s i g n a l t u r n e d on. The s u b j e c t lowered the l e v e l o f the s i g n a l t o t h r e s h o l d and then r e p e a t e d l y b r a c k e t e d , o r t r a c k e d , h i s t h r e s h o l d over time by a d j u s t i n g the l e v e l o f the s i g n a l u n t i l he c o u l d not hear i t , then r a i s i n g i t s l e v e l u n t i l he c o u l d hear i t , e t c . When the a u t o m a t i c a l l y r e c o r d e d l e v e l s were s t a b l e , the s i g n a l l e v e l was r a i s e d by the experimenter and the s i g n a l t u r n e d o f f . The range o f the s u b j e c t ' s adjustments i s about 4 dB on the average, and o n l y r a r e l y g r e a t e r than 6 dB. A f t e r each t r i a l , a v i s u a l average o f the r e c o r d i s immediately r e c o r d e d on a graph. Such v i s u a l averages a r e v e r y r e -l i a b l e , d e t e r m i n a t i o n s o f such averages by d i f f e r e n t ex-per i m e n t e r s b e i n g w i t h i n % dB. A f t e r each t r i a l , the f r e q u e n c y o f the s i g n a l was changed and a d j u s t e d t o the d e s i r e d f r e q u e n c y w i t h an -27-a c c u r a c y o f *1 Hz. A f t e r one or more t h r e s h o l d s ( i n c l u -d i n g t h a t o f a tone a t the masker frequency) a r e o b t a i n e d i n the q u i e t , t h r e s h o l d s i n the presence o f one or more maskers a r e determined. When d e t e r m i n i n g t h r e s h o l d s i n the presence o f a masker, the masker i s turned on f i r s t , b e f o r e the s i g n a l . A f t e r a t h r e s h o l d i s o b t a i n e d , the s i g n a l i s t u r n e d o f f f i r s t , b e f o r e the masker. -28 RESULTS The r e s u l t s o f t h i s study w i l l be c o n s i d e r e d under two main headings: (a) masking o f and s e l e c t i v e l y , and (b) masking o f and D 1 s i m u l t a n e o u s l y . Greenwood (1971b, F i g . 1 1 ) , when masking and s i m u l t a n e o u s l y (and over almost the whole fr e q u e n c y range i n which they e x i s t ) , showed t h a t the d i p i n the masking p a t t e r n b e t -ween the main and secondary peak o f the pure-tone masked audiogram i s consequently completely e l i m i n a t e d (see F i g . 2). That i s , when d e t e c t i o n o f and i s . p r e c l u d e d and the masked t h r e s h o l d s one measures a r e consequently those a t which the s i g n a l i t s e l f i s d e t e c -t e d , no notch i s e v i d e n t i n the masked audiogram, the t r a n s i t i o n from main t o secondary peaks b e i n g smooth. S i n c e t h i s e x p e r i m e n t a l f a c t has been demonstrated q u i t e c l e a r l y , i t was not c o n s i d e r e d n e c e s s a r y t o r e p l i c a t e i t f o r e v e r y s u b j e c t i n the p r e s e n t s t u d y . N e v e r t h e l e s s , i t i s important t o keep t h i s f i n d i n g i n mind d u r i n g an exa-m i n a t i o n o f the f o l l o w i n g f i n d i n g s on s e l e c t i v e masking, s i n c e the band-pass n o i s e i n t r o d u c e d below the masker o f t e n has o n l y a s m a l l e f f e c t i n terms o f e l e v a t i n g s i g -n a l t h r e s h o l d s above the pure-tone masker. A s m a l l i n -cr e a s e i n t h r e s h o l d w i t h the i n t r o d u c t i o n o f the n o i s e does not n e c e s s a r i l y i n d i c a t e t h at the p a r t i c u l a r combi-n a t i o n tone which i s masked by the n o i s e i s any the l e s s - 2 9 c r i t i c a l i n d e t e r m i n i n g masked t h r e s h o l d on the h i g h -f r e q u e n c y s i d e of the masking tone. A s m a l l e f f e c t on t h r e s h o l d s a f t e r the a d d i t i o n o f a s e l e c t i v e - m a s k i n g n o i s e simply i n d i c a t e s t h a t the i n t e n s i t i e s o f D^ ^ and a r e more n e a r l y comparable than i n a case where the t h r e s h o l d i n c r e a s e a f t e r s e l e c t i v e masking i s l a r g e . I n o t h e r words, any one f i n d i n g cannot be c o n s i d e r e d i n i s o l a t i o n , but must be c o n s i d e r e d i n c o n j u n c t i o n w i t h r e s u l t s o f o t h e r , complementary e x p e r i m e n t a l s i t u a t i o n s . In c o n s i d e r i n g the f o l l o w i n g f i n d i n g s , another ex-p e r i m e n t a l f a c t must be kept i n mind; namely, the v a r i a -t i o n i n the shape o f the h i g h - f r e q u e n c y s i d e o f the pure-£one «masked ^ auddLogzam w i t h .small changes i n masker. ,j_n.ten-s i t y (see F i g . 1; and Wegel and Lane, 1924, F i g . 2 ) . Fxom t h i s expeximental f a c t we know t h a t . s m a l l changes from s e s s i o n t o s e s s i o n , i n the " e f f e c t i v e " i n t e n s i t y o f the tone masker a t the ear w i l l x e s u l t i n r e l a t i v e l y laxge changes i n the shape o f the high-fxequency s i d e o f the masked audiogxam. Such changes i n " e f f e c t i v e " i n t e n -s i t y axe l i k e l y those due t o random v a r i a t i o n s o f the q u i e t t h r e s h o l d from day t o day due t o s m a l l c o n d u c t i v e changes i n the middle eax and other f a c t o x s such as v a x i -a t i o n , on d i f f e x e n t o c c a s i o n s , i n the p o s i t i o n o f the headphones w i t h xespect t o the eax (which a l t e x s i n t e n -s i t y l e v e l s a t the tympanic membxane). Although the -30-e l e c t r i c a l l y measured i n t e n s i t y of the tone as d e l i v e r e d  t o the earphone remained constant i n each experiment, s m a l l random v a r i a t i o n s i n the " e f f e c t i v e " i n t e n s i t y o f the tone may have a marked e f f e c t on masked t h r e s h o l d a t h i g h e r s i g n a l f r e q u e n c i e s o f h i g h - i n t e n s i t y pure-tone masked audiograms. T h i s v a r i a t i o n i n " e f f e c t i v e " ( a c t u -a l ) l e v e l s , and hence shape, reduces the e x a c t i t u d e o f d i r e c t comparisons o f audiograms o b t a i n e d d u r i n g d i f f e r -ent e x p e r i m e n t a l s e s s i o n s . A l s o , even more marked d i f -f e r e n c e s i n the shape o f the h i g h - f r e q u e n c y s i d e o f the masked audiogram can occur between s u b j e c t s , because o f d i f f e r e n c e s i n q u i e t t h r e s h o l d and l e v e l s o f the combi-n a t i o n toneSt, ^ among oth e r , f a c t o r s . D.uring the ,same exp e r i m e n t a l s e s s i o n , however, any confounding e f f e c t s o f changes i n the " e f f e c t i v e " i n t e n s i t y of the pure-tone masker w i t h r e s p e c t t o the c r i t i c a l v a r i a b l e under o b s e r -v a t i o n , were minimized as f a r as p o s s i b l e . To do so, "tone a l o n e " and "tone p l u s n o i s e " c o n d i t i o n s were present-ed on c o n s e c u t i v e t r i a l s f o r each s i g n a l f r e q u e n c y . Masking and S e l e c t i v e l y The d a t a on s e l e c t i v e masking i s c o n s i d e r e d i n two s e c t i o n s : (1) moderately i n t e n s e m a s k e r s — t h e case i n which the secondary peak o f the masked audiogram i s near or below 1.5 times the masker f r e q u e n c y (f^)» t h i s peak - 3 1 -s h i f t s toward h i g h e r f r e q u e n c i e s as masker i n t e n s i t y i n c r e a s e s ; (2) more i n t e n s e m a s k e r s — t h e case i n which the secondary peak o f the masked audiogram has p r o g r e s s e d above 1.5 times f ^ s u f f i c i e n t l y t o permit s e l e c t i v e mask-i n g o f or when s i g n a l f r e q u e n c i e s are p r e s e n t e d t h a t l i e i n the i n t e r v a l between 1.5 times and the secondary peak. and D^ , cannot be s e l e c t i v e l y masked when s i g n a l s a r e near 1.5 times f ^ , s i n c e the two combi-n a t i o n tones c o i n c i d e when s i g n a l and masker a r e a t t h i s f r e q u e n c y r a t i o (If f 2=1.5f^ C1=2f^^-1 „5f 1=1.5f 1 ~ f 1=D 1) „ Moderately i n t e n s e maskers (secondary peak near or below 1.5f^) A t moderately h i g h i n t e n s i t i e s (about 70 t o 80 dB SL) o f a pure-tone masker, the secondary peak o f the masked audiogram i s l o c a t e d near or below 1.5 times f ^ . When i s s e l e c t i v e l y masked f o r s i g n a l f r e q u e n c i e s between the main and secondary peak o f the masking p a t t e r n , the masked t h r e s h o l d a t these s i g n a l f r e q u e n c i e s i n c r e a s e s c o n s i d e r a b l y (see F i g . 4 ) . The l e v e l o f the new masked t h r e s h o l d i s t h a t a t which e i t h e r the s i g n a l , or another combination tone (most l i k e l y D^), becomes a u d i b l e . The i n c r e a s e i n masked t h r e s h o l d , a f t e r the a d d i t i o n o f the n o i s e , o c c u r s a t s i g n a l f r e q u e n c i e s between the main and secondary peak of the masked audiogram. A l s o , because o f the magnitude o f the o b t a i n e d t h r e s h o l d e l e v a t i o n , one may i n f e r t h a t the l e v e l s o f D- and any other combination t o f a c e page 32 F i g . 