studies on the aural reflex. iii. reflex latency as inferred from reduction of temporary threshold...

THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 34, NUMBER 8 AUGUST 1962

Studies on the Aural Reflex. III. Reflex Latency as Inferred from Reduction of Temporary Threshold Shift from Impulses

W. Dixon WArn)

Subcommittee on Noise, 327 South Alvarado, Los Angeles, California (Received May 10, 1962)

The effective protection provided by arousal of the acoustic reflex against temporary threshold shift (TTS) at 4 kc from exposure to impulse noise was studied by comparing the rate of growth of TTS produced by 2-min exposures to clicks of successively higher peak levels under two conditions: first, when the clicks were heard alone, and second, when each click was preceded at various intervals from 25 to 150 msec by a 1000-cps 100-dB pure tone presented to the contralateral ear. The results imply that the effective attenuation amounts to 1 dB at 25 msec, 5 dB at 62 msec, and 13 dB at 100 msec. Individual differences were large; some of the most slowly responding ears did not show significant protection until the delay reached 150 msec. The practical conclusion reached is that a reflex-arousal stimulus to be used to protect gun crews should precede firing by 150 msec or more. In regard to theory, the results support the hypothesis that the value of the acoustic impedance of an ear at any instant is an accurate indicator of the protection provided by the middle-ear muscles. However, there is also evidence that part of the protection may stem from a change, with reflex arousal, in the peak-limiting characteristics of the middle ear.

INTRODUCTION

HEN the normal ear is first exposed to sound of over 85 dB or so, the middle-ear muscles leap

into action, tightening the ossicular chain and providing up to 20 dB or more of effective protection. Unfor- tunately, if the sound in question is an impulsive sound such as a gunshot or the drop of a forge, the muscles have only locked the barn door after the horse has been stolen. Because of the latency of the reflex, the damage has been done by the time any protection is available. In order to obtain reflex protection against such sounds, it is necessary to pre-arouse the muscles by means of another loud sound that precedes the impulse.

What are the characteristics of the optimum reflex- arousal stimulus (RAS)? Clearly, two opposing con- siderations are involved here. The frequency, intensity, and duration of the RAS, and the time by which its onset precedes the impulse, should be designed to provide maximum attenuation of the impulse sound for the maximum number of persons. On the other hand, the RAS should not itself cause a TTS (temporary threshold shift) greater than the TTS it eliminates. In problems involving simultaneous maximization of one factor and minimization of another, we can usually expect that some sort of compromise will have to be made. No doubt that will also be the case here. We know

that the impedance change produced by a short impulse increases fairly linearly with SPL • once the reflex threshold has been reached, up to about 120 dB SPL. A similar condition has been shown to be true for TTS:

for a given spectrum and duration of stimulation, the TTS is proportional to the number of dB by which the level of the stimulus exceeds a base level of about 75

dB SPL? So as we raise the RAS level, the impedance

• A. R. M•ller, "Bilateral Contraction of the Tympanic Muscles in Man," Ann. Otol. Rhinol. & Laryngol. 70, 735-753 (1961).

2 W. D. Ward, A. Glorig, and D. L. Sklar, "Temporary Thresh- old Shift from Octave-Band Noise: Applications to Damage-Risk Criteria," J. Acoust. Soc. Am. 31, 522-528 (1959).

change will grow (and with it, presumably, the effective attenuation afforded by the muscles), but, alas, so will the TTS. Fortunately, if the RAS need be on only a short fraction of the time, the TTS produced by even a very high-intensity tone or steady noise may be tolerable, since TTS has been shown to be proportional to the fraction of the time that the steady tone or noise is on. 3 Therefore RAS level may not be a crucial problem.

In regard to spectral characteristics of the RAS, we have a happier set of circumstances. The most recent evidence is that the strength of short-term arousal of the reflex is constant for a given sensation level or perhaps loudness, 4,• regardless of frequency. Therefore we can probably use almost any loud pure tone or noise band, or, for that matter, a klaxon.

