temporary threshold shift and damage-risk criteria for intermittent noise exposures

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Received 16March 1970 4.4.5• 4.6;8.1 Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures W. Dxxo• WA•x> IIearing Research Laboratory, University of Minnesota, Minneapolis, Minnesota 55455 Twelve normal-hearing students were exposed forupto 8 h to steady and intermittent noises that,according to the CHABA damage-risk criteria (DRC) proposed in 1966, should produce an average TTS2(temporary threshold shift measured 2 min after exposure) no greater than 10 dBat 1kHz orbelow, 15 dBat 2 kHz, or 20dBat 3 kHz orabove. This objective appears tohave been attained forsingle uninterrupted exposures and forintermittent exposures involving short (below 3-5 min)noise bursts and recovery periods. However, the cited limits ofTTS2 tend to beexceeded when thenoise bursts arequite long (10 minormore), owing to an erroneous previous assumption about the course of recovery during the quieter intervals between successive bursts. Modification of the DRC is therefore indicated. However, a more vexing finding is that intermittent exposures to high-frequency high-energy noise--either long bursts or short--often produce a delayed recovery; that is,more than16h of rest is required to restore theinitialsensitivity. Thus, in an industrial situation, some workers in such a noise may in essence have a chronic TTS. In case such a chronic auditory fatigue may lead to permanent damage, extreme caution should beexercised in exposing men to levels above 100 dB in the!500-Hz range orhigher, even when the ratio ofnoise duration topause duration isquite small. It is proposed that limiting values of TTSa0 or TTSx000 ratherthanTTS2might be more validindicators of relative noxiousness of various exposure patterns. Alternate DRC presently under consideration arealso discussed. INTRODUCTION In 1965 the Committee onHearing, Bioacoustics and Biomechanics of the NationalAcademy of Sciences National Research Council(CHABA), an advisory group to the U.S. Armed Forces, proposed a set of damage-risk criteria (DRC) for both continuous and intermittent exposures to steady •(asopposed to im- pulsive) noise (Kryter et al., 1966). These D RC were based on two fundamental assumptions. The first was that a certain degree of noise-induced permanent thres- hold shift (NIPTS) could be tolerated; the CHABA working groupsomewhat arbitrarily decided that a sound-exposure pattern would beconsidered acceptable if a lifetime of daily exposure produced no morethan 10dB of NIPTS at 1 kHz or below, 15dB at 2 kHz, or 20 dB at 3 kHz andabove in the average worker. The second assumption was that the eventualNIPTS from habitual exposure to a given noise will not exceed the temporary threshold shift (TTS) produced by a single- day exposure. For purposes of defining the D RC in question, it was simply assumed that thetwoare equal, or moreprecisely, that the average TTS2 (TTS mea- sured 2 min after leaving the noise) will be identical to theaverage NIPTS resulting froma lifetime of exposure. Thus, the CHABA DRC should,in theory, indicate those patterns of exposure that produce precisely 10 dB of TTS2 at 1 kHz or below, 15 dB at 2 kHz, or 20 dB at 3 kHz or higher. Enough empiricaldata on TTS from singleunin- terrupted exposures was on hand to allow construction of a setof curves indicating the permissible duration of a single exposure to variouslevelsof octave bandsof noise (Ward, 1966). The resulting criterion for an 8-h exposure, for example, was quite comparable to previous DRC--viz., near 85 dB SPL for octavebands centered at 1000 Hz and above, but somewhat greater for lower frequencies, reaching 100 dB SPL for 50-100-Hz noise. The lesser noxiousness of lowfrequencies was even more pronounced for shorter exposures, 15-minexposures to 1200-2400-Hz noise at 100 dB SPL and to 150-300-Hz noise at 125 dBSPLbeing equally undesirable. However, some extrapolation from short exposures to longer ones was involved here. Two different procedures were usedto develop the criteria for time-varying noise. Equations expressing the growth of TTS as a function of level and duration of The Journalof the Acoustical Society of America 561 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 155.33.16.124 On: Sat, 22 Nov 2014 10:15:01