4. Masked t h r e s h o l d s above the pure-tone masker ( f ^ 1.3 kHz, top and middle; f 1 = 2 kHz, bottom) b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a band o f n o i s e j u s t below t h e tone masker ( f . ^ ) . Squares r e p r e s e n t t h r e s h o l d s i n the n o i s e a l o n e . As can be seen from the schematic a t the top l e f t o f the F i g u r e , the f r e q u e n c y l o c i o f both and change s y s t e m a t i c a l l y as the f r e q u e n c y o f the s i g n a l ( f 2 ) i s changed. For example, i n the t o p graph, s i g n a l s a t f r e q u e n c y (1) p l a c e on t h e s k i r t o f the n o i s e , and s i g n a l s a t (2) generate a f a l l i n g i n the c e n t e r o f the n o i s e . In both cases f a l l s o u t s i d e the i n f l u e n c e o f the n o i s e . The same i n f o r m a t i o n i s more c o m p l e t e l y determined from the curves a t the bottom r i g h t - h a n d c o r n e r o f each graph. These curves d i s p l a y the masked t h r e s h o l d s (squares) i n the n o i s e , which i s l o c a t e d below f ^ . Hence they show the masking e f f e c t o f the n o i s e a t those f r e q u e n c i e s where ( s o l i d l i n e ) o r (dashed l i n e ) w i l l f a l l when s i g n a l s a r e p r e -s e n t e d i n the f r e q u e n c y range o f the n o t c h to which these curves have been s h i f t e d . Thus, f o r a g i v e n s i g n a l f r e -quency, the s h i f t e d curves show to what ext e n t C^, D^, or both w i l l be s u b j e c t t o the masking e f f e c t of the n o i s e below the masker. (The apparent narrowing i n the shape o f these curves i s due s o l e l y t o the l o g a r i t h m i c c o n t r a c t i o n on the f r e q u e n c y a x i s ) . The n o i s e i s p l a c e d so t h a t s i g n a l t o f a c e page 32 f r e q u e n c i e s between the main and secondary peaks o f the masked audiogram produce a which f a l l s i n the n o i s e . S i g n a l s i n t h i s same fr e q u e n c y range produce a D 1 f a l l i n g o u t s i d e the i n f l u e n c e o f the n o i s e . The masked t h r e s h o l d s o f s i g n a l s p r e s e n t e d i n the "tone p l u s n o i s e " c o n d i t i o n (open c i r c l e s ) show t h a t the n o t c h i s e l i m i n a t e d by the n o i s e , which masks , but not D 1, when s i g n a l f r e q u e n c i e s a r e below l . S f ^ . The low-frequency s l o p e o f the masked audiogram ( ) i s measured i n the p r e s e n c e of the tone a l o n e . L e v e l of. the n o i s e i s i n d i c a t e d on each graph i n terms of i t s t o t a l SPL (sound p r e s s u r e l e v e l ) . The l e v e l o f the'tone masker i s i n d i c a t e d ' i n "both d e c i b e l s sound p r e s s u r e l e v e l -(dB SPL) and d e c i b e l s s e n s a t i o n l e v e l (dB S L ) . D e c i b e l s s e n s a t i o n l e v e l a r e the number o f d e c i b e l s the tone i s above i t s q u i e t t h r e s h o l d . 3 2 -SIGNAL FREQUENCY IN KHZ _| p r^ —>—i 1 1—i—i—i—i ' r SIGNAL FREQUENCY IN KHZ 05 0.7 1.0 1.5 20 3.0 4.0 SIGNAL FREQUENCY IN KHZ FIGURE 4 t o f a c e page 33 F i g . 5. Masked t h r e s h o l d s above the tone masker ( f ^= 1.3 kHz, top; f^= 2 kHz, bottom)—before ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a band o f n o i s e j u s t below a fre q u e n c y h a l f t h a t o f the masker ( i . e . . 5 f ^ ) . I n t h e s e audiograms, the l o c a t i o n o f the secondary peak i s a t 1.5f^. The audiograms, when compared t o those a s s o c i a t e d w i t h a d i f f e r e n t l y s i t u a t e d n o i s e i n F i g u r e 4, do not show such e l e v a t e d t h r e s h o l d s on the hi g h - f r e q u e n c y s i d e o f the masker. The audiograms i n d i c a t e t h a t i s not r e l e v a n t i n d e t e r -mining masked t h r e s h o l d on t h e hi g h - f r e q u e n c y s i d e o f the pure-tone masked audiogram. The l e v e l s o f the tone masker a r e i n d i c a t e d i n both dB SPL ( d e c i b e l s sound p r e s s u r e l e v e l ) and dB SL ( d e c i b e l s s e n s a t i o n l e v e l ) . F or an e x p l a n a t i o n o f the curves and symbols, see F i g u r e 4 . -33 0.2 03 0.5 07 1.0 15 2.0 SIGNAL FREQUENCY IN KHZ SIGNAL FREQUENCY IN KHZ FIGURE 5 34-tones, a r e c o n s i d e r a b l y below t h a t o f Ci» R e s u l t s s i m i l a r t o those i n F i g . 4 a r e r e v e a l e d i n audiograms (not shown here) o b t a i n e d from t h r e e o t h e r s u b j e c t s , when the stimu-l u s parameters were the same as those i l l u s t r a t e d a t the bottom o f F i g u r e 4. C o n v e r s e l y , when a n o i s e i s p l a c e d w i t h i n the f r e -quency r e g i o n s u r r o u n d i n g the f r e q u e n c y o f t h e which i s s i m u l t a n e o u s l y generated by the same s i g n a l s t h a t generate the which i s masked i n F i g u r e 4, no compa-r a b l e t h r e s h o l d e l e v a t i o n i s o b t a i n e d . Instances o f t h i s r e s u l t a r e shown i n F i g u r e 5. S i m i l a r r e s u l t s (not shown) were o b t a i n e d from t h r e e other s u b j e c t s . More Intense maskers (secondary peak above 1.5 f ^ ) A t l e v e l s of the pure-tone masker g r e a t e r than about 75 dB SL, the secondary peak of the masking p a t -t e r n o c c u r s above 1.5 times f ^ . A t these h i g h e r i n t e n -s i t i e s o f the pure-tone masker, the meaning o f the term "secondary peak", as i t i s used here, i s not as obvious as i t i s a t lower l e v e l s o f the masker, s i n c e o t h e r peaks may be seen i n the masking p a t t e r n a t frequency l o c i lower than t h a t o f the secondary peak, and, a t t i m e s , even a t h i g h e r f r e q u e n c i e s . T h e r e f o r e , the "secondary peak" must be more s p e c i f i c a l l y d e f i n e d as the n e a r e s t peak below 2 times f ^ . The d a t a i n d i c a t e t h a t the secon-dary peak, so d e f i n e d , marks the frequency a t and above -35-which the s i g n a l i t s e l f c o n s t i t u t e s the "event" d e t e c t e d a t masked t h r e s h o l d . A t these h i g h e r masker i n t e n s i t i e s , s i g n a l f r e q u e n -c i e s j u s t below the secondary peak but s u f f i c i e n t l y above 1.5f^, produce C 1 and components t h a t a r e s u f f i c i e n t l y s e p a r a t e d t o a l l o w t h e i r s e l e c t i v e masking, and an examina'tron o f t h e i r r e s p e c t i v e r o l e s . Masked t h r e s h o l d s between and 1.5f^ D i f f e r e n c e s i n the e f f e c t of the s e l e c t i v e masking n o i s e , as compared t o i t s e f f e c t on pure-tone masked audiograms o b t a i n e d a t lower i n t e n s i t i e s , a r e seen i h the s i g n a l f r e q u e n c y r e g i o n between and 1.5 times That i s t o say , w i t h the e x c e p t i o n of the d a t a from s u b j e c t TT ( F i g . 6 ) , which was o b t a i n e d a t a lower masker f r e q u e n c y (1.3 kHz), F i g u r e s 7 through 12 i l l u s t r a t e t h a t the s e l e c -t i v e masking o f C 1 produces s m a l l e r t h r e s h o l d e l e v a t i o n s , r e l a t i v e t o the depth of the n o t c h , a t the h i g h e r l e v e l s , t h a n a t t h e lower l e v e l s , o f the tone masker. That i s , s e l e c t i v e masking o f r a i s e s measured t h r e s h o l d by a s m a l l e r amount s i n c e , as s i g n a l l e v e l i s r a i s e d , the unmasked reaches t h r e s h o l d b e f o r e the t h r e s h o l d of the s i g n a l per se i s reached. At lower masker l e v e l s , the r e v e r s e was t r u e . Such l e s s e r t h r e s h o l d e l e v a t i o n s , then, r e p r e s e n t the l e v e l s o f the s i g n a l f o r which i s a t t h r e s h o l d below the tone masker. When the n o i s e i s p l a c e d i n the r e g i o n t o f a c e page 36 Fig» 6» Masked t h r e s h o l d s above the tone masker (f^=1.3kHz) b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a band o f n o i s e , e i t h e r j u s t below f ^ ( t o p ) , or_ j u s t below .5f^ (bottom) f o r s u b j e c t XT. Top: T h i s n o i s e masks C 1 when s i g n a l f r e q u e n c i e s a r e below 1950 Hz ( 1 . 5 f 1 ) , and i t masks D 1 when s i g n a l f r e q u e n c i e s are above-1950 Hz. Bottom; T h i s n o i s e masks when s i g n a l f r e q u e n c i e s a r e below 1950 Hz ( 1 . 5 f ^ ) , and i t masks when s i g n a l f r e -q u encies a r e above 1950 Hz. Masked t h r e s h o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n c i e s t h a t p l a c e i n the n o i s e . That i s , the n o t c h i s e l i m i n a t e d when i s s e l e c t i v e l y masked. The l e v e l o f the tone masker i s i n d i c a t e d i n bot h d e c i b e l s -sound p r e s s u r e l e v e l (dB SPL) and d e c i b e l s sensa-t i o n l e v e l (dB S L ) . F o r an e x p l a n a t i o n o f the curves and symbols, see F i g . 4. -36 0.5 0.7 1.0 1.5 SIGNAL FREQUENCY IN KHZ FIGURE 6 to f a c e page 37 F i g . 7. Masked t h r e s h o l d s above the tone masker ( f 1 = 2 kHz) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a band o f n o i s e , e i t h e r j u s t below f 1 ( t o p ) , o r j u s t below .5f^ (bottom) f o r s u b j e c t KL. Top: T h i s n o i s e masks when s i g n a l f r e q u e n c i e s a r e below 3 kHz ( 1 . 