More uncertainty surrounds the question of the delay between onset of the RAS and the impulse. It is clear from M•ller's records 6 that the temporal course of the change of impedance associated with a short tone-pulse varies greatly from person to person. Al- though latency of the first action potentials in the stapedius is known to be on the order of 10 msec, 7 appreciable changes in acoustic impedance are not manifest until after 30 to 40 msec, and maximum change is not reached in some subjects for nearly 300 msec. Metz 8 has shown that the original balance is not re-

3 W. D. Ward, A. Glorig, and D. L. Sklar, "Dependence of Temporary Threshold Shift at 4 kc on Intensity and Time," J. Acoust. Soc. Am. 30, 944-954 (1958).

4 A. M611er, "Bilateral Contraction of the Tympanic Muscles in Man," Rept. No. 18, Speech Transmission Lab., Royal Inst. of Technology, Stockholm, Sweden (February 1961).

50. Jepsen, "Studies on the Acoustic Stapedius Reflex in Man," thesis, Aarhus University, Denmark (1955).

6 See reference 4, particularly his Figs. 5 and 15. ? H. B. Perlman and T. J. Case, "Latent Period of the Crossed

Stapedius Reflex in Man," Ann. Otol. Rhinol. & Laryngol. 48, 663-675 (1939).

80. Metz, "Studies on the Contraction of the Tympanic Muscles as Indicated by Changes in the Impedance of the Ear," Acta Oto-Laryngol. 39, 397-405 (1951).

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AURAL REFLEX. I I I. REFLEX LATEN CY 1133

stored until about 1 sec after cessation of the arousal stimulus.

The cutaneous stapedius reflex studied by Klockhoff ø shows even longer latency. In a group of 10 subjects, the time from onset of the shock stimulus applied to the ear canal to the first reflex contraction (presumably of the stapes) varied from 120 to 190 msec. However, this increased latency is attributed to the need for substan- tial temporal summation of the afferent impulses from the skin in order to initiate the muscle action. More

germane here is the fact that some 150 msec were required after onset of reflex action before the maximum was reached.

On the other side of the coin, Terkildsen •ø reports that in cases where he believes the observed impedance change to represent only stapedius action, the latency of arousal is as short as he can measure (about 10 msec). The tensor tympani has a much longer latency (on this point all studies agree). If it is true, therefore--and it seems likely from animal experimentsn--that the stapedius is primarily responsible for the protection, perhaps a RAS-impulse interval somewhat shorter than that implied by impedance measurements would be better. Terkildsen puts it as follows: "The rather long latencies found with measurements of the impedance are then best explained through the assumption that a real fixation of the whole ossicular chain does not take

place until both muscles are in action. As the tensor muscle is the slower of the two, such measurements actually record the latency of the muscle, which for protective purposes is of minor importance. "•'

The best temporal relation between RAS and impulse, then, is not at all obvious. Indeed, M•ller's results indicate that, as we might expect, the optimum spacing will vary with the other characteristics of the RAS. Although no systematic study of the distribution of latencies was made, M•ller •a indicates that the impedance change increased more swiftly when the RAS was a 500-cps tone than when it was 1500 cps. Similarly, the time required to reach maximum shift in impedance decreased as the RAS level increased.

So in an actual practical situation, as around a gun emplacement, it appears that the optimum RAS-impulse interval may have to be determined empirically once the frequency and duration of the RAS has been fixed.

One way of doing this would be to record the course of impedance changes in many individuals as a function

0 I. Klockhoff, "Middle Ear Muscle Reflexes in Man," Acta Oto-Laryngol. Suppl. 164 (1961).

•0 K. Terkildsen, "The Intra-Aural Muscle Reflexes in Normal Persons and in Workers Exposed to Intense Industrial Noise,' Acta Oto-Laryngol. 52, 384-396 (1960).

xx F. B. Simmons, "Middle. Ear Muscle Activity at Moderate Sound Levels," Ann. Otol. Rhinol. & Laryngol. 68, 1126-1143 (1959).

•' See reference 10, p. 395. •a A. R. M•ller, "Intra-Aural Muscle Contraction in Man,

Examined by Measuring Acoustic Impedance of the Ear," Laryngoscope 68, 48-62 (1958).

of time for the particular RAS chosen. (The intensity of the RAS to be used would be the average intensity to be expected at various positions around the gun. Clearly, the level at each crew member's ears will be different, so that where one may be stimulated with 120 dB, another may get only 100.) The RAS-impulse interval, which, for all practical purposes, could be made the duration of the RAS, would be that interval just long enough to allow maximum impedance change to be reached by even the slowest ears.