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Page 1: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

Received 16 March 1970 4.4.5• 4.6; 8.1

Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

W. Dxxo• WA•x>

IIearing Research Laboratory, University of Minnesota, Minneapolis, Minnesota 55455

Twelve normal-hearing students were exposed for up to 8 h to steady and intermittent noises that, according to the CHABA damage-risk criteria (DRC) proposed in 1966, should produce an average TTS2 (temporary threshold shift measured 2 min after exposure) no greater than 10 dB at 1 kHz or below, 15 dB at 2 kHz, or 20 dB at 3 kHz or above. This objective appears to have been attained for single uninterrupted exposures and for intermittent exposures involving short (below 3-5 min) noise bursts and recovery periods. However, the cited limits of TTS2 tend to be exceeded when the noise bursts are quite long (10 min or more), owing to an erroneous previous assumption about the course of recovery during the quieter intervals between successive bursts. Modification of the DRC is therefore indicated. However, a more vexing finding is that intermittent exposures to high-frequency high-energy noise--either long bursts or short--often produce a delayed recovery; that is, more than 16 h of rest is required to restore the initial sensitivity. Thus, in an industrial situation, some workers in such a noise may in essence have a chronic TTS. In case such a chronic auditory fatigue may lead to permanent damage, extreme caution should be exercised in exposing men to levels above 100 dB in the !500-Hz range or higher, even when the ratio of noise duration to pause duration is quite small. It is proposed that limiting values of TTSa0 or TTSx000 rather than TTS2 might be more valid indicators of relative noxiousness of various exposure patterns. Alternate DRC presently under consideration are also discussed.

INTRODUCTION

In 1965 the Committee on Hearing, Bioacoustics and Biomechanics of the National Academy of Sciences National Research Council (CHABA), an advisory group to the U.S. Armed Forces, proposed a set of damage-risk criteria (DRC) for both continuous and intermittent exposures to steady •(as opposed to im- pulsive) noise (Kryter et al., 1966). These D RC were based on two fundamental assumptions. The first was that a certain degree of noise-induced permanent thres- hold shift (NIPTS) could be tolerated; the CHABA working group somewhat arbitrarily decided that a sound-exposure pattern would be considered acceptable if a lifetime of daily exposure produced no more than 10 dB of NIPTS at 1 kHz or below, 15 dB at 2 kHz, or 20 dB at 3 kHz and above in the average worker. The second assumption was that the eventual NIPTS from habitual exposure to a given noise will not exceed the temporary threshold shift (TTS) produced by a single- day exposure. For purposes of defining the D RC in question, it was simply assumed that the two are equal, or more precisely, that the average TTS2 (TTS mea- sured 2 min after leaving the noise) will be identical to

the average NIPTS resulting from a lifetime of exposure. Thus, the CHABA DRC should, in theory, indicate those patterns of exposure that produce precisely 10 dB of TTS2 at 1 kHz or below, 15 dB at 2 kHz, or 20 dB at 3 kHz or higher.

Enough empirical data on TTS from single unin- terrupted exposures was on hand to allow construction of a set of curves indicating the permissible duration of a single exposure to various levels of octave bands of noise (Ward, 1966). The resulting criterion for an 8-h exposure, for example, was quite comparable to previous DRC--viz., near 85 dB SPL for octave bands centered at 1000 Hz and above, but somewhat greater for lower frequencies, reaching 100 dB SPL for 50-100-Hz noise. The lesser noxiousness of low frequencies was even more pronounced for shorter exposures, 15-min exposures to 1200-2400-Hz noise at 100 dB SPL and to 150-300-Hz

noise at 125 dBSPL being equally undesirable. However, some extrapolation from short exposures to longer ones was involved here.