5 f ^ ) , and i t masks D 1 when s i g n a l f r e q u e n c i e s a r e above 3 kHz. Bottom; T h i s n o i s e masks when s i g n a l f r e q u e n c i e s a re below 3 kHz, and i t masks when s i g n a l f r e q u e n c i e s a r e above 3 kHz. T h r e s h o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n c i e s below 3 kHz (1.5f^) t h a t p l a c e i n the n o i s e . The l e v e l o f the tone masker i s i n d i c a t e d i n both dB SPL and dB SL. F o r an e x p l a n a t i o n o f the curves and symbols, see F i g . 4. FIGURE 7 t o f a c e page 38 F i g . 8. Masked t h r e s h o l d s above the tone masker ( f ^ = 2 kHz) - — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a band o f n o i s e , e i t h e r j u s t below ( t o p ) , ojr j u s t be-low . 5 ^ (bottom) f o r s u b j e c t FK. Top: T h i s n o i s e masks when s i g n a l f r e q u e n c i e s a r e below 3 kHz ( 1 . 5 f ^ ) , and i t masks when s i g n a l f r e q u e n c i e s a r e above 3 kHz. Bottom: T h i s n o i s e masks when s i g n a l f r e q u e n c i e s a r e below 3 kHz, and i t masks when s i g n a l f r e q u e n c i e s a r e above 3 kHz. Masked t h r e s h o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n c i e s below 3 kHz ( l . S f ^ ) t h a t p l a c e C 1 i n the n o i s e , and a t s i g n a l . f r e q u e n c i e s above 3 kHz t h a t p l a c e i n the n o i s e . Masked t h r e s h o l d s i n the n o i s e a l o n e ( M i — r ) were measured u s i n g a v e r y slow sweep method (see Methods). T h i s method was used o n l y f o r t h i s s u b j e c t , FK, and o n l y f o r the " n o i s e a l o n e " c o n d i t i o n . The l e v e l o f the 90 dB SPL tone masker i s a l s o i n d i c a t e d i n dB SL. For an e x p l a n a t i o n o f the curves and symbols, see F i g . 4, -38-FIGURE 8 -39 where f a l l s when s i g n a l f r e q u e n c i e s a r e below l,5f^ (see bottom o f F i g s . 6 t o 12), no t h r e s h o l d e l e v a t i o n i s observed. T h e r e f o r e , as i s the case f o r the l e s s i n t e n s e maskers, i t i s C^, and not D^t t h a t determines the measured masked t h r e s h o l d when s i g n a l s l i e i n the fr e q u e n c y r e g i o n below 1 . 5 f 1 and above f ^ . Masked t h r e s h o l d s between 1 . 5 ^ and the secondary peak -From F i g u r e s 6 t o 12 i t can be seen t h a t , as one o f the combination tones, o r i s s u b j e c t t o mask-i n g by a g i v e n band o f n o i s e when s i g n a l f r e q u e n c i e s a r e g r e a t e r than 1 , 5 f 1 , i t i s n e c e s s a r i l y the oth e r o f these two combination tones t h a t i s masked s e l e c t i v e l y by the same n o i s e when s i g n a l f r e q u e n c i e s a r e below 1.5f^. That i s t o say, the and D 1 i n t e r c h a n g e p l a c e s when the s i g n a l i s s h i f t e d as d e s c r i b e d . When s i g n a l f r e q u e n c i e s a r e g r e a t e r than 1.5f^, and e i t h e r o r D 1 (depending on the freque n c y l o c a t i o n o f the n o i s e ) i s s e l e c t i v e l y masked, i t can be seen from F i g u r e s 6 t o 12 t h a t masked t h r e s h o l d e l e v a t i o n s o f these s i g n a l s a r e g e n e r a l l y s m a l l and comparable, o r absent under both e x p e r i m e n t a l c o n d i t i o n s . That s m a l l t h r e s h o l d e l e v a t i o n s a r e e v i d e n t i n the same s i g n a l f r e q u e n c y r e g i o n , when C^> as w e l l as when D^, i s s e l e c t i v e l y masked, may be accounted f o r as f o l -lows. L e s s than p e r f e c t l y s e l e c t i v e masking o f the -40-i n t e n d e d d i f f e r e n c e tone i s o b t a i n e d when the medium-w i d t h (350- or 600-Hz) bands o f n o i s e invade, t o some exte n t , t h e f r e q u e n c y range i n which the o t h e r d i f f e r e n c e tone i s l o c a t e d . The i n t r u s i o n o f the s k i r t s o f the s e l e c t i v e - m a s k i n g n o i s e on a f r e q u e n c y range i n which the o t h e r "unmasked" combination tone i s l o c a t e d , w i l l t end t o produce s m a l l i n c r e a s e s i n " s i g n a l " t h r e s h o l d , due t o the masking o f the other combination tone. T h i s i s t he case when s i g n a l f r e q u e n c i e s a r e near 1.5f^. Then, D^, i f p r e s e n t , i s near i n frequ e n c y , and both and may f a l l on the s k i r t o f the n o i s e and be s u b j e c t t o masking a t the same time. Consequently, the s k i r t o f the n o i s e may, i n a d d i t i o n , i n t e r f e r e w i t h any summation or b e a t i n g i n t e r a c t i o n t h a t o c c u r s , and can n o r m a l l y be de-t e c t e d between C^ and D^. Summation, f o r example, has been demonstrated by G a s s i e r (1954), who showed t h a t tones l e s s than a c r i t i c a l bandwidth a p a r t summate t o reduce t h r e s h o l d . The p e r c e p t i o n o f be a t s between C^ and D^, when they a r e c l o s e t o g e t h e r , may a l s o be h i n -d e r e d by the pres e n c e o f the n o i s e and consequently l e a d t o t h r e s h o l d e l e v a t i o n s when the n o i s e i s added. Re d u c t i o n i n the l e v e l o f the band o f n o i s e (not i l l u s t r a t e d ) or r e d u c t i o n o f the bandwidth o f the n o i s e (see F i g s . 10 and 12) p r e c l u d e d such e f f e c t s by the s k i r t s o f t h e n o i s e by r e s p e c t i v e l y l o w e r i n g the l e v e l o f the t o f a c e page 41 F i g . 9 . Masked t h r e s h o l d s above the tone masker ( f ^ = 2kHz) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a 600-Hz band o f n o i s e , e i t h e r j u s t below £^ ( t o p ) , o r j u s t below <>5f^ (bottom) f o r s u b j e c t RB. Top; T h i s n o i s e masks when s i g n a l f r e q u e n c i e s a r e below 3 kHz ( 1 . 5 f ^ ) , and i t masks when s i g n a l f r e q u e n c i e s a r e above 3 kHz. Bottom; T h i s n o i s e masks when s i g n a l f r e q u e n c i e s a r e below 3 kHz, and i t masks when s i g n a l f r e q u e n c i e s a r e above 3 kHz. Masked t h r e s h o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n c i e s below 3 kHz (1.5f^) t h a t p l a c e i n the n o i s e . The l e v e l o f the tone masker, as i n d i c a t e d , i s 80 dB SPL and 78 dB SL. For an e x p l a n a t i o n o f the curves and symbols, see F i g . . - 4 1 -SIGNAL FREQUENCY IN KHZ SIGNAL FREQUENCY IN KHZ FIGURE 9 t o f a c e page 42 F i g . 10. Masked t h r e s h o l d s above the tone masker 2 kHz) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a narrow, 50-Hz, band o f n o i s e , e i t h e r j u s t below f ^ ( t o p ) , or j u s t below . 5 f ^ (bottom) f o r s u b j e c t RB. Masked t h r e s h -o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n c i e s below 3 kHz (1.5f^) "that p l a c e i n the n o i s e . The l e v e l o f the tone masker, as i n d i c a t e d , i s 80 dB SPL and 80 dB SL. For an e x p l a n a t i o n o f the curves and symbols, see. F i g . 3. - 4 2 -(fi I 1 1 1 — i — i — i — i 1 1 • 1 — 1 — r SIGNAL FREQUENCY IN KHZ SIGNAL FREQUENCY IN KHZ FIGURE 10 t o f a c e page 43 F i g . 11. Masked t h r e s h o l d s above the tone masker ( f ^ = 2 — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a 600-Hz band o f n o i s e , e i t h e r j u s t below f ^ ( t o p ) , o r j u s t below »5f^ (bottom) f o r s u b j e c t RB. Masked t h r e s h o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n c i e s below 3 kHz ( 1 . 5 ^ ) t h a t p l a c e i n the n o i s e . The l e v e l o f the 85-dB SPL masker i s a l s o i n d i c a t e d i n dB SL. F o r an e x p l a n a t i o n o f the curves and symbols, see F i g . 3. 43 SIGNAL FREQUENCY IN KHZ FIGURE 11 t o f a c e page 44 F i g . 12. Masked t h r e s h o l d s above the tone masker (f-^- 2 kHz) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a narrow, 50-Hz, band o f n o i s e , e i t h e r j u s t below f ^ ( t o p ) , o r j u s t below .5f^ (bottom) f o r s u b j e c t RB. Masked t h r e s h -o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n c i e s below 3 kHz ( 1 . 5 f 1 ) -that ^ pla-ee-C^ i n the- ^noise. The l e v e l o f the tone masker, as i n d i c a t e d i s 85 dB SPL and 83 dB SL. For an e x p l a n a t i o n o f the curves and symbols, see F i g . 3. -44 0.4 0.5 07 1.0 15 2.0 3.0 SIGNAL FREQUENCY IN KHZ i r i i r Q_ CO 60f GQ Q 501 40 Q UJ cn < 30 O X cn _ 201 ioh oh R.B. 40 DB 85 DB 83 DB SL J L 0.4 0.5 0.7 1.0 1.5 2.0 SIGNAL FREQUENCY IN KHZ FIGURE 12 45 s k i r t s , o r removing them from the a r e a i n which they have the s e e f f e c t s . R e s u l t s o f r e d u c i n g n o i s e l e v e l were o b t a i n e d , a t v a r i o u s widths and l o c a t i o n s o f the n o i s e , from t h r e e s u b j e c t s . The e f f e c t o f narrowing the s e l e c t i v e masking n o i s e was observed i n two s u b j e c t s . In both c a s e s , the s m a l l t h r e s h o l d e l e v a t i o n s o f s i g n a l s near and above 1.5f^ were e i t h e r c o n s i d e r a b l y reduced o r e l i m i n a t e d (see F i g s . 