However, in view of the apparent uncertainty about the relation between impedance and protection, it may also be worthwhile to measure the effective attenuation

at a particular time more directly (although more laboriously) by actually measuring the change in TTS- producing ability associated with different RAS-impulse intervals. The present experiment was designed to study the course of the reflex in this way.

APPARATUS

The apparatus consisted of a high-intensity speaker (Altec 20801) to which a short square pulse of variable voltage was delivered. The listener placed his ear in a holder near the mouth of an exponential horn that coupled the speaker to the air. A microphone (Altec 21-BR-180) and impact noise meter (General Radio 1556-A) were used to measure and monitor the output click level. This apparatus, and the acoustic pulse produced, are described in more detail in an earlier article. •4

Opposite the other ear was a speaker used to generate the RAS, which in the present study was an unshaped (abrupt onset) 1000-cps tone of 250-msec duration at 100 dB SPL (measured at the position of the ear but with listener removed). The time between the onset of the RAS and the high-intensity pulse was controlled by means of a timing system (Tektronix 161 and 162 waveform generators). This timing system also controlled the entire recycling rate, which was in this case set at 1 pulse every 2.4 sec (25 pulses per min). The use of an interpulse interval this long guarantees that no residual reflex arousal from one pulse remains by the time the next pulse arrives.

SUBJECTS Ai•ID PROCEDURE

If one is to measure the effective attenuation provided by the RAS, one must use listeners who show some TTS from the pulses when no RAS is present. Furthermore, they must show this effect when the pulse level is well below the maximum obtainable; otherwise we cannot estimate the effective attenuation, which is the difference between (1) the pulse level required to produce a given TTS when the RAS is present and (2) the level needed to produce the same TTS when the RAS is omitted.

•4 W. D. Ward, W. Selters, and A. Glorig, "Exploratory Studies on Temporary Threshold Shift from Impulses," J. Acoust. Soc. Am. 33, 781-793 (1961).


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1134 W. DIXON WARD

An earlier study •5 had indicated that an average attenuation of at least 10 dB could be expected from this particular RAS. Since the maximum peak level of the pulses obtainable was 155 dB, only those subjects were used who showed, in at least one ear, a TTS at 4 kc of 20 dB or more, 30 sec after a 2-min exposure to pulses at 145 dB peak level. In order to get 6 new subjects who met this criterion (4 were already known to be "tender-eared" from earlier studies) it was necessary to test 14 individuals. Therefore the listeners used here represent approximately the upper half of the distribution in regard to susceptibility to TTS from clicks. In agreement with earlier results, the absolute thresh- olds for these subjects differed in no way from those of the rejected ("tough-eared") group.

The procedure was as follows. After a complete audio- gram at fixed frequencies from 500 to 13 000 cps, using an interrupted tone, the ear was given a 2-min exposure to pulses at 133 dB (that is, 50 pulses). Then the ear- phone was replaced, and the threshold at 4000 cps was measured for nearly 1 min. The listener then put his ear back in the holder, and he was given another 2-min exposure at the next higher level (steps were about 3.5 dB), following which the threshold at 4000 cps was again measured for 1 min. This sequence was followed until the TTS 30 sec after cessation of exposure exceeded 20 dB, whereupon the other frequencies were tested twice.

All subjects were first tested in this sequence without any RAS. On the following three sessions (one week apart), they were tested with delays of 25, 62, and 100 msec, in random order. At a final session, some of the ears were tested with a 150-msec delay, if it appeared feasible (if the 100-msec delay had not resulted in "complete" protection--i.e., no TTS after the highest level obtainable). Other ears were retested with no RAS in order to make sure no progressive changes in either stimulus or susceptibility had occurred (none had, as it turned out).

RESULTS

Figure 1 shows typical results, both the unequivocal and the ambiguous. Subject MK displayed no protection in the right ear for the 25- and 62-msec delays. At 100 msec, however, he was able to tolerate the full range of intensities (i.e., through 155 d•B) before reaching the criterion TTS0.5 (TTS 0.5 min after cessation of exposure) at 4 kc. The curve for 100-msec delay lies 13 dB to the right of the other three curves; therefore it is reasonable to say that there was an effective attenuation of 13 dB for this delay. This subject's left ear showed similar behavior.