Two different procedures were used to develop the criteria for time-varying noise. Equations expressing the growth of TTS as a function of level and duration of

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Page 2: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

W. D. WARD

exposure all implied that no TTS2 would be developed by octave-band noises whose levels were about 75 dB or below (Ward, Glorig, and Sklar, 1959a). If, then, one defines a quantity EL (for "effective level") as the amount by which the momentary SPL exceeds 75 dB (with negative values excluded; levels below 75 dB SPL are equivalent to EL--O), it is found that for noise bursts whose duration is short relative to the total

exposure time (up to 2 min or so for 2-h exposures) the TTS2 is proportional to the average EL over the entire exposure period; that is,

TTS•. =-- (EL)dt. T 0

Thus, if a noise alternated regularly between 85 and 105 dB SPL, the TTS2 would be the same as that produced by a steady exposure at 95 dB (not by one at 102 dB as predicted if the average energy were what was impor- tant). Similarly, if a noise of 105 dB were on only one- third of the time and off two-thirds (i.e., had an on- .fraction of one-third), then the TTS2 after a given dura- tion of exposure would be the same as that produced by continuous 85 dB--namely, one-third of the TTS2 after the same duration of exposure at 105 dB continuously. I have called this the on-fraction rule.

By using this relation, it was easy to determine, for a given on-fraction, how much greater than for a continuous noise the duration or intensity could be before the criterion TTS2 of 20 dB (or 15, or 10, as the case might be) were reached. Again, however, extrapola- tion was involved. The on-fraction rule had only been shown to hold for noises up to 110 dB SPL for octave- band noises below 1000 Hz in center frequency and only up to 100 dB for those bands above 1000 Hz (Ward, 1962a); whether or not it held for higher intensities was unknown. Yet CHABA criteria were given for inter- mittent exposure to levels of up to 125 or even 130 dB SPL, for octave bands of noise between 300 and 4800 Hz.

For exposures involving on- and off-periods of more than 2 min, a more complicated procedure was necessary. Suppose, for example, that we have an exposure pattern of 10 min of noise alternating with 30 min of effective quiet, and we want to know what level can be tolerated if the TTS2 is not to exceed the criterion

by the end of the last 10-min exposure of the working day. Here an iterative procedure is used; one must: (1) calculate, using growth curves relating TTS to duration of exposure, the TTS2 produced by the first 10-rain exposure; (2) by means of generalized recovery curves, calculate the residual TTS remaining at the end of the 30-min quiet period; (3) determine how long an exposure would have had to be in order to have produced a TTS2 equal to this residual TTS; (4) add this time (the exposure equivalent--see Ward et al., 1959c) to the 10 min of the second exposure and again apply the equation for growth--this total time will accurately

predict the TTS2 observed at the end of the second exposure; and (5) again calculate the residual TTS at the end of the second 30-min quiet period, as in step (2), and so on.

Now, a crucial assumption in this iterative procedure is that the rate of recovery of TTS is independent of how the TTS was produced, so that a universal set of re- covery curves can indeed be used. It had been shown, for example (Ward et al., 1959b), that recovery from a TTS2 of 20 dB proceeded at the same rate no matter whether the TTS was produced by 12 min of noise at 106 dB or by 117 min at 90 dB. This lack of dependence, on either test frequency or exposure frequency, of the course of recovery from a particular value of TTS2 produced by a single continuous exposure has recently been confirmed by Smith and Loeb (1969). Thus, a single set of re- covery curves was used in calculating the residual TTS at the end of the rest periods. This procedure is des- cribed more completely elsewhere (Ward, 1966).

Unfortunately, this crucial assumption cannot be correct for the interrupted exposures of concern here, because it can lead to an absurd prediction if successive noise exposures are not at the same level. This was pointed out by Dr. Arndt von Ltipke of the Bundes- institut for Arbeitsschutz in Koblenz (personal com- munication). His argument can be summarized in the following way.