10 and 12). The q u e s t i o n , i n any i n d i v i d u a l case, o f whether i t i s or t h a t determines masked t h r e s h o l d o f s i g n a l s g r e a t e r  than 1.5 times f ^ ( t h e masker frequency) i s d i f f i c u l t t o answer, s i n c e the t h r e s h o l d e l e v a t i o n s a t these s i g n a l f r e -q u encies a r e n e a r l y comparable whether the n o i s e s e l e c t i v e -l y masks o r D^. The r e s u l t s shown i n F i g u r e 6 suggest t h a t s u b j e c t TT d e t e c t s when s i g n a l f r e q u e n c i e s a re above 1.5 times s i n c e the l e v e l o f i s f a r below t h a t o f i n the case when s i g n a l f r e q u e n c i e s a r e l e s s than 1.5f A d d i t i o n a l experiments w i t h t h i s s u b j e c t and the same tone masker con f i r m e d t h i s c o n c l u s i o n . F o r example, n a r -row masking n o i s e d i d not produce the s m a l l t h r e s h o l d e l e v a t i o n o b t a i n e d when wider n o i s e bands (e.g. F i g . 6, top) s e l e c t i v e l y masked components generated by s i g n a l f r e q u e n c i e s g r e a t e r than 1.5f^. Narrow-band n o i s e d i d , however, produce a masked t h r e s h o l d e l e v a t i o n when i t s e l e c t i v e l y masked C- components generated by s i g n a l s -46 p r e s e n t e d i n the same fr e q u e n c y r e g i o n above 1 . 5 ^ . That i t i s , i n f a c t , t h a t i s a c t u a l l y d e t e c t e d i n t h i s case, when s i g n a l s a r e a t masked t h r e s h o l d , was a l s o c o n f i r m e d by more d i r e c t measurement o f and D^ ^ l e v e l s . T h i s was accomplished by s u b s t i t u t i n g a narrow (50-Hz) band o f n o i s e a t an i n t e n s i t y s l i g h t l y g r e a t e r than mea-sur e d masked t h r e s h o l d f o r a s i g n a l near 1.6 times the f r e q u e n c y of the masker Masking by and combi-n a t i o n bands o f n o i s e components r e v e a l e d t h a t C 1 d i s t o r -t i o n was a t a h i g h e r l e v e l i n t h i s c ase. T h e r e f o r e , f o r t h i s p a r t i c u l a r s u b j e c t (TT) and f o r the masker f r e q u e n c y o f 1.3 kHz p r e s e n t e d a t the l e v e l s here employed, i s most l i k e l y the o n l y combination tone t o be d e t e c t e d a t masked t h r e s h o l d as measured when the s i g n a l f r e q u e n c y l i e s between the main and secondary peaks (see F i g . 6 ) . As w i l l be shown i n subsequent paragraphs, f o r o t h e r s u b j e c t s the r e l a t i v e l e v e l s of and D^, and hence the c o n c l u s i o n drawn i n t h i s e x p e r i m e n t a l s i t u a t i o n , can d i f f e r . I t can, f o r example, i m p l i c a t e i n s t e a d . As mentioned e a r l i e r , narrow (50-Hz) s e l e c t i v e mask-i n g n o i s e s were used w i t h s u b j e c t RB. From F i g u r e s 10 and 12, i t may be seen t h a t the use o f narrow s e l e c t i v e masking bands does not produce t h r e s h o l d e l e v a t i o n s f o r s i g n a l s i n the f r e q u e n c y range between 1.5f^ (3 kHz) and the secondary peak. T h i s f i n d i n g suggests t h a t C 1 and -47-l e v e l s a r e e q u a l , or v e r y n e a r l y so, when the l e v e l of the s i g n a l i s a t masked t h r e s h o l d . A l s o , measurements o f masking by and combination bands o f n o i s e p r o v i d e independent ev i d e n c e o f t h i s c o n c l u s i o n f o r the s u b j e c t RB (Greenwood, 1972c; see D i s c u s s i o n ) . The same c o n c l u s i o n may be drawn from the audiograms o f s u b j e c t KL, which show comparable t h r e s h o l d e l e v a t i o n s a t s i g n a l f r e q u e n c i e s j u s t below the secondary peak, and above 1.5£\^, whether i t i s or D^ ^ t h a t i s s e l e c t i v e l y masked (see F i g . 7 ) . F o r s u b j e c t FK, however, (see F i g . 8 t h r e s h o l d e l e v a t i o n s o b t a i n e d by s e l e c t i v e masking o f D 1 a r e somewhat g r e a t e r than those o b t a i n e d by s e l e c t i v e masking of c^. T h i s r e s u l t i n d i c a t e s t h a t , here a g a i n , and D 1 l e v e l s a r e comparable f o r s i g n a l s above 1.5f^, s i n c e t h r e s h o l d e l e v a t i o n s a r e s m a l l , but i t a l s o suggests t h a t D^ i s the combination tone t h a t determines " s i g n a l " masked t h r e s h o l d i n t h i s f r e q u e n c y r e g i o n and f o r t h i s s u b j e c t . .. Masking and S i m u l t a n e o u s l y As d i s c u s s e d i n the p r e v i o u s s e c t i o n , the s e l e c t i v e masking o f o n l y one o f the two s u s p e c t e d combination tones, o r D^, o f t e n produces e i t h e r u n d r a m a t i c a l l y s m a l l i n c r e a s e s o f s i g n a l t h r e s h o l d s , o r none a t a l l , e s p e c i a l l y when the pure-tone masker i s o f h i g h i n t e n s i t y . to f a c e page 48 F i g . 13. Masked t h r e s h o l d s above the pure-tone masker (f^= 2 k H z ) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open o c i r c l e s ) a d d i t i o n o f a wide (1400-Hz bandwidth), e f f e c -t i v e l y low-pass, band of n o i s e below f . . T h i s n o i s e masks bot h and s i m u l t a n e o u s l y when s i g n a l f r e q u e n -c i e s -axe'-foetween t'tie^main and 'secondary peaks. - A f t e r a d d i t i o n o f the n o i s e , masked t h r e s h o l d s are c o n s i d e r -a b l y e l e v a t e d a t s i g n a l f r e q u e n c i e s between the main and secondary peaks. The l e v e l o f the tone masker i s i n d i c a t e d i n b o t h dB SPL and dB SL. For an e x p l a n a t i o n o f the curves and symbols, see F i g . 4. -48-SIGNAL FREQUENCY IN KHZ FIGURE 13 -49 The c o n s i s t e n t l y supported reason g i v e n f o r the s m a l l s i z e o f t h i s e l e v a t i o n i s t h a t , as s i g n a l i n t e n s i t y i n c r e a s e s , b o t h combination tones become more i n t e n s e and more compar-a b l e i n l e v e l . Whichever combination tone i s l e f t un-masked, i s the one t h a t becomes a u d i b l e b e f o r e the s i g n a l i t s e l f can r e a c h an a u d i b l e i n t e n s i t y on the h i g h - f r e q u e n c y s i d e o f the masker. When both of the most prominent combination tones, and D 1, are masked s i m u l t a n e o u s l y , i t f o l l o w s t h a t the much g r e a t e r s i g n a l t h r e s h o l d s on the h i g h - f r e q u e n c y s i d e o f the masker i n d i c a t e the l e v e l s a t which the s i g n a l i t s e l f ( o r , c o n c e i v a b l y , y e t another combination tone) .becomes ..audible.. Wide band o f n o i s e T h i s t h r e s h o l d e l e v a t i o n c e r t a i n l y o c c u r s w i t h the i n t r o d u c t i o n of a wide band o f n o i s e below the masker (see. F i g . 13). T h i s n o i s e , however, must a l s o mask combination tones o t h e r than C 1 and D^, t h a t may be p r e s e n t . As d i s c u s s e d i n consequent s e c t i o n s , the use o f narrower bands of n o i s e which mask and s i m u l -t a n e o u s l y circumvents t h i s problem. As p r e v i o u s l y mentioned, a d i p i n the masking p a t t e r n between the main and secondary peaks was p r o g r e s s i v e l y e l i m i n a t e d w i t h i n c r e a s i n g l e v e l s o f a low-pass n o i s e l o c a t e d below the masker (Greenwood, 1971b, F i g . 11, p. 515) 50 as shown i n F i g u r e 2 o f t h i s paper. Such a n o i s e masks both and throughout the f r e q u e n c y range o f t h e i r e x i s t e n c e . The same r e s u l t has been confirmed f o r h i g h -i n t e n s i t y masked audiograms o f two s u b j e c t s used i n the p r e s e n t s t u d y (see F i g . 13), u s i n g a wide band o f n o i s e . The purpose o f masking and s i m u l t a n e o u s l y and c o n f i r m i n g t h a t the e f f e c t o f the wide or low-pass n o i s e does not have another e x p l a n a t i o n may be accomplished by u s i n g narrow bands o f n o i s e t h a t w i l l mask w i t h i n more r e s t r i c t e d f r e q u e n c y ranges. Then, o n l y those s i g -n a l f r e q u e n c i e s t h a t generate C^and-D^ components, bot h o f which f a l l s i m u l t a n e o u s l y w i t h i n the masking i n f l u e n c e o f the n o i s e , should be e l e v a t e d . The e l e v a t i o n , moreover, s h o u l d be comparable t o t h a t o b t a i n e d when the wide band o f n o i s e i s used. T h i s l a t t e r l e v e l o f e l e v a t i o n i s the l e v e l a t which i t w i l l then be most r e a s o n a b l e t o conclude t h a t the s i g n a l i t s e l f i s heard. Medium-width bands o f n o i s e Masking and s i m u l t a n e o u s l y w i t h medium-width (350- or 600-Hz) bands o f n o i s e may be accomplished by c e n t e r i n g the n o i s e a t a f r e q u e n c y h a l f t h a t o f the masker (.5 times **^)» There, the n o i s e w i l l c o n s e q u e n t l y mask both and D 1 s i n c e they c o i n c i d e or a r e c l o s e t o g e t h e r when they f a l l w i t h i n t h i s r e g i o n near .5f^. S i n c e s i g n a l s near 1.5f- p l a c e C- and D.. i n t h i s n o i s e , -51-masking the d i f f e r e n c e tones near «5f^ i s expected to a f f e c t the t h r e s h o l d s o f s i g n a l s near 1.5f 1» As may be seen from F i g u r e s 14 t o 17, and 19, measured t h r e s h o l d s a r e e l e v a t e d i n the f r e q u e n c y r e g i o n i n which the s i g n a l , t o g e t h e r w i t h the masker, i s r e s p o n s i b l e f o r the gener-a t i o n o f and components which b o t h f a l l w i t h i n the n o i s e . As e v i d e n t from the F i g u r e s , the t h r e s h o l d e l e -v a t i o n i s r e s t r i c t e d t o s i g n a l f r e q u e n c i e s which p l a c e both and i n the n o i s e . The r e s t r i c t i o n o f the e f f e c t t o the t h r e s h o l d s o f a l i m i t e d range o f s i g n a l f r e q u e n c i e s demonstrates t h a t o n l y and C^, and no o t h e r type o f combination tone, a r e d e t e c t e d i n the f r e q u e n c y range over which the i n t r o d u c t i o n o f the n o i s e l e d t o t h r e s h o l d i n c r e a s e s . E f f e c t o f n o i s e bandwidth By u s i n g s t i l l narrower widths o f n o i s e , the r e s t r i c -t i o n o f t h r e s h o l d i n c r e a s e s t o s i g n a l f r e q u e n c i e s whose and a r e s i m u l t a n e o u s l y masked may be somewhat more d r a m a t i c a l l y demonstrated. F i g u r e 17 compares the e f f e c t s o f t h r e e n o i s e s o f d i f f e r e n t w i dth. The r e s p e c t i v e narrow-i n g o f the r e g i o n o f e l e v a t e d t h r e s h o l d s i s c l e a r l y demon-s t r a t e d . The F i g u r e shows t h a t t h r e s h o l d e l e v a t i o n s a r e comparable f o r the d i f f e r e n t widths o f n o i s e used, w h i l e the e x t e n t o f the f r e q u e n c y r e g i o n over which masked t h r e s h o l d s a r e e l e v a t e d i s reduced as n o i s e bandwidth i s reduced. t o f a c e page 52 F i g . 