temporary threshold shifts following monaural and binaural exposure

Received 12 April 1965 4.4.5, 4.3

Temporary Threshold Shifts following Monaural and Binaural Exposure

W. D•xo• W•

Department of Otolaryngology, University of Minrlesota, Minneapolis, Minnesota

The temporary threshold shifts (TTS) following monaural and binaural exposure to three different high- intensity stimuli were compared in 49 listeners. In all cases, the binaural exposures produced less TTS than the monaural, though the difference was greatest at lower frequencies. Results are interpreted to indicate that the middle-ear muscles contract more vigorously for binaural exposure than for monaural, thus producing more reduction of the effective intensity of lower-frequency stimulus components. However, the possi- bility of influences exerted by efferent or cochleo-cochlear systems cannot be ruled out. The relation of these results to the fact that industrial hearing loss first develops at 4000 cps is discussed.

INTRODUCTION

ECENTLY, in an experiment concerning the difference between octave-band noises and pure tones in ability to produce temporary threshold shift (TTS), the evidence indicated that much of the difference could be attributed to the action of the middle-ear muscles. •

Figure 1, which is taken from the earlier study, illustrates the point. The top (dashed) curve shows the course of recovery at 1000 cps following exposure to a 700-cps tone at 125 dB SPL presented monaurally for 5 min. On the other hand, when a narrow band of noise (about -• oct in width) at the same level is used instead, a much lower TTS is obtained (solid curve). Now if, concomitantly with the tone, the noise is presented to the contralateral ear, the TTS produced by the tone (dot-dashed curve) is reduced to the value produced by the noise alone. From these results, it seemed clear that the greater efficiency of sinusolds in producing TTS could be ascribed to the reflex action of the middle-ear

muscles' noise, because of its time-varying character, produces a stronger or at least more sustained contraction of the muscles than a pure tone, and this contraction provides more protection by peak-clipping or attenuating the incoming signal to a greater degree. In the present instance, because reflex action is con- sensual (the strength of the contraction in the unexposed ear is nearly as great as that in the exposed ear)? the

•W. D. Ward, "Damage-Risk Criteria for Line Spectra," J. Acoust. Soc. Am. 34, 1610-1619 (1962).

• A R •[ 11er • . . • • , "B'lateral Contraction of the Tympanic Muscles in Man," Ann. Otol. Rhinol. Laryngol. 70, 735-753 (1961).

noise caused the magnitude of the contraction to be that appropriate to the noise, and so reduced the "effective level" of the tone (the level reaching the inner ear). It was also found that the TTS produced in the ear receiving noise, under the dichotic conditions, was not different from that following noise alone, which implied that the presence of tone in the opposite ear did not affect the strength of the contraction in the noise ear.

Analogous experiments with noises and tones at 1400 cps showed a similar though less dramatic reduction in TTS. However, the TTS produced by a 2000-cps tone was unchanged by simultaneous presentation of narrow-band noise of the same level and center fre-

quency. All these observations, and others, a are con- sistent with the notion that only the transmission of low

• 30

Fro. 1. Course of recovery • from TTS at 1000 cps fol- o 2o lowing a 5-min exposure at o o 125 dB SPL to tone and -

narrow-band noise. [From • •o Ward.•']

o I 25

-- •"•'-700-CPS TONE ALONE ,.

-- 700-CPS TONE; "• '•.NOISE IN CONTRALATERA'•""..,..,

•"' •,,..E A R

5 I 2 4

RECOVERY TIME (MIN)

3 p.O. Thompson and R. S. Gales, "Temporary Threshold Shifts from Tones and Noise Bands of Equivalent rms Sound- Pressure Level," J. Acoust. Soc. Am. 33, 1593-1597 (1961).

121

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122 W.D. WARD

frequencies is significantly affected by the acoustic reflex.

At this point, I would have been willing to support the thesis that the strength of bilateral contraction of middle-ear muscles was determined by the more prepo- tent of the stimuli presented to the right and left ears, respectively. That is, if stimulus A produced reflex action of strength X, and stimulus B produced Y, then in the dichotic situation, the strength would be either X or Y, whichever was the greater. However, later reflection indicated that this would have been a care-

less and illogical conclusion, since it would postulate that the reflex center, which apparently sends equivalent efferent signals to the muscles of both ears, considers only one or the other of the afferent input channels. Neurologically, at least, it would seem more reasonable to expect that its output would be influenced by both inputs.