Figure 1 shows some curves used in the calculation of the CHABA DRC. On the left are a family of curves showing the growth of TTS2 as a function of time, with level the parameter, and on the right are recovery curves that fit the actual recovery data when exposure has been to a single noise burst. Let us suppose that the worker first is exposed to 100-dB noise for 17 min. This will produce a TTS2 of 15 dB (point A). It can be seen that, if he now stayed out of the noise for the rest of the day, complete recovery would occur in about 420 min (point X, the intercept on the abscissa of the 'line originating at 15 dB TTSs). Suppose, however, that he is out for only 30 min. At the end of that time, the residual TTS will be 7.5 dB (point B). Now he enters a noise of only 90 dB, say. If his residual TTS upon entering the noise is 7.5 dB, this is the same TTS• that would have been produced by a 13-min exposure at 90 dB (point B'). Therefore, if he now stays in the 90-dB noise for 17 min, his total exposure will be 13-t-17=30 min, which will eventuate in a TTS• of 11 dB (point C). But if, at this point, one asks how much time will be needed for complete recovery from this 11 dB (i.e., from C' to D), it can be seen that the general recovery curves predict that only about 200 min will be required. In other words, the second exposure to noise will have shortened the recovery time from 7 h to 4 h! It would be difficult indeed to construct a model of the

auditory mechanism that would produce such a whimsical result.

The implication is, therefore, that the process of recovery is not completely independent of the time it

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Page 3: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

TTS FOR INTERMITTENT NOISE

FIG. 1. Growth and recovery curves for TTS, as a function of time, used in deriva- tion of the CHABA DRC. For explanation of the heavy dashed lines, see text.

5 I0 20 50 I00 2 5 I0 20 50 I00 200 500 EXPOSURE TIME (MIN) RECOVERY TIME (MIN)

takes to produce the TTS--or, if you will, of the time that TTS has been greater than zero. There must be some sort of cumulative effect that produces a pro- gressive delay of recovery as the ear is repeatedly exposed. Indeed, I am somewhat embarrassed to confess that yon Liipke pointed out that my colleagues and I had already shown that with intermittent exposure, the recovery between noise bursts did not proceed in the manner predicted by these curves--in the sanhe article in which the exposure-equivalent rule had been shown to be true (Ward et al., 1959b).

That the D RC for repeated long noise bursts must be in error was also clear from the results of Botsford's

(1967) simplification of the final complicated CHABA D RC for intermittent exposure consisting of two com- plete sets for each octave band: one for long bursts and one for short. Noting that the shape of the curves defining limits for intermittent exposure was about the same for the several octave bands concerned, he developed a single set of generalized curves relating total on-time (cumulated exposure time) to either the number of exposure cycles per day or the off-time per cycle. However, these curves were not monotonic as one would expect, indicating instead that more total noise could be tolerated when the off-time was between 3 and 15 min than for either shorter or longer rests. This was contrary to the one relevant study that had been per- formed, in which it had been shown that--at least for an exposure time of 52 min and a constant on-fraction of 0.5 (on-time equal to off-time)--the final TTS•. gradually increased as the noise bursts were pro- gressively lengthened (Selters and Ward, 1962).

The original purpose of the present series of experi- ments was therefore to determine the degree to which the CHABA DRC were in error. The specific questions to be attacked were:

(1) Do the DRC proposed for single exposures to steady noise in fact produce TTS2s of 10 dB at 1 kHz

or below, 15 dB at 2 kHz, and/or 20 dB at 3 kHz or above?

(2) Is this same TTS2 produced by the criterial 8-h exposure to short noise bursts?

(3) To what extent must we make the criteria for longer noise bursts more stringent in order to limit the final TTS• to these values?

(4) How does the process of recovery from a given value of steady-noise-induced TTS• differ from that produced by intermittent noise?

And, if I may anticipate the results, one can ask a final question'

(5) Is TTS• a sensible index at all?

I. METHOD

A. Subjects and Apparatus

Twelve normal-hearing young adults (seven male and five female), mainly medical students at the University of Diisseldorf, were recruited. They were paid approxi- mately $1.50 per hour for their participation, with one- third of their salary withheld until completion of the series of experiments. The students were divided into two groups of six each, referred to later as groups A andB.

Threshold sensitivity at 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 9, and 10 kHz was initially mea- sured in order to determine normality of hearing (i.e., within 15 dB of ISO normal). At the beginning of each session, preexposure thresholds were taken at those frequencies at which TTS was to be measured.