14. Masked t h r e s h o l d s above the pure-tone masker (f^= 1.3 k H z ) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r c l e s ) a d d i t i o n o f a band o f n o i s e (350-Hz bandwidth) c e n t e r e d a t »5f^f f o r s u b j e c t TT. T h i s n o i s e masks both and s i m u l t a n e o u s l y when s i g n a l f r e q u e n c i e s a r e near 1950 'Hz ( l . S f ^ ) . A f t e r a d d i t i o n o f the n o i s e , masked t h r e s h o l d s a r e c o n s i d e r a b l y e l e v a t e d a t s i g n a l f r e q u e n -c i e s near 1950 Hz. Masked t h r e s h o l d s o b t a i n e d i n the presence o f the tone p l u s the n o i s e (diamonds) demonstrate an a d d i t i v e e f f e c t o f the n o i s e on the t h r e s h o l d s o f s i g n a l s j u s t below the tone. The l e v e l o f the tone masker i s i n d i -c a t e d i n b o t h dB SPL and dB S L . For an e x p l a n a t i o n o f the curves and the symbols, see F i g . 4. MASKED THRESHOLD .IN DB SPL _ ro o o w o o O P CM CO CD > r* JO m o CZ m 5 o -< ^ ro -L b N 0) o 3 H H o 00 ID O o CD 3 8 o o X MASKING BY NOISE IN FREQUENCY REGION WHERE C, 8 D, FALL IN DB SPL o <T> O al MASKED THRESHOLD IN DB SPL t o f a c e page 53 F i g . 15. Masked t h r e s h o l d s above the pure-tone masker (f^= 2 kHz)—before ( s o l i d c i r c l e s ) and a f t e r (open c i r -c l e s ) a d d i t i o n of a band o f n o i s e (600-Hz bandwidth) c e n t e r e d a t 1 kHz („5f^), f o r s u b j e c t TC. T h i s n o i s e masks bot h and s i m u l t a n e o u s l y when s i g n a l f r e -q u e n c i e s a r e near 3 "kHz ' ( l . S f ^ ) . A f t e r a d d i t i o n o f "the n o i s e , masked t h r e s h o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n -c i e s near 3 kHz. The l e v e l o f the tone masker i s i n d i c a t e d i n bo t h dB SPL and dB SL. For an e x p l a n a t i o n o f the curves and symbols, see F i g . 4. -53 FIGURE 15 t o f a c e page 54 F i g . 16. Masked t h r e s h o l d s above the pure-tone masker (fjj= 2 k H z ) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r -c l e s ) a d d i t i o n o f a band o f n o i s e (600-Hz bandwidth) c e n t e r e d a t 1 kHz ( , 5 f ^ ) , f o r s u b j e c t RB. T h i s n o i s e masks b o t h and s i m u l t a n e o u s l y when s i g n a l f r e -quenrcies *are*near *-3 kHz ( l . S f ^ ) . A f t e r a d d i t i o n o f the n o i s e , masked t h r e s h o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n -c i e s near 3 kHz. The l e v e l o f the tone masker i s i n d i c a t e d i n both dB SPL and dB SL. F o r an e x p l a n a t i o n o f the curves and the symbols, see F i g . 4. -54-SIGNAL FREQUENCY IN KHZ FIGURE 16 to f a c e page 55 F i g . 17. Masked t h r e s h o l d s above a pure-tone masker ( f 1 = 2 kHz) b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r -c l e s ) a d d i t i o n o f noise-bands o f d i f f e r e n t w i d t h (1400 Hz, top; 600 Hz, middle; 50 Hz, bottom), f o r s u b j e c t RB. A l l bands a r e c e n t e r e d a t 1 kHz ( . S f ^ ) . These n o i s e s mask -both *c^*«and *D^* simultaneous 1 y when «signals a r e w i t h i n v a r i o u s i n t e r v a l s about 3 kHz ( 1 . 5 f 1 ) , the e x t e n t o f the i n t e r v a l depending on the width o f the n o i s e . A f t e r d i t i o n o f the n o i s e , masked t h r e s h o l d s a r e e l e -v a t e d a t those s i g n a l f r e q u e n c i e s which p l a c e b o t h and i n the n o i s e . The l e v e l o f the tone masker i s i n d i c a t e d i n both dB SPL and dB SL. For an e x p l a n a t i o n o f the curves and the symbols, see F i g . 4. 55 FIGURE 17 t o f a c e page 56 F i g . 18 . Masked t h r e s h o l d s above a pure-tone masker ( f 1 = 2 k H z ) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r (open c i r -c l e s ) a d d i t i o n o f two narrow (50-Hz) bands o f n o i s e below the tone masker. These n o i s e s mask both and s i m u l t a n e o u s l y when s i g n a l f r e q u e n c i e s a r e i n the r e g i o n ^ i n d i c a t e d i n •"the bo t torn " r i g h t - h a n d corner o f each -graph. A g a i n , t h r e s h o l d s a r e e l e v a t e d a t s i g n a l f r e q u e n c i e s which p l a c e both and D 1 i n the n o i s e . The l e v e l o f the tone masker i s i n d i c a t e d i n both dB SPL and dB SL. For an e x p l a n a t i o n o f the cu r v e s and symbols, see F i g . 4. 56-1 1 1 1—i—i—i •> 1 • 1 1—i—1—r—n Z SIGNAL FREQUENCY IN KHZ FIGURE 18 57-Two narrow bands o f n o i s e Two narrow bands o f n o i s e may a l s o be used to mask and s i m u l t a n e o u s l y i n r e s t r i c t e d f r e q u e n c y r e g i o n s o t h e r than those c e n t e r e d around .5f^. The e f f e c t o f s i m u l t a n e o u s l y masking and may, consequently, be observed i n r e s t r i c t e d segments o f the hi g h - f r e q u e n c y s i d e o f the masked audiogram ot h e r than those c e n t e r e d a t 1.5f^. To accomplish t h i s purpose, two narrow bands o f n o i s e a r e used and p l a c e d so t h a t when the s i g n a l i s i n the f r e q u e n c y range o f the audiogram under s c r u t i n y ( i . e . a chosen range i n the notch a r e a ) , the w i l l f a l l w i t h -i n one narrow band o f n o i s e and the w i t h i n the oth e r band o f n o i s e . F i g u r e 18 shows the r e s u l t s o f t h i s ex- > periment. Marked e l e v a t i o n s i n the measured masked t h r e s h -o l d o c c u r o n l y a t those s i g n a l f r e q u e n c i e s g e n e r a t i n g and components bot h o f which f a l l i n the n o i s e and a r e masked. When e i t h e r o f the two n o i s e s i s used by i t s e l f , t h r e s h o l d e l e v a t i o n s i n the same " s i g n a l " ranges a r e s m a l l e r o r absent (see F i g s . 10 and 12) s i n c e a s i n g l e band of n o i s e masks o n l y one combination tone i n one o f these r e g i o n s , l e a v i n g the oth e r unmasked. In t h i s case, as p o i n t e d out e a r l i e r , masked t h r e s h o l d e l e v a t i o n s c o r -respond t o a l e v e l a t which the unmasked combination tone generated by s i g n a l and masker becomes a u d i b l e . When both C. and D- a r e masked by the two narrow bands of n o i s e , -58-masked t h r e s h o l d e l e v a t i o n s correspond t o a l e v e l a t which the s i g n a l i t s e l f becomes a u d i b l e . We know t h i s s i n c e masked t h r e s h o l d e l e v a t i o n s correspond t o a l e v e l o b t a i n e d when the wide band o f n o i s e was used (see F i g . 13). T h i s l a r g e d i f f e r e n c e o f t h r e s h o l d e l e v a t i o n s b e t -ween s e l e c t i v e and simultaneous masking of C 1 and demonstrates c o n c l u s i v e l y t h a t and a r e o f comparable i n t e n s i t y near t h r e s h o l d when masker and s i g n a l parameters are those r e p r e s e n t e d i n h i g h - l e v e l pure-tone masked audiograms, p a r t i c u l a r l y when s i g n a l f r e q u e n c i e s a r e between 1 . 5 f 1 and the secondary peak. In o t h e r words, and D 1 s i n g l y o r j o i n t l y produce the d e t e c t e d " e v e n t s " a t masked t h r e s h o l d v a l u e s o f the pure-tone masked a u d i o -gram, as measured i n the notc h or t r o u g h r e g i o n o f the audiogram. E f f e c t s of Noise L e v e l As the i n t e n s i t y o f the n o i s e below the masker i s lowered, the e l e v a t i o n o f masked t h r e s h o l d on the h i g h -f r e q u e n c y s i d e o f the masker i s not expected to show a co r r e s p o n d i n g decrease u n t i l the i n t e n s i t y o f the n o i s e i s lowered below a l e v e l a t which e i t h e r one or both combination tones a r e no longer masked. Then the lower masked t h r e s h o l d on the h i g h f r e q u e n c y s i d e o f the tone masked r e f l e c t s the s i g n a l i n t e n s i t y a t which the most t o f a c e page 59 F i g . 