Subject KA, on the other hand, showed a gradual increase in the effective protection as the delay became greater. The implied attenuation was about 4, 8, and

•5 W. D. Ward, "Studies on the Aural Reflex. I. Contralateral Remote Masking as an Indicator of Reflex Activity," J. Acoust. Soc. Am. 31t, 1034-1045 (1961).

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................. 150

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POSITIVE PEAK SPL (DB)

FIG. 1. Typical examples of the growth of TTS0.5 at 4 kc as the level of successive 2-min exposures to pulses at 25 pulses per min is raised from 133 dB SPL to a level sufficient to produce 20 dB or more of TTS. The short-dashed line shows the TTS produced when no reflex-arousal tone is present. The other curves illustrate the changes that occur when a 1000-cps arousal tone at 100 dB SPL is presented to the contralateral ear. The parameter is the delay, in msec, between onset of the arousal tone and presentation of the pulse.

14 dB with increasing delay (25, 62, and 100 msec, respectively) for the right ear, and 1, 6, and at least 13 for the left ear.

Unfortunately, the picture is not always this clear. This is, the curve for a given delay is not always shifted bodily to the left. Instead, the shape may be quite different. This was true, for example, for EW's left ear (Fig. 1). In this vein, the most bizarre results of all are those of YT. The TTS in the left ear showed an

abrupt rise in TTS from about 2 to 20 dB at some point for all delay times. However, going on to the next step (or even two steps) did not result in any further increase in TTS. Possible reasons for these deviant results will be discussed later.

Figure 2 shows the results from the three subjects who had so little protection at 100-msec delay that 150 msec was also used. In JM's case, the right ear just seems to have an unusually long latency, since about 13 dB of effective attenuation was produced by the 150-msec delay. However, the results for HM's right ear imply that he has a weak muscle; apparently full contraction is reached by 100 msec, since there is no increase in going to 150 msec, but the maximum attenuation is only about 5 dB.

The right ear of VA differs from the average in two ways. First, it is unusually susceptible (notice the large TTS produced by even the lowest intensity). Second, no attenuation could be demonstrated even with 150-


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AURAL REFLEX. I I I. REFLEX LATENCY 1135

msec delay. Both of these observations suggest that the reflex is inoperative in this ear. This supposition has been confirmed by my colleague, Robert Fleer, with an impedance bridge. Although VA's left ear shows normal response to a short high-intensity tone presented to the contralateral ear, not the slightest change in the balance of the bridge can be produced in the right ear when the arousal stimulus is in the left.

DISCUSSION

It was probably naive to expect that the protection against TTS produced by pulses afforded by the reflex could be summarized in a single number. In the first place, arousal of the reflex has been shown to attenuate low frequencies more than high. •6 Thus the reflex will not only "attenuate" the pulse, but will also change its shape. With arousal, therefore, one might reasonably expect that the change in TTS at 4 kc will not tell the whole story. It is conceivable, that is to say, that although the curve for TTS at 4 kc was shifted, say 13 dB by the RAS, the TTS at some other frequency might be just as great with RAS as without.

The plan of the present experiment did not permit a direct test of this possibility, since the other frequencies were tested only at the end, after the 20-dB criterion of TTS0.5 at 4 kc or the maximum obtainable click level had been reached. However, some indication of whether or not a shift in locus of maximum TTS is caused by reflex activation can be derived by comparing the terminal audiograms with 0- and 100-msec delay. This was done, and no such tendency was found. When the terminal TTS at 4 kc was greater, so was that at 1 and 10 kc, for example, and when the TTS at 4 kc was less (which it usually was, you will note from Figs. 1 and 2), the TTS at other frequencies was also reduced in roughly the same proportion.

A second reason that the expectation of a single number might be overoptimistic is the possibility that the effective protection may be in part a function of the click level. Loeb and Riopelle •7 obtained results that implied that there was more attenuation of a tone well above threshold than of one just at threshold, and sug- gested that the reflex activation may protect as much changing the peak-limiting characteristics of the middle ear as by reflection of energy due to stiffness increase. Certainly this hypothesis is tenable in view of the fact that the physical orientation of the ossicles may change when the muscles are energized, as indicated by studies that measure the inward or outward displacement of

•0 S. N. Reger, "Effect of Middle Ear Muscle Action on Certain Psycho-Physical Measurements," Ann. Otol. Rhinol. & Laryngol. 69, 1179-1198 (1960).