Threshold-testing equipment consisted of a Peters audiometer with a B•k•sy attachment (attenuator speed 5 dB/sec) and an auxiliary 90-f• resistor in series with each earphone to cut down the level by an addi- tional 20 dB in order to measure thresholds below 5 dB

SPL. The audiometer calibration was measured by

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Page 4: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

w. D. WARD

means of a Brtiel & Kja•r 4153 artificial ear, a 4134 «-in. condenser microphone and 2603 microphone com- plement, a frequency meter, vacuum-tube voltmeters, and an oscilloscope. Fixed-frequency audiomerry was used. The test frequencies were set by hand, and built-in detents assured repeatability within a few hertz. The audiometer was left turned on throughout the course of the experiments (six weeks) for stability to within -t-0.5 dB. The test tone was internally interrupted at a rate of 2 per sec; tone bursts, with a rise-time of 20 msec and a decay-time of 30 msec, were supposed to be 250 msec in duration, but were actually only about 125 msec long (half-amplitude to half-amplitude). All audiometric testing was performed with the listener in an anechoic chamber, with the experimenter and apparatus in the adjoining control room.

Noise for the exposure was produced by a Wandel and Gol•ermann 531C noise generator and 513B -}-oct passive filter, followed by one variable-gain and 4 balanced fixed-gain Telefunken amplifiers feeding a 64-element array of 4-in. speakers. This speaker array consisted of four 16-speaker panels hinged together so that they did not all point in the same direction. This array was placed at one end of the reverberant room of the Akustische Laboratorium, a room with high-re- flectance non-parallel walls and ceiling, and with 5 large cones scattered around the floor. The combination of

the nondirectional speaker array and the room char- acteristics provided a noise field of up to 118 dB SPL that varied by only q-1 dB unless one got within a foot of the walls or speaker array itself. This uniformity is important because it allows the subjects to move about (a necessity for 8-h exposures!) The noise level was continually monitored by means of a Hewlett-Packard microphone and complement assembly which also included a 8054A real-time audio spectrum analyzer. Adjustment of the amplifier gain was necessary only when the number of persons in the room was less than seven (the reverberant room had no window in the door, so an assistant--wearing ear plugs--was always in the room to make sure, for example, that nobody went to sleep with his hand over his ear).

The noise was made intermittent by means of a cam- microswitch assembly. For short noise bursts, of course, the excellent reverberation characteristics of the room

proved to be a complicating effect, since the rate of decay even with the absorption provided by seven listeners was only about 55 dB/sec. However, the effective duration of the noise bursts was actually measured in terms of the average effective level, EL, as defined in the Introduction. For example, when 105-dB noise bursts were to be used (that is, 30 dB above the "no-TTS-producing" level), the effective burst duration was the time during which the level exceeded 90 dB SPL. This, of course, also changed slightly when some of the subjects were not in the room, but no easy solution was apparent; it is hoped that the error thus introduced was negligible.

B. Procedure

After the daily preexposure threshold tests, the six listeners of panel A or panel B were introduced into the noise at 2.5-min intervals. This staggered schedule allowed testing periods every 15 min if desired, although for these experiments the first test usually came after half an hour of exposure (or, for some noises, after the first hour). The TTS was determined at three fre- quencies in each ear during these testing periods; the subject was sent out by the assistant, who had a clock synchronized with that of the experimenter, at a pre- determined time. Audiometric testing, 20 sec per test condition, began 1 min later and took 2 min to complete, whereupon the subject hurried back into the noise room. Hence the subject was out of the noise for just over 3 min at least once an hour (during the 8-h tests, the 3-min breaks after 3, 5, and 7 h of exposure were "free time"). In order to attempt to "cancel out" the effect of these 3-min breaks on the TTS from the 8-h

noise exposures, the noise level was raised by 5 dB for 6 min immediately after all the listeners were back in the room. No such level changes were used for the inter- mittent exposures involving short noise bursts; in this case, the final calculated on-fraction took the 3-min interruptions into account. For long bursts and pauses, of course, no correction was necessary.