19 0 Masked t h r e s h o l d s above a pure-tone masker (f^= 2 k H z ) — b e f o r e ( s o l i d c i r c l e s ) and a f t e r , (open c i r c l e s ) a d d i t i o n o f a 50-Hz band o f n o i s e a t d i f f e r e n t l e v e l s . T h i s n o i s e , which i s c e n t e r e d a t 1 kHz ( . 5 ^ ) , masks bot h and s i m u l t a n e o u s l y when the f r e q u e n c y ->ox*-"tire -"s-'ignal *±s' n e a r " 3 1 "kHz ?5f^) • As the l e v e l o f the n o i s e i s lowered, the amount by which s i g n a l t h r e s h -o l d s are e l e v a t e d does not show a c o r r e s p o n d i n g r e d u c t i o n u n t i l the l e v e l o f the n o i s e i s lowered below 35 dB SPL. The l e v e l o f the tone masker, as i n d i c a t e d , i s 85 dB SPL and 86 dB SL. For an e x p l a n a t i o n o f the curves and symbols, see F i g . 4. 59-MASKED THRESHOLD IN DB SPL — ro o o cn o CD > r o ZD _ m o O c m -z. o _ 7^  "E o O J o o Ul o (J) o 3} CD _L MASKING BY NOISE IN FREQUENCY REGION g o o WHERE C, a D, FALL MASKED THRESHOLD IN DB SPL IN DB SPL MASKED THRESHOLD IN DB SPL FIGURE 19 -60-prominent combination tone i n the n o i s e i s j u s t d e t e c t -a b l e a t i t s masked t h r e s h o l d v a l u e . F i g u r e 19 shows the e f f e c t o f l o w e r i n g the i n t e n s i t y o f a narrow band o f n o i s e which masks and s i m u l t a -n e o u s l y . I t i s r e a d i l y e v i d e n t t h a t the l e v e l of the combination tones f a l l i n g w i t h i n the masking i n f l u e n c e o f the n o i s e i s w e l l below t h a t o f the masking n o i s e used i n the remainder o f the experiments r e p o r t e d here. S p e c i -f i c a l l y , the most i n t e n s e combination tone i s a t l e a s t 5 dB, but l e s s than 15 dB below the 40 dB masking n o i s e o f F i g u r e 19, s i n c e l o w e r i n g the l e v e l of t h e n o i s e 5 dB d i d not lower masked t h r e s h o l d e l e v a t i o n s , whereas low-e r i n g , i t , 1 5 ,dBjdid lower the amount by which t h r e s h o l d s were r a i s e d . T h i s r e s u l t was c o n f i r m e d i n another ex-periment (not shown), s i m i l a r t o t h a t i l l u s t r a t e d i n F i g u r e 19, except t h a t a wider (600-Hz) n o i s e (as i n F i g . 16, bottom), c e n t e r e d a t the same fr e q u e n c y , was lowered 10 dB from 45 dB t o 35 dB t o t a l SPL. The lower-l e v e l n o i s e produced comparable t h r e s h o l d e l e v a t i o n s as the one a t 45 dB so l o n g as and f e l l i n the c e n t e r o f the n o i s e , i m p l y i n g they were s t i l l masked and t h a t the s u b j e c t was d e t e c t i n g the s i g n a l p e r s e . The lower-l e v e l n o i s e , however, produced s m a l l e r t h r e s h o l d e l e v a -t i o n s a t those s i g n a l f r e q u e n c i e s which p l a c e d both d i f -f e r e n c e tones on the s k i r t s of the n o i s e , i m p l y i n g t h a t 61-the s u b j e c t was then d e t e c t i n g the combination tones be-f o r e the s i g n a l became a u d i b l e . S i n c e the l o w e r - l e v e l n o i s e was 10 dB below the 45-dB SPL n o i s e , one may i n f e r t h a t combination tones produced by s i g n a l s near 1 . 5 f 1 a r e about 10 dB below the masking i n f l u e n c e o f the 45-dB n o i s e , a t these masker and s i g n a l l e v e l s , which, as the h i g h e s t used f o r t h i s s u b j e c t (RB), produce the l a r g e s t combination t o n e s . From the f o r e g o i n g , one can conclude t h a t the e l e v a t e d masked t h r e s h o l d s near 1.5f^ a r e those o f the s i g n a l i t s e l f . In o t h e r experiments (not shown), u s i n g TT and TC as w e l l as RB as s u b j e c t s , the l e v e l o f a n o i s e l o c a t e d near the f o o t o f the pure-tone masked audiogram was low-er e d 10 dB below the l e v e l s used i n the experiments r e -p o r t e d h e r e . T h i s n o i s e masks when s i g n a l f r e q u e n c i e s a r e below,1.5 times f ^ . Lowering the l e v e l o f the n o i s e lowered the e l e v a t e d masked t h r e s h o l d s , which were ob-s e r v e d when the h i g h e r - l e v e l n o i s e was used, but o n l y a t s i g n a l f r e q u e n c i e s which p l a c e d a combination tone on the s k i r t o f the n o i s e . Thus, combination tones near the c e n t e r o f the h i g h e r - l e v e l n o i s e must have been more than 10 dB below t h e i r t h r e s h o l d s . S p e c i f i c a l l y , f o r i n t e n s e tone maskers, l e s s e r t h r e s h o l d e l e v a t i o n s , i n the presence o f the l o w e r - l e v e l n o i s e , occur a t s i g n a l f r e q u e n c i e s near and above 1.5f-, which had p l a c e d C- and D- on the s k i r t s -62-o f the h i g h e r - l e v e l n o i s e . In a d d i t i o n , l e s s e r masked t h r e s h o l d s , accompanying the r e d u c t i o n i n n o i s e l e v e l , o c c u r r e d a t s i g n a l f r e q u e n -c i e s j u s t above the masker, i . e . on the s t e e p l y d e c l i n i n g p a r t o f the audiogram. These t h r e s h o l d e l e v a t i o n s a r e s m a l l e r s i n c e the upper s k i r t o f the h i g h e r - l e v e l n o i s e can have an a d d i t i v e e f f e c t on t h r e s h o l d s j u s t below the tone masker. T h i s r e d u c t i o n i n e l e v a t i o n s j u s t above the masker i n d i c a t e s t h a t determines masked t h r e s h o l d even when the s i g n a l i s q u i t e c l o s e t o the masking tone. T h i s a d d i t i v e masking e f f e c t o f the n o i s e , below the tone masker, on masked t h r e s h o l d s a t or near the tone w a s . p r e v i o u s l y r e p o r t e d by Greenwood (1972a), and i s i l l u s t r a t e d i n F i g u r e 2 and the top o f F i g u r e 14. The top o f F i g u r e 14 shows the a d d i t i v e e f f e c t o f the s k i r t o f the n o i s e on t h r e s h o l d s below the tone masker and the consequent s m a l l t h r e s h o l d e l e v a t i o n s j u s t above the masker i n r e l a t i o n t o the masking produced by the tone a l o n e . 63-DISCUSSION S i g n i f i c a n c e o f Masked T h r e s h o l d s i n the Notch In p r e v i o u s r e s e a r c h (Greenwood, 1971b), the notch observed on the high- f r e q u e n c y s i d e o f the pure-tone mask-er was e l i m i n a t e d a f t e r the i n t r o d u c t i o n o f a low-pass •"noise, "which masked-'all-eombination tones below the masker. Greenwood, t h e r e f o r e , a t t r i b u t e d masked t h r e s h o l d s mea-sured a t s i g n a l f r e q u e n c i e s w i t h i n the notch r e g i o n t o the d e t e c t i o n o f the combination tones below the masker, spe-c i f i c a l l y t o the d e t e c t i o n o f a t low t o moderate masker l e v e l s and p o s s i b l y a l s o t o a t high e r masker l e v e l s when was a l s o generated. The r e s u l t s , r e p o r t e d here, o f s e l -e c t i v e l y masking d i f f e r e n t combination tones, or masking two combination tones s i m u l t a n e o u s l y over s h o r t frequency ranges below the masker, c o n f i r m and extend Greenwood's f i n d i n g s . That i s , r e s u l t s from the p r e s e n t study, conduc-te d a t h i g h e r masker l e v e l s , a l s o i n d i c a t e t h a t , over the e n t i r e notch r e g i o n (the freque n c y i n t e r v a l between the main and secondary peaks of the masked audiogram), measured masked t h r e s h o l d s a c t u a l l y r e p r e s e n t s i g n a l i n t e n s i t i e s a t which the combination tones produced by the s i g n a l and the masker, r a t h e r than the s i g n a l i t s e l f , a r e d e t e c t e d . A l s o , as Greenwood (1971b) demonstrated i n the case o f low t o moderate masker l e v e l s , i t i s the d e t e c t i o n o f C. , t h a t , - 6 4 -i n most cases, determines masked threshold at s i g n a l f r e -quencies i n the notch region. Although, f o r one subject and over a short frequency range (Greenwood, unpublished r e s u l t s , personal communication), C 2 was most probably detected at masked threshold at low masker l e v e l s , C 2 or were not so implicated at the higher masker l e v e l s used i n the present study, the r e s u l t s of which suggest that constitutes the detected "event" even when the frequency of the sign a l i s ju s t above that of the masker. And, i t i s only at high i n t e n s i t i e s of the masker that the detection of can, i n some cases, determine masked threshold. I n s t a n c e s i n which -D^ i s -sometimes implicated .occur when the frequency of the s i g n a l i s i n the high-frequency p o r t i o n of the notch, i . e . below the secondary peak but above 1.5f^. That i s , detection of can sometimes de-termine masked thresholds, as normally measured at f r e -quencies i n the high-frequency portion of the notch above 1.5^. The secondary peak of the masked audiogram occurs at a frequency locus of 1.5f^ or higher only when the masker i s at a high i n t e n s i t y , greater than about 75 dB SL. Then, the s e l e c t i v e masking of or elevates thresholds measured at higher frequencies i n the notch only s l i g h t l y , or not at a l l , since C. and D- are of approximately equal 65 amplitude. In t h i s c i r c u m s t a n c e , i t may be e i t h e r o r D^, or both, t h a t a re d e t