•7 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).

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Fro. 2. Growth of TTS0.• at 4 kc in 3 observers who showed little protective effect of the reflex at delays of 100 msec and less between arousal tone and impulse, and who were therefore tested with a 150-msec delay.

the eardrum. •8.•ø If this were the case, we might well expect that in some ears, an increase in click level might not produce a greater TTS. That is, once the pulse level reaches the point at which the ossicles are being driven as far as they will go, further increases will not have any additional effect.

Now there is some evidence in the present data that this maybe so. The best example is YT's left ear (Fig. 1), where the TTS after 152- and 155-dB clicks was no

greater than immediately after the 148-dB exposure. Other ears display similar characteristics, for example HM's left ear and both of JM's in Fig. 2. I suspect that the effect could be seen more plainly if the experiment had not involved successive exposures to increasing levels, but instead the subjects had been given a single level on a single day. As it is, you will notice, the TTS after a given click level represents not only the effect of 2 min of exposure to that level but also the residual effects of all the preceding levels. Therefore one would expect a slight growth of TTS with level in this par-

•s E. S. Mendelson, "Improved Method for Studying Tympanic Reflexes in Man," J. Acoust. Soc. Am. 33, 146-152 (1961).

•9 K. Terkildsen, "Acoustic Reflexes of the Human Musculus Tensor Tympani," Acta Oto~Laryngol. Suppl. 158, 230-236 (1960).


18:39:48

1136 W. DIXON WARD

TABLE I. Effective protection, in dB, against TTS at 4 kc from impulses, afforded by prior activation of the acoustic reflex by a 1000-cps tone at 100 dB SPL. N--20 ears.

Delay between arousal tone and pulse (msec) Statistic 25 62 100

Least -- 2 -- 5 -- 2

25 %ile --0.5 2 5 Median 1 5 13

75 %ile 2.5 10 > 15 Most 4.5 •15 > 20

ticular case even if level per se were not particularly relevant because of peak clipping.

The notion that peak limiting may be occurring, therefore, has considerable support. Additional evidence can be found in an earlier article. TM In several cases (the procedure was essentially the same as in the present study) the TTS at 4 kc showed a plateau similar to that seen here, even when there was no reflex-activating tone. For example, one subject, when exposed to successive 3-min series of clicks at 25 per min, showed no TTS0.5 at 4 kc through click levels of 142 dB, then showed 14 dB after 145, 148, 152, and 155 dB pulses.

So there are good reasons for hesitating to get an average value for the effective protection here. Indeed, I really cannot talk meaningfully about an average at all for the 100-msec delay, since many ears displayed so little TTS that the amount by which the curves were shifted to the left is simply indeterminate. However, the median protection can be calculated with more confidence, since the large indeterminate shifts are somewhat balanced by the ears whose results imply little protection.

Accordingly, the "protection" for each value of RAS delay was determined for each ear by measuring the difference in SPL between the curves at TTS0.5-- 20 dB. Medians of these values, together with the 25 and 75 centile values and the extremes are given in Table I.

It is clear, then, that at least 100 msec should be allowed for the reflex to become activated, and 150 will certainly be even better, since some ears apparently operate rather sluggishly. It is reassuring to find that a tone of short duration and moderate level can provide nearly 15 dB of effective protection against damage from impulses whose peak level is as high as 155 dB SPL. With such a moderate RAS, there is absolutely no danger that the RAS will itself produce an appreciable TTS.

These average values, and the large inter-subject variability, are in excellent agreement with the sample impedance-change records of M•ller's, •ø which indicate that in some individuals the reflex latency to a 100-dB tone may be over 100 msec, while in others it may be less than 40 msec. Therefore the present results do tend

•0 See reference 13, especially Fig. 5.

to confirm the hypothesis that relative changes in the acoustic impedance accurately reflect the relative re- jection of acoustic energy by the middle ear and therefore indicate the degree of protection of the inner ear. And since the stapedius has been shown to be by far the most important muscle in protection of the ear from permanent loss, at least in cats, •'• the net implica- tion is that even though an initial twitch of the stapedius may occur very early after onset of an acoustic stimulus, as Terkildsen maintains, full contraction is not reached until some time later. It is even possible that the latency of the muscle action in the contralateral ear may be greater than that of the ipsilateral ear; M•ller • has recently shown that strength of contraction is greater for the ipsilateral ear, so perhaps there is also a difference in latency.