At the end of the exposure for the day, the same three frequencies were tested immediately, after 15 min, after 30 min, and every 30 min thereafter, for 4 h. Thus, TTS2, TTS•7, TTSa2, TTS02, etc. were determined. In addition, 3 more frequencies above and below the main ones were tested at 45 min, 75 min, etc.

Although a few of the experiments involving less than 8-h exposures were run while school was in session, scheduling difficulties dictated that the bulk of the experiments be run during the 1969 spring break. During this time, each test panel was exposed on alternate days (including Sundays) so that about 40 h always inter- vened between noise exposures. When the residual TTS was still measurable (5 dB at 2 adjacent frequencies) after 4 h of recovery, it was also measured the following morning (i.e., after 16-20 h of recovery) at the same time as the preexposure tests of the other panel. If the TTS persisted until the day after that (i.e., after 40-44 h of recovery), a noise exposure was used that did not affect the frequency in question (the recovery of TTS at high frequencies is not altered by low-frequency noise exposure, and vice versa; see Ward, 1961).

C. Noise Exposures

Because the cutoff frequencies of the «-oct filter available did not coincide with those provided on old American filters, it was necessary to use noise bands differing slightly from those whose limits are specified by the CHABA DRC. The bulk of the experiments to be reported therefore are for a «-oct band of noise whose upper cutoff was 2000 Hz, although several experiments

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Page 5: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

TTS FOR INTERMITTENT NOISE

FzG. 2. Recovery from TTS at 3 kHz produced by various moderate exposures to 1400-2000-Hz noise. Curve 3 is an idealized recovery curve used in deri- vation of the CHABA DRC. Average of 12 subjects, 24 ears.

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I I I I I 'I ' ' '1 ' ' I I I I NOISE'1400-2000 CPS _ 12 LISTENERS

... l:8H•9SDBi10 MIN ON.,S MIN OFF -- •'. 2:8H•95DB, 25MIN ON, 2SMIN OFF --

I :"'. """ 3.AVE RECOVERY CURVE _ '•",. """-... 4:8 H•85 DB CONT (CHABA) ?.,..'"'•", "'... 5:35 MIN•95 DB CONT (FIRST) ?,•",._"•,..", '"-., 6:8H•95 DB•ON-FRACTION =0.5 -- •.•"'-,•,."', ".,. 7:12 MIN.105 DB CONT --

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. I I I , I , ,I, ,,1,, I , I I I 4 10 20 40 100 200 400 1000 2000 4000

RECOVERY TIME (MIN)

involved a 500-700-Hz band, and one exposure to 125- 250 Hz was given. The fact that the range was less than an octave is unimportant, for the "critical bandwidth" at these high intensities is about an octave anyway (Ward, 1926b; Shoji et al., 1966; Weissing, 1968). The CHABA curves were consulted, and a list of exposure conditions indicated to be equally tolerable was com- piled, by interpolation when necessary. For the 1400- 2000-Hz noise, these included' (1) continuous exposures' 8 h at 85 dB SPL, 35 min at 95 dB, or 12 min at 105 dB; (2) short-burst intermittent exposure' 8 h at either 95 dB with an on-fraction of 0.5 (5 sec on, 5 sec off or 37.5 sec on, 37.5 sec off; panel "B" was run with the former, panel "A" with the latter) or 105 dB with an on-fraction of 0.32, i.e., 3.2 sec on, 6.8 sec off (this condi- tion was terminated after 6 h because of large values of TTS in some ears as described later); (3) repeated longer-burst exposures' 8 h to 95 dB with either 10 min on, 5 min off or 25 min on, 25 min off. In order to get a better picture of the growth of TTS as the noise-burst duration increases, with on-fraction held constant, Group A was also run at 95 dB with 5 min on, 5 min off.