e c t e d a t measured masked t h r e s h -o l d a t these f r e q u e n c i e s , depending on the i n d i v i d u a l s u b j e c t . The f i n d i n g t h a t and are app r o x i m a t e l y equal i n l e v e l when parameters o f the masker and s i g n a l a r e those o f the s i t u a t i o n j u s t d e s c r i b e d , i s c o n s i s t e n t w i t h more d i r e c t d e t e r m i n a t i o n s of the l e v e l s o f and D1 o b t a i n e d by Greenwood (1972c) and H a l l (1971, 1972a,b). More s t r i c t l y s p eaking, Greenwood e s t i m a t e d the l e v e l s o f and D 1 bands o f n o i s e from the amount o f masking produced by combination bands of the two c o r r e s p o n d i n g t y p e s . H a l l , on the other hand, used the l e v e l s o f can-c e l l a t i o n tones as e s t i m a t e s o f and i n t e n s i t i e s . A t primary i n t e n s i t y l e v e l s s u f f i c i e n t l y h i g h t o a l l o w measurement of D^, both i n v e s t i g a t o r s r e p o r t n e a r l y equal l e v e l s o f and D^. Such i s p a r t i c u l a r l y the case when frequen c y and i n t e n s i t y parameters of the p r i m a r i e s approx-imate those o f the masker and s i g n a l i n the audiograms r e p o r t e d here. With r e s p e c t t o the s t i m u l u s parameters as they o b t a i n i n the audiograms r e p o r t e d i n t h i s paper, one can add t h a t the l e v e l o f i s s l i g h t l y g r e a t e r than t h a t o f when the freque n c y of the s i g n a l i s i n the lower-frequency p o r t i o n o f the notch r e g i o n ( s i g n a l f r e -quency l e s s than 1 . 5 ^ ) , w h i l e C, and D. are n e a r l y the 66-same l e v e l when the f r e q u e n c y o f the s i g n a l i s i n the h i g h e r - f r e q u e n c y p a r t o f the notch. That the l e v e l o f the D 1 i s o n l y s l i g h t l y below t h a t o f i s determined from measurements o f s i g n a l l e v e l s i n those s i t u a t i o n s i n which i t i s t h a t i s d e t e c t e d a t masked t h r e s h o l d , as d e s c r i b e d i n the f o l l o w i n g s e c t i o n . S i g n a l i n t e n s i t i e s a t which C^, or the S i g n a l I t s e l f are D e t e c t e d a t T h r e s h o l d When o n l y one one o f the two d i f f e r e n c e tones, or D^, i s masked and the o t h e r i s not, measured t h r e s h -o l d above the masker i s e l e v a t e d t o a l e v e l a t which i t i s e i t h e r the s i g n a l , o r the unmasked combination tone, t h a t f i r s t reaches d e t e c t i o n a t masked t h r e s h o l d . When both C 1 and D^ are masked s i m u l t a n e o u s l y , the new, e l e -v a t e d masked t h r e s h o l d i s t h a t of the s i g n a l i t s e l f . I t f o l l o w s , then, t h a t i f the masked t h r e s h o l d measured a f -t e r s e l e c t i v e masking i s lower than t h a t o b t a i n e d a f t e r -simultaneous masking, then t h i s lower t h r e s h o l d l e v e l r e p r e s e n t s the s i g n a l l e v e l a t which the unmasked combi-n a t i o n tone reaches t h r e s h o l d o u t s i d e the s e l e c t i v e mask-i n g n o i s e . In t h a t these d a t a d i s p l a y l e v e l s a t which C^, D^, and the s i g n a l i t s e l f a r e heard, the F i g u r e s , t o g e t h e r , c o n s t i t u t e a more complete and c o r r e c t r e p r e s e n t a t i o n than Wegel and Lane's (1924, F i g . 5) schematic o f a -67-pure-tone masked audiogram which i l l u s t r a t e s s i g n a l l e -vels at which a "difference tone" and the s i g n a l become audible. Their schematic shows that the primary and a "difference tone" are heard when signals are a t , and some-what above, masked thresholds measured between main and secondary peaks. At s t i l l higher s i g n a l l e v e l s , the p r i -mary, a "difference tone", and the s i g n a l i t s e l f are shown to be heard. The experiments reported here extend these findings i n that they i d e n t i f y two d i f f e r e n c e tones, and d i r e c t l y measure the s i g n a l l e v e l s at which, respectively, each of the diff e r e n c e tones and the s i g -nal i t s e l f become audible. The sig n a l l e v e l at which becomes audible i s generally that of the measured masked threshold between the main and secondary peak of the masked audiogram. In some cases, when the masker i s intense, may become audible at l e v e l s s l i g h t l y higher than maske threshold i n the higher-frequency part of the notch. When the l e v e l of the masker i s less than approxi-mately 70 to 75 dB SL, and the secondary peak i s at or below 1.5f 1, i s not generated at s u f f i c i e n t i n t e n s i t y to appear above threshold before the s i g n a l i t s e l f reaches i t s masked threshold. In other words, the l e v e l of the s i g n a l , i n t h i s case, must exceed the l e v e l at which the sign a l i t s e l f i s detected, before D- can become audible. - 6 8 -T h i s l a t t e r l e v e l i s not i l l u s t r a t e d i j i the F i g u r e s . U n l e s s the masker, and a l s o the s i g n a l , a r e a t h i g h l e v e l s , does not e x i s t . T h e r e f o r e , the be-comes r e l e v a n t o n l y when masking by h i g h - l e v e l s t i m u l i i s measured. In t h i s case, when the secondary peak i s above 1 . 5 f 1 , D 1 i s l i k e l y t o become a u d i b l e and, a f t e r s e l e c t i v e masking o f , t o determine a measured t h r e s h -o l d . T h i s l e v e l o f the s i g n a l a t which D^ becomes a u d i -b l e i s g r e a t e r than masked t h r e s h o l d as n o r m a l l y measured, when s i g n a l f r e q u e n c i e s a r e below 1.5f^. The g r e a t e r the masker i n t e n s i t y , the sooner D 1 becomes a u d i b l e as s i g n a l l e v e l s a r e r a i s e d above masked t h r e s h o l d . When ...-signal frequencies a r e ,in the upper p a r t o f the notch, i . e . above 1.5f^ but below the secondary peak, e i t h e r a l o n e i s d e t e c t e d a t masked t h r e s h o l d , as n o r m a l l y measured, o r , i f not, D^ becomes a u d i b l e a t a l e v e l o n l y s l i g h t l y h i g h e r than t h a t of masked t h r e s h o l d . D i p s i n the Masking P a t t e r n D i p s , or abrupt changes, are sometimes observed i n the masking p a t t e r n . These are the r e s u l t o f the beat-i n g between two d i s t o r t i o n components, or the b e a t i n g between a d i s t o r t i o n component and the s i g n a l . A d i p , due t o b e a t i n g between the d i s t o r t i o n com-ponents and T>^» i s o f t e n seen a t 1.5 times f 1 , p a r -t i c u l a r l y f o r s u b j e c t s KL and FK (e.g. F i g s . 7 and 8 ) . 69-A t t h i s frequency r a t i o o f the p r i m a r i e s ( f 2 / f ^ = 1.5), and both f a l l a t ,5f^» Since the s i g n a l i s not u s u a l l y s e t e x a c t l y a t 1 . 5 f 1 , the r e s u l t i n g slow beats between and a l l o w t h e i r d e t e c t i o n a t lower s i g n a l l e v e l s . That t h i s monaural phase e f f e c t i s due to the i n t e r a c t i o n o f and has been amply demonstrated by H a l l (1972b), and i t s occurence a t masked t h r e s h o l d has been r e p o r t e d by C l a c k e t . a l . (1972). A d i p i n the masking p a t t e r n i s a l s o e v i d e n t a t the second harmonic ( 2 f ^ ) i R some h i g h - l e v e l pure-tone masked audiograms. T h i s d i p i s the r e s u l t o f b e a t i n g between the s i g n a l and the second harmonic. I t was f i r s t r e p o r t e d by-.wegel and Lane (-1924). T^hey p l a c e d the secondary peak o f the masked audiogram a t t h i s l o c u s o f b e a t i n g w i t h the second harmonic, s i n c e they assumed the secondary peak t o be due t o masking by the a u r a l harmonic. A l t h o u g h the secondary peak o f Wegel and Lane's audiograms was, i n g e n e r a l , m i s - c e n t e r e d a t the second harmonic, i t may occur t h e r e f o r the h i g h e s t masker l e v e l s used (see F i g -ures 8 and 13 )<> Evidence o f the presence o f the second harmonic i s a l s o seen i n some of the audiograms o f Green-wood (1971b) as a d i p i n masked t h r e s h o l d a t the second harmonico In the p r e s e n t paper, a l s o , a d i p a t the second harmonic i s seen i n F i g u r e s 8 and 13 (bottom) f o r the sub-j e c t FK. I t a l s o appears i n other audiograms o f t h i s -70-s u b j e c t which a r e not i l l u s t r a t e d h e r e e In a d d i t i o n , the same s u b j e c t heard the s i g n a l beat when i t was near the fre q u e n c y o f the second harmonic* That the a u r a l harmonic i s n o r m a l l y not f a r below masked t h r e s h o l d was demonstrated by C l a c k e t , a l , (1972) who e s t i m a t e d the amplitude o f the a u r a l harmonic from d a t a showing a monaural phase e f f e c t when a t h r e s h o l d -l e v e l s i g n a l was p r e s e n t e d a t the second harmonic o f t h e masker and v a r i e d i n phase. L i m i t i n g P o s i t i o n o f the Secondary Peak The s h i f t i n l o c a t i o n o f the secondary peak of the masked audiogram t o a freque n c y near the a u r a l harmonic, however, seems t o be an upper l i m i t . That i s , the secon-d a r y peak o f the masked audiogram does not p r o g r e s s beyond a freq u e n c y twice t h a t o f the masker ( i . e . 