THE EFFECT OF VARYING THE LEVEL OF THE STIMULUS TONE

8.4.1 Results at different levels

SFOAE suppression curves were measured at levels of stimulus tone of 2 0 , 3 0 , 4 0 , 50 & 60 dB SPL at a frequency of 1599Hz in the left ear of subject DB, 2 0dB SPL was the lowest level of stimulus at which the residual could be easily measured. Below this level it becomes immeasurable due to noise in the measurement system used in these experiments. 60 dB SPL is the highest level of stimulus that can be measured on the system

used in these experiments. Above this level microphone

distortion causes artefactual results.

Measurements were made at all these levels of stimulus tone with stimulus to suppressor tone frequency ratios (F1:F2) of 1.5:1, 1.3:1, 1.1:1, 1:1, 1:1.1, 1:1.3, 1:1.5. These ratios are given when the suppressor tone frequencies are 1062Hz,

1233Hz, 1453Hz, 1599Hz, 1746Hz, 2047Hz and 2454Hz,

respectively. Levels of suppressor tone are used which give [level of suppressor tone - level of stimulus tone], (level

F2-F1), of +12dB, +6dB, OdB, -6dB, -12dB. For example, if the

level of stimulus tone is 30 dB SPL then levels of suppressor tone of 42, 36, 30, 24 & 18 dB SPL are used.

D . N . B r a s s SFOAEs and c o c h l e a func t i o n

Figure 8.3 shows the results of these measurements. The results, for each level of stimulus tone, are presented in three forms:

(i) Iso-frequency suppression curves, which are as described above and are useful for showing the effect of variation of level of suppressor tone at a given frequency.

(ii) Iso-stimulus-level suppression curves, which show the variation of residual level with suppressor tone frequency at a given level of suppressor tone. These curves are useful for showing the relative effects of different frequencies of suppressor tone.

(iii) Iso-residual suppression curves, which show the level of suppressor tone necessary to evoke a given level of residual at various frequencies of suppressor tone. If the level of residual represents the amount of suppression then this type of presentation is analogous to the usual form of presentation for two tone suppression of nerve-fibre responses.

8.4.2 The variation of results at various levels of stimulus

Although all the general features of the suppression curves, noted above, were present at all levels of stimulus tone, there were minor variations. These are detailed below:

It can be seen from the iso-stimulus-level curves in figure 8.3 that the level of suppression is more dependent upon the frequency of the suppressor tone at lower levels of stimulus tone. This effect can be seen in the gradients of the iso­ stimulus curves becoming steeper as the stimulus tone level decreases from 60dB SPL to 20dB SPL.

This increased dependence of suppression upon the suppressor tone frequency is particularly marked when the suppressor tone frequency is less than the stimulus tone frequency. At stimulus tone levels of less than 30dB SPL the residual level is so low as not to be measurable at suppressor to stimulus frequency ratios of 1:1.5.

When the suppressor tone frequency is greater than the stimulus tone frequency the increase of dependence of the suppression on the frequency of the suppressor tone is less marked. However, at stimulus tone levels of 60dB SPL the suppression is hardly frequency dependent at all compared to the measurements made with stimulus tone levels of 50dB SPL or less.

D . N . B r a s s SFOAEs and cochlea function

Figure 8.3 Iso-frequency, iso-stimulus and iso-residual

suppression curves for a stimulus tone of frequency 1599Hz at levels of 20, 30, 40, 50 & 60 dB SPL. The measurements were made in the subject DB's left ear.

ISa-FREOUENCY SUPPRESSION CURVES

DB L E T T EAR n . 1S9BHZ < 2 0 d B S P L 0 ■13 18 re L E V EL - FL L E V E L (dB) .N -»L3

ISO-STIMULUS SUPPRESSION CURVES

DB L E FT EAR FL ' L S 9 8 H z 8 SOdB SPL 0 L8 DB L E F T EAR FL - L I ^ H z « 3 W B SP L DB L E FT EAR FL • L 5F 9H z « 3 9dB SPL -L8 F a L E V EL - FL L E V E L ( d B ) OB L E T T EAR FL • LT9SH z « *OdB S P L DB. L E F T EAR FL • L 5 9 8 H z « W dB SP L

OB. LETT EAR Fl . l599Hz « 60dB SPL

RATIO FL F g 0 -6 F a L E V EL - FL L E V E L (dB) I 3-L8 -12 F a L E V EL - FL L E V E L ( d B ) 1 d -L2 F 2 LEVÜ. - n LEVEL (dB) »L3 I»L3 -La -L2 RATIO F I f a

DB L EFT EAR FL - L T M H z < «OdB SPL

a DB LETT EAR FL ■ L T M H z < 3 0 d B SPL DB LETT EAR F\ - lS99Hz « 60dB SPL 0 -La •ta I -LS L-L3 1 d 2

Figure 8.3 (continued) r S D - R E S P O N S E S U P P R E S S IO N C U R V E S DB L EFT EAR F l « 1599H z g 2 0 d B SPL 0 -12 DB LEFT EAR F l » 1599H z 0 3 0 d B SPL

DB LEFT EAR F l * 1599H z 0 4i)c^B SPL

DB LEFT EAR F l - l5 9 9 H z 0 5 ù d B SPL RATIO F1.F2 DB LEFT EAR F l = 15 9 9 H z 0 6 0 d B SPL RES. LEVEL > RES. LEVEL -12 0 -12 PhD t h e s i s s u b m i t t e d F e b r u a r y 1995 p a g e 171

D . N . B r a s s SFOAEs and c o c h l e a funct i o n