Whether or not this same RAS would provide 13 dB of effective attenuation of gunfire from heavy arms, where peak levels may exceed 170 dB, is still open to question. Certainly reflex arousal cannot be considered exactly equivalent to a well-fitting earplug or muff, even if the effective attenuation were shown to be the

same, which it has not? I am referring here to the fact that with the RAS, the middle ear is still exposed to the full force of the impulse and may suffer mechanical damage. Thus, although the middle ear may be better prepared to withstand high pressures when the muscles have been tightened, it is hard to believe that they would not be safer with an earplug or muff instead. Certainly the pain associated with high-level pulses is not reduced by the RAS; in the present study, as in our earlier one, •4 the 155-dB pulses were usually painful even when they were not producing the slightest TTS. If pain is a warning of impending structural failure, it may be found that eardrum rupture may occur just as often from heavy gunfire with the use of a RAS as without one. Such a study should be performed with experimental animals.

As far as future research is concerned, the main conclusion I have come to as a result of the present experiment is that this method should be used only as

• D. A. Hilding, "The Protective Value of the Stapedius Reflex: an Experimental Study," Trans. Am. Acad. Ophthalmol. & Oto- laryngol. 65, 297-306 (1961).

• Although Fletcher claims to have demonstrated that a RAS very similar to the one used here provides nearly as much effective protection as the V-51R earplug, his results can be interpreted otherwise. What he actually found was that either the V-51R earplug or the 1000-cps RAS reduced the average TTS produced by 100 rounds of 30-caliber gunfire. The average TTS from 400 to 8000 cps was reduced from 19.2 to 2.5 dB by the plug, from 19.2 to 6.3 dB by the RAS. However, it should be clear from the present results that such an outcome could be produced by a RAS that provides an effective attenuation of only 5 dB or even less. After all, there is no direct relation between the decrease of TTS produced by some arbitrary number of rounds of gunfire and the effective attenuation. They are both measured in dB, it is true, but there the similarity ends; a 5-dB attenuation can change the TTS produced by 5, 25, or even 50 dB, depending on the duration of the exposure. See: J. L. Fletcher, "Comparison of the Attenua- tion Characteristics o{ the Acoustic Reflex and the VSI-R Ear-

plug," J. Aud. Research 1, 111-116 (1961).


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AURAL REFLEX. III. REFLEX LATENCY 1137

a check on the results of other methods of studying reflex activity. To find the optimum characteristics of a RAS (frequency, level, duration, and delay) would require months of tedious experimentation using this threshold shift technique. With the impedance bridge, however, one can rather quickly determine the RAS characteristics that give a maximum impedance change. Only then should one set up an experiment to verify that maximum reduction in TTS is indeed achieved at the same time.

CONCLUSIONS

(1) If a 1000-cps tone at 100 dB SPL is used to arouse the acoustic reflex and thus protect the inner ear from impulse noise such as gunfire, it should precede the impulse by at least 150 msec if full contraction of the middle-ear muscles is to be attained in some of the more

slowly responding ears. (2) However, some degree of protection exists, on

the average, for shorter delays' the effective protection was found to be about 1 dB when the delay was 25 msec, 5 dB for a 62-msec delay, and 13 dB at 100 msec.

(3) Individual differences are large; the maximum protection measured here varied from zero, in an ear that is suspected to have an inoperative stapedius muscle, to more than 15 dB.

(4) Although estimates of the effective protection can thus be obtained using the present technique, this technique is not recommended for routine work, since it is quite cumbersome and time-consuming.

(5) Finally, there is even reasonable doubt that the protection against impulses can be subsumed under a single number. Not only is the attenuation produced by reflex activation probably a function of the particular wave shape (i.e., spectral characteristics of the impulse), but in addition there is mounting evidence that part of the protective effect may be due to a change in the limits of excursion of the ossicles, which thus produce peak clipping at a lower sound level.

ACKNOWLEDGMENT

This investigation was supported by research grants from the National Institutes of Neurological Diseases and Blindness, Public Health Service.


18:39:48

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