Suppose, for example, that the strength of contraction is determined by the loudness of the total percept, or, in view of the short latencies involved, 4 by the amount of neural activity at some subcortical center fed by both ears--neural activity that, transmitted up the auditory chain, eventually provides the basis for loudness judgments. If so, then in the dichotic situation, the amount that each of the two signals will contribute to the determination of the reflex strength should be correlated with its contribution to the over-all loudness. In this formulation, the results of the earlier experiments can be given a first-order explanation in terms of perstimulatory fatigue. It is well known that tones show more perstimulatory adaptation than noise. 5 Presumably this means that the reduction in neural activity with continued stimulation will be greater for tone than noise, if both are at the same level. Thus, after the first few seconds of the 5-min dichotic exposure, the noise contributed most of the afferent signal to the reflex center--such a large proportion, indeed, that the contribution from the tone became negligible.

There are several implications of this hypothesis. For example, what happens if the same noise or tone is presented to both ears simultaneously? Since the loudness of a binaural stimulus is about twice that of a monaural signal, 6 the schema above requires that there should therefore also be more reflex contraction, and hence a reduction in the TTS produced by stimuli below 2 kc/sec in frequency.

This prediction was examined in three experiments embedded in a series of tests of auditory fatigue concerned with the prediction of individual differences in susceptibility to permanent loss. In this series, the same 24 male and 25 female young, normal-hearing adults

4 H. B. Perlman and T. J. Case, "Latent Period of the Crossed Stapedius Reflex in Man," Ann. Otol. Rhinol. Laryngol. 48, 663-675 (1939).

5 E. C. Carterette, "Loudness Adaptation for Bands of Noise," J. Acoust. Soc. Am. 28, 865-871 (1956).

o G. S. Reynolds and S.S. Stevens, "Binaural Summation of Loudness," J. Acoust. Soc. Am. 32, 1337-1344 (1960).

were tested weekly for a period of about six months. Pre- and post-exposure thresholds were determined via B•k•sy audiomerry (50% duty cycle, 0.5-sec period, 20-msec rise and fall times, threshold defined as average of tracing during 20-sec observation period). All pre- testing, exposure to fatiguing stimulus, and postexposure testing was done without moving the PDR-8 earphones in MX-41/AR muffs (calibrated on ASA 6-cc coupler using both Altec and Brtiel & Kjeer con- denser microphone systems).

I. EXPERIMENT 1

On weeks 14 and 15, the listeners were exposed for 3 min to a 1400-cps tone at 115 dB SPL. Half of the listeners were exposed monaurally [right ear (RE) first-] on the 14th week, binaurally on the 15th, with the other half getting the reverse order. Following the binaural exposures, the threshold at 1400 cps in the right ear was tracked for 1 min. Beginning at 1 min postexposure, the following test sequence was followed (20 sec each): 1.4 (kc/sec) LE, 1.4 RE, 2.0 RE, 2.0 LE, 2.8 LE, 2.8 RE, 4.0 RE, 4.0LE. The series after monaural exposure followed the same sequence, except that intermediate frequencies (1.7, 2.4, 3.3, 4.7 kc/sec) were tracked on the exposed ear, replacing tests on the contralateral ear. Then, by means of generalized recovery curves based on the entire 21 weeks of testing (to be published elsewhere), the TTS measured at each point was converted to TTS•. (TTS 2 min after cessation of exposure).

Table I gives the average values of TTS2 for the 98 ears. They are also plotted in the left panel of Fig. 2. Although the differences between the binaural and monaural shifts are not large, the level of statistical significance at 2 and 2.8 kc/sec exceeds 0.001 (chi- square). The maximum effect occurs at 2 kc/sec, where the binaural exposure gave 4.1 dB less TTS (a reduction of 21%). To interpret this TTS reduction, we can consider some unreported data gathered recently in order to establish damage-risk criteria for short exposures. A 3-min monaural exposure to a 1200-cps tone at 117 dB SPL gave 19 dB of TTS•. at 1.7 kc/sec, while 112 dB gave 12 dB. Thus a 7-dB difference in maximum TTS•. was produced by a 5-dB change in level. If the intensity versus TTS functions for 1400 and 1200 cps are identical, the present 4-dB change in TTS would represent a difference in input level of about 3 dB.