With 500-710-Hz noise, theoretically permissible exposures were 8 h at 88 dB, 30 min at 105 dB, and 8 h of 105 dB at 5 sec on, 5 sec off. Finally, for the 125-250- Hz noise, the CHABA curves indicated that the maxi- mum obtainable 117 dB SPL could be tolerated for

about 45 min; however, a 25-min exposure fitted into the schedule better (it was given one day while following the recovery of TTS after a short exposure at a higher frequency, so the two recovery schedules had to be sandwiched properly).

Experiments involving intermittent exposures in which each listener had different exposure durations were also conducted near the end of the series. These

"variable exposure" tests will be described in Sec. II-B below.

The last experiment in the series was a repetition of the first' a 35-min exposure to 95 dB of 1400-2000 Hz.

At the conclusion of the entire study, including some exposures to be discussed later, the preexposure thresh- olds at a given frequency over the entire six weeks of experimentation were averaged in order to give a mean resting threshold; all group TTS data presented here are based on this long-term average. No significant shifts in resting threshold at a particular frequency were found, when the first and last preexposure thresh- olds were compared (i.e., no NIPTS was produced).

II. RESULTS

A. 1400-2000-Hz Noise' Planned Exposures

Figure 2 shows the average recovery curves for all 12 subjects at 3000 Hz (the frequency at which, as ex- pected, the maximum TTS from 1400-2000-Hz noise was found) after the four continuous-noise exposures (curves 4, 5, 7, and 8). The solid straight line (curve 3) is the generalized 20-dB-TTS2 recovery curve used in the CHABA DRC derivation. It is clear that for all of

these, the DRC are correct, and the recovery proceeds to a first approximation according to prediction. For the 8-h exposures to 95 dB with an on-fraction of 0.5 (curve 6-here the data from Group B, who were given 5-sec bursts, were combined with that from Group A, who received 37.5-sec bursts), the criteria seem if anything to be overprotective. Only the exposure in- volving 10-min exposures with 5-min recovery periods (curve 1) shows a TTS2 more than 10% greater than the intended 20 dB, as feared. The problem is, however, that the 4 dB by which the target TTS•. was exceeded persisted for at least 12 h; recovery was not complete after 16 h (1000 min) of recovery. I shall return to this point later.

In general, Fig. 2 shows that the rank order of TTS is maintained fairly well over the course of recovery. That is, those exposures producing the most TTSs will also produce the most TTS100, for example. In the case of intermittent 105-dB noise, however, even this general-

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Page 6: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

26

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W. D. WARD

GROUP A 6 LISTENERS _ x 1 6 H,32 SEC ON, 6.8 SEC OFF x x 2 8 H,CA 5MIN ON,45 MIN OFF -- _""•'•,• 3: ISMIN CONT • '"x •• 4:8 H•CA 3MIN ON,4? MIN OFF '""N•"•xx •• 1 --AVE RECOVERY CURVES (Cb

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--AVE RECOVERY CURVES(CHABA) --

10 20 /-,0 100 200 400

RECOVERY TIME (MIN)

1000 2000 4000

FIG. 3. Recovery from TTS at 3 kHz produced by exposures to 1400-2000-Hz noise at 105 dB SPL in listening Group A (6 subjects, 12 ears).

ization does not hold. Figure 3 shows four different recovery patterns from 105-dB 1400-2000-Hz noise for test panel A. Recovery from a 15-min steady exposure follows the generalized recovery curves reasonably well, but this is not so for the intermittent exposure. Although an on-fraction of one-third should be tolerable for a full

8 h, the exposure was terminated after 6 h because some of the listeners were getting too close to the 40-dB TTS,. established as the top limit I would permit. And this was just as well, because full recovery for the last ear was complete only after four days of recovery.

The recovery of this most sensitive ear for this exposure is shown in the uppermost curve of Fig. 4; it can be seen that, at the end of four full hours of recovery, the TTS was exactly the same as TTS2. Both this figure and Fig. 3 illustrate the breakdown of the invariance-of-order notion; although this intermittent exposure produced only the same TTS2 as a 15-min

continuous exposure, the recoveries rapidly diverged, so that TTSa0 after continuous