2 f ^ ) . The upper end o f the n o t c h r e g i o n , and hence the b e g i n n i n g o f s i g n a l - d e t e r m i n e d masked t h r e s h o l d s a t and beyond the secondary peak, i s reached a t a s i g n a l f r e q u e n c y e q u a l t o a p p r o x i m a t e l y 1.8 t o 1*85 times f ^ . T h i s f r e q u e n c y r a t i o o f s i g n a l t o masker ( f ^ / f ) corresponds t o the same r a t i o , i n a F i g u r e o f Greenwood's (1972c, F i g u r e 1 ) , a t which the masking e f f e c t o f a combination band o f n o i s e merges, and d i s a p p e a r s i n t o , the low-frequency f o o t o f the masking p a t t e r n produced by the lower, pure-tone 71-component o f the primary masker. A t the same time, C^, whose amplitude decreases w i t h i n c r e a s i n g f r e q u e n c y sepa-r a t i o n between masker and s i g n a l , recedes i n t o i n a u d i b i l i t y i n the low-frequency r e g i o n . C^, o f course, ceases t o be d e f i n e d when f ^ reaches 2 f 1 (C^= 2 f 1 ~ f 2 ) . G e n e r a l i t y and I m p l i c a t i o n s o f the R e s u l t s The r e s u l t s o f the p r e s e n t study demonstrate t h a t masked t h r e s h o l d s measured above a tone o f moderate t o h i g h i n t e n s i t y a r e a c t u a l l y those o f combination tones (C^, D^, or both) below the masker. Thus, an i n a u d i b l e , masked s i g n a l above the masker, can generate, by i n t e r -a c t i n g .with .the....masker,, .audible, unmasked combination tones below the masker. Measurements o f masking by com-p l e x s t i m u l i such as n o i s e s a l s o a r e i n f l u e n c e d by the presence o f combination tones o f the same type i n a manner t o be d e s c r i b e d below. Fo r c e r t a i n purposes, a l s o d e t a i l e d below, i t i s important t o determine the masked t h r e s h o l d o f the s i g n a l i t s e l . T h e r e f o r e i t i s o f v a l u e t o know which combina-t i o n tones appear above t h r e s h o l d , and a t which f r e q u e n -c i e s o f the s i g n a l they can appear above t h r e s h o l d w h i l e the s i g n a l i t s e l f i s a t a l e v e l below t h a t o f i t s own t h r e s h o l d . Then, such a u r a l d i s t o r t i o n can be l o c a l i z e d , and consequently e l i m i n a t e d so t h a t the t h r e s h o l d o f the 72-s i g n a l i t s e l f can be determined. D e t e r m i n a t i o n o f the t h r e s h o l d o f the s i g n a l i t s e l f i s o f g r e a t importance t o the t a s k o f r e l a t i n g p s y c h o a c o u s t i c and n e u r o p h y s i o l o g i c a l d a t a . Masking by complex s t i m u l i When measuring the masking by complex s t i m u l i , such as n o i s e s , the presence o f combination components o f the type C n and D^ i s r e l e v a n t , as d i s c u s s e d a t l e n g t h by Greenwood (1971b). H i s c o n c l u s i o n s a r e b r i e f l y r e s t a t e d h e r e . As mentioned i n the I n t r o d u c t i o n , Greenwood (1971b, 1972a,b,c) p o i n t e d out t h a t the c o n s t i t u e n t f r e q u e n c y com-ponents o f one o r more bands o f n o i s e , o r o f t o n e - n o i s e combinations, generate combination tone -components i n t h e same way t h a t two or more tones do. The r e s u l t i n g combi-n a t i o n bands o f n o i s e , produced by n o i s e or n o i s e - t o n e p r i m a r i e s , a r e a t l e v e l s c o r r e s p o n d i n g t o those o f com-b i n a t i o n tones, produced by tone p r i m a r i e s a t s i m i l a r l e v e l s t o those o f the n o i s e - t o n e p r i m a r i e s (Greenwood, 1972b,c). A l s o , combination bands (and combination tones) produce masking as do bands o f n o i s e o f e x t e r n a l o r i g i n (Greenwood, 1972a). T h e r e f o r e , combination bands o f n o i s e w i l l a l s o have e f f e c t s on measurements o f masking, the s p e c i f i c e f f e c t s b e i n g determined by the c h a r a c t e r i s t i c s o f the combination components produced by the complex s t i m u l i , and by the parameters of the l a t t e r . 7 3 -Narrow bands o f n o i s e , f o r example, produce a notched form o f the masking p a t t e r n s i m i l a r t o t h a t o f pure-tone masked audiograms* The r e s u l t s o f the p r e s e n t study a p p l y a l s o t o the n o t c h observed i n the masking p a t t e r n above a narrow band o f n o i s e , except t h a t combination bands o f n o i s e , r a t h e r than combination tones, a r e d e t e c t e d i n s t e a d o f the s i g n a l when s i g n a l s a r e a t f r e q u e n c i e s i n the n o t c h . For somewhat wider bands o f n o i s e , the notched form d i s -appears, but the s l o p e o f d e c r e a s i n g t h r e s h o l d s above the n o i s e i s s t i l l determined by the d e t e c t i o n o f combination components. A t s t i l l wider widths o f the n o i s e , the s i g -n a l i t s e l f i s d e t e c t e d a t masked t h r e s h o l d above the n o i s e (Greenwood, 1971b). In a d d i t i o n t o the combination components generated by a band o f n o i s e and a s i g n a l , a band o f n o i s e by i t s e l f produces combination components which are the r e s u l t o f the i n t e r a c t i o n o f f r e q u e n c y components w i t h i n the n o i s e . T h e r e f o r e , i f the s k i r t s o f the n o i s e a r e s u f f i c i e n t l y s t e e p , the c o m b i n a t i o n a l aggregate w i l l be p r e s e n t a t g r e a t e r amplitude than the s k i r t and w i l l produce masking j u s t below the band o f n o i s e and hence determine masked t h r e s h o l d a t s i g n a l f r e q u e n c i e s j u s t below t h e band o f n o i s e . For a more complete d e s c r i p t i o n , see Greenwood (1971b). -74-Masked t h r e s h o l d o f the s i g n a l i t s e l f As mentioned e a r l i e r , i t i s sometimes important to determine the masked t h r e s h o l d o f the s i g n a l i t s e l f . For example, the t h r e s h o l d l e v e l s o f the s i g n a l i t s e l f a r e needed t o r e l a t e p s y c h o a c o u s t i c measures of masking t o a u d i t o r y mechanisms a t the p e r i p h e r y o f the system, spe-c i f i c a l l y near the l o c u s on the b a s i l a r membrane a t which the s i g n a l produces i t s maximum e f f e c t . Zwicker (1955, 1968, 1969), f o r example, bases h i s d e t e r m i n a t i o n o f the " p s y c h o a c o u s t i c a l e x c i t a t i o n " o f a sound on masked t h r e s h o l d s o f the s i g n a l i t s e l f . He p r e s e n t s e v i d e n c e t o suggest t h a t t h i s " e x c i t a t i o n " , and hence a l s o masked t h r e s h o l d s o f the s i g n a l i t s e l f , r e f l e c t p r i m a r i l y a hydromechanical f i l t e r i n g p r o c e s s , w i t h i n the c o c h l e a , t h a t o c c u r s b e f o r e t r a n s d u c t i o n to n e u r a l s p i k e s . Furthermore, he demonstrates t h a t the " e x c i t a t i o n l e v e l " (comparable t o masked t h r e s h o l d l e v e l o f the s i g n a l i t s e l f ) , as a f u n c t i o n o f f r e q u e n c y ( i n c r i t i c a l - b a n d u n i t s ) , i s important t o the u n d e r s t a n d i n g o f JND's ( j u s t n o t i c e a b l e d i f f e r e n c e s ) , l o u d n e s s , p i t c h and timbre (Zwicker, 1969). Masked t h r e s h o l d s o f the s i g n a l i t s e l f may a l s o be u t i l i z e d i n making comparisons w i t h measurements of mask-i n g d e r i v e d from the r e c o r d i n g of the responses o f s i n g l e nerve c e l l s near the p e r i p h e r y o f the a u d i t o r y system. - 7 5 -Masking, i n the c o n t e x t o f the d i s c h a r g e o f a s i n g l e u n i t , i s d e f i n e d by Greenwood and Maruyama (1965) as : "the a c t i o n o f one s t i m u l u s i n p a r t i a l l y o r c o m p l e t e l y p r e v e n t -i n g an o t h e r w i s e e f f e c t i v e s t i m u l u s from a c t i v a t i n g the u n i t i n q u e s t i o n , " The s i n g l e u n i t d e r i v e s i t s major i n -put from one l o c u s on the b a s i l a r membrane, which i s i n d i -c a t e d by the nerve c e l l ' s b e s t f r e q u e n c y . The masking o f a neuron's response to a s i g n a l a t i t s b e s t frequency, by i n t r o d u c t i o n o f a second s t i m u l u s , may then be compared t o the p s y c h o p h y s i c a l l y measured masked t h r e s h o l d a t a f r e q u e n c y analogous t o t h a t o f the b e s t f r e q u e n c y o f the neuron. To r e l a t e such n e u r o p h y s i o l o g i c a l d a t a , f o r ex-ample, t o p s y c h o p h y s i c a l r e s u l t s , i t i s important t o understand the meaning o f masked t h r e s h o l d s as measured p s y c h o p h y s i c a l l y ( i . e . what i s d e t e c t e d and a t what l e v e l ) , b e f o r e o t h e r fundamental i n f e r e n c e s can be drawn from the d a t a o b t a i n e d i n t h i s way. -76-BIBLIOGRAPHY American N a t i o n a l Standard Standard S l . l (1960). American  N a t i o n a l Standard A c o u s t i c a l Terminology (Ameri-can N a t i o n a l Standards I n s t i t u t e , I nc., New York) von Bek^sy, G. 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