Test

frequency (kc/sec) Mortaural Binaural

1.4 4.2 5.1 2 19.8 15.7 2.8 14.4 11.6 4 9.1 8.0

TABLE I. Temporary threshold shifts, in dB, 2 min after 3-min exposure, monaurally or binaurally, to a 1400-cps tone at 115 dB SPL. Mean of 98 ears of 49 listeners.


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TTS FROM MONAU'RAL AND B INAURAL EXPOSURE 123

• BIN 1.4 2 2.8 4 1.4 2 2 8 4

FREQUENCY (KC)

Fro. 2. Comparison of temporary threshold shifts produced by monaural and binaural exposures to (a) a 1400-cps pure tone at 115 dB SPL (left panel); (b) shifts produced by monaural and binaural exposures to 120-dB noise, 700-1400 cps, and by dichotic exposure to noise and tone (right panel). ----: Monaural. ----: Binaural (noise both ears). •: Dichotic (120-dB tone in other ear). ½: TTS from 120-dB tone with noise in other ear.

However, this figure of 3 dB is probably the result of a fortuitous combination of stimulus conditions. It does

not necessarily imply, for instance, that twice as much excitation is reaching the reflex center under binaural as under monaural conditions, with a simple summation of excitation from the right and left channels. The reason that it does not is that we have here a feedback loop. With the increased input when the second ear is stim- ulated, the total activity of the reflex center also in- creases, and this produces an increase in middle-ear- muscle activity, which produces a reduction in the effective intensity of the input, so that the total activity in the reflex center must be somewhat less than twice as

much as that under monaural stimulation, even if there were simple summation.

Besides, the 3 dB is only an average figure. The actual difference in the effective level of cochlear

stimulation might be much greater at the beginning of the 3-min exposure than at the end, or vice versa (or, it could be 3 dB throughout).

II. EXPERIMENT 2

At any rate, it is clear that binaural exposure to pure tones below 2 kc/sec will give less TTS than monaural exposure. In the next experiment, the basic 3-min exposure stimulus was an octave band of noise (700- 1400 cps) at 120 dB SPL. On either week 16 or 17, each listener was exposed to the noise monaurally (again RE first). On the other week, half of the listeners were exposed to binaural noise, and half received a dichotic exposure: noise in the RE but a 120-dB 1400-cps tone in the LE. TTS was calculated as before.

Average result.s are shown in the right panel of Fig. 2. Where the monaural noise produces a TTS2 of 10.5 dB at 2 kc/sec, only 6.1 dB is found after binaural exposure, a reduction of 42%. Only 3 of the 24 ears in this comparison showed a difference in the opposite direction, so

chi-square shows this to be significant beyond the 0.001 level. The 120-dB tone in the other ear reduces the

noise-induced TTS2 at 2 kc/sec to 7.4 dB, a reduction of 30% (p=0.01), showing once more that, although a pure tone does not arouse the reflex to the same extent as a noise of the same level, its contribution to reflex arousal is far from negligible.

Also shown in Fig. 2 is the TTS produced by the 120- dB tone in the dichotic situation (upper dotted curve). The fact that this TTS is greater than that produced by the noise alone appears at first glance to contradict the earlier study (as in Fig. 1) in which addition of contralateral noise reduced the tone-induced TTS to the

same value as that produced by noise alone. Figure 3 shows the earlier results in which the stimuli had the

same frequency characteristics as in Expt. 2. However, it must be noted that the points in Fig. 3 ("tone; noise in other ear") refer to the situation in which the contralateral noise was about -} oct in width. The disparate findings can be reconciled by assuming that, although one gets approxi-mately the same TTS from a narrow band of noise as from an octave band (implied by the congruence of the three lower curves in Fig. 3), the narrow-band noise has greater reflex-arousing ability. I was therefore evidently wrong in concluding from the earlier results that "the critical-band concept is not relevant at high intensities." It appears that, in the absence of the action of the middle-ear muscles, the narrow-band noise, with its higher critical-band level, would indeed produce more TTS--but, because it induces a stronger reflex contraction, its TTS effectiveness is self-limited to about that of the octave-band

noise. (Let me hasten to add, however, that although I may recant in regard to the appropriateness of the critical-band concept, I am not retreating from my position • that damage-risk criteria for pure tones should be determined empirically rather than by applying a single "correction factor" such as the 10-dB rule of AFR 160-3 (Ref. 7), or my belief that assuming a «-oct band of noise to be 5 dB more dangerous than a 1-oct band of noise at the same level is a gross oversimplifica- tion. Although the latter assumption was incorporated

Fro. 3. Course of recovery of TTS at 2000 cps after 5-min exposures to a 1400- cps tone and to noises of various widths at 113 dB

SPL. All noises had the same upper cutoff frequency, rather than a com- mon center frequency. •From Ward. •-] . 1400-cps tone. ß ..... •-oct band.- .... «-oct band. --' 1-oct band. •' Tone; noise in other ear.

.25 5 I 2 4

RECOVERY TIME (MIN)

* Anon., "Hazardous Noise Exposure," USAF Regulation No. 160-3 (29 Oct. 1956).


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124 w.D. WARD

into a recent set of damage-risk criteria 8 developed by a group of which I was a member, it was accepted only over my protests, which I reiterate now. The fact that an isolated -}-oct band would probably be 5 dB more effective than a 1-oct band in producing TTS if there were no middle-ear muscles to consider does not alter

the fact that their action equalizes the two stimuli, at least for low-frequency stimuli. After all, damage-risk criteria should be based on normal ears, not those with inoperative middle-ear muscles.)

So much for what the TTS2 from tone in Expt. 2 was not; now let us consider what it was. Comparison of the right and left panels of Fig. 2 will show that the TTS2 from a 120-dB tone with a 120-dB 1-oct-band noise in the contralateral ear was about the same as that from a 115-dB tone when a 115-dB tone was in the contralateral

ear. If equal values of TTS imply equal activities at the basilar membrane, then the present equivalence means that the joint effectiveness of the middle-ear-muscle activity from 120-dB-tone-plus-120-dB-noise was 5 dB greater than that from 115-dB-tone-plus-l15-dB-tone. Because the relative contribution to the activity in the reflex center is not the same for the 120-dB tone in the first instance and the 115-dB tone in the second, it is not necessarily true that a 120-dB noise produces 5 dB more of effective reduction of the input signal than a l15-dB tone; it is clearly more, but how much more is yet unknown. In order to establish such values, it would be necessary to use a wide range of different contralateral stimuli.

III. EXPERIMENT 3

A final comparison between monaural and binaural stimulation involved a 3-min exposure to a composite noise designed to generate moderate but not excessive values of TTS• from 1 to 8 kc/sec. This noise was generated by mixing three octave bands: 700-1400 cps at 125 dB SPL, 1400-2800 at 116, and 2800-5600 at 110 dB. On week 21, all listeners were given binaural exposures, and then on week 22 the left ears were exposed monaurally (the right ears were used in a different experiment altogether).

The resulting adjusted values of TTS• are shown in Table II and Fig. 4. The average reduction of TTS•

Test

frequency (kc/sec) Monaural Binaural

1 7.3 3.1 1.4 11.6 7.0 2 12.6 7.8 2.8 13.8 10.4 4 14.7 12.9 5.6 9.5 8.9

T^BL•, II. Temporary threshold shifts, in dB, 2 min after 3-min monaural or binaural

exposure to a composite noise (700-1400 cps at 125 dB, 1400-2800 cps at 116 dB, 2800-5600 cps at 110 riB). Mean of 49 left ears.

8 K. D. Kryter (Chairman), "Hazardous Exposure to Inter- mittent and Steady-State Noise," Natl. Acad. Sci.-Natl. Res. Council Comm. Hearing, Bioacoust., Biomech. Working Group 46 Rept. (Jan. 1965).

(males and females combined) from monaural to binaural amounted to about 40% at 1.4 and 2 kc/sec (p<0.001), 25% at 2.8 kc/sec (p=0.01), 12% at 4 kc/sec, and 6% at 5.6 kc/sec (not significant).

IV. DISCUSSION

One important conclusion from these results is that TTS studies employing monaural exposures are not directly applicable in the development of damage-risk criteria where the working assumption is that the permanent losses from a given noise are proportional to (or at least cannot exceed) the TTS, and therefore that equal-risk criteria will correspond to equal-TTS contours. If these contours are based on monaural ex-

posures, then they will tend to be overprotective, in general, for real-life binaural exposures, especially at low frequencies.

The only direct comparison of monaural and binaural exposures that I have been able to find in the literature is that of Hirsh, 9 who concluded' "In general, there appears to be little difference between a binaural and a monaural exposure so far as the TTS in the average ear is concerned. What difference there may be suggests that a binaural exposure has less of a fatiguing effect than a monaural exposure." This somewhat equivocal conclusion arose because, although he found a significant difference in TTS at 1400 cps following monaural versus binaural 1-min exposures to a 1000-cps tone at 100 dB SPL, as would be expected from the present results, the difference at 4 kc/sec following a 3-min exposure to white noise at 110 dB SPL was not appre- ciable. A white-noise stimulus and a 4-kc/sec test frequency, however, happens to be a combination that will not reveal the difference between monaural and binaural exposures. White noise has a spectrum with a downward slope of only 6 dB/oct; therefore, after the effectiveness of the lows has been diminished by the middle-ear muscles, most of the TTS produced at 4 kc/sec will be a result of the energy in the 2.5- to

/ BIN ,.

5-- /

I 1.4 2 2.8 4 5.6

FREQUENCY (KC)

Fro. 4. Comparison of temporary threshold shifts produced by monaural and binaural 3-min exposures to composite noise (over-all level about 126 dB).

9 I. J. Hirsh, "Monaural Temporary Threshold Shift following Monaural and Binaural Exposures," J. Acoust. Soc. Am. 30, 912-914 (1958).


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TTS FROM MONAURAL AND BINAURAL EXPOSURE 125

3-kc/sec range, which is unaffected by the strength of the reflex contraction.

The picture that emerges is that contraction of the middle-ear muscles depends on the over-all loudness of the stimulus, and this contraction reduces the effective intensity of stimulus components below 2 kc//sec, thus reducing the TTS produced by these components. These results may be relevant to the so-called "boilermaker's notch"; a permanent hearing loss whose maximum occurs at about 4 kc/sec. This tonal gap is produced by steady and by impact noises, by noises with both rising and falling spectral shapes. One reason for this par- ticular maximum is that the auditory system has resonances in the 2- 3-kc/sec range, so that energy in this region is enhanced. Thus, since auditory fatigue is greatest half an octave above the exposure frequency, the most TTS is to be expected to occur at about 4 kc/sec. However, the differential effect of the middle- ear muscles may also contribute to the phenomenon. As Fig. 4 shows, even a noise with much greater energy in the 700- to 1400-cps band than in higher bands produces the most effect at 4 kc/sec, more so for binaural than for monaural exposure. According to the present hypothesis, this occurs because the greater the over-all intensity, the greater the reduction in effective intensity of the lower-frequency components relative to that of the high frequencies. Thus, the maximum effect would be expected to occur «-1 oct above the lowest frequency that is unaffected by the middle-ear muscles--that is, again at 3 or 4 kc/sec.

I suppose that the acoustic reflex is not the only

possible explanation for the monaural-binaural differences. One could jump on the current bandwagon and call on the olivo-cochlear efferent bundle and

cochleo-cochlear pathways to account for the results. It would, however, be a somewhat strained hypothesis by the time that it finished explaining why low-frequency fatigue is more suppressed by efferent signals from the opposite ear than is high-frequency fatigue. The reflex theory seems at the moment to be simpler, though efferent factors cannot be completely discounted.

The question still remains, however, as to how the reduction of effective intensity occurs. To what extent is this reduction a simple attenuation of the stimulus signal caused by the shift in acoustic impedance of the middle ear, and how much instead must be attributed to some form of peak clipping, as Loeb and Riopelle •ø suggested a few years ago (peak clipping perhaps associated with the change in mode of vibration of the stapes •) ? There seems at the present to be no compelling evidence that either mechanism can, alone, fully account for the relevant empirical observations.

ACKNOWLEDGMENT

This research was supported by the Public Health Service, U.S. Department of Health, Education, and Welfare.

•0 M. Loeb and A. J. Riopelle, "Influence of Loud Contralateral Stimulation on the Threshold and Perceived Loudness of Low- Frequency Tones," J. Acoust. Soc. Am. 32, 602-610 (1960).

n G. von B(•k(•sy, "The Structure of the Middle Ear and the Hearing of One's Own Voice by Bone Conduction," J. Acoust. Soc. Am. 21,217-232 (1949).


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temporary threshold shifts following monaural and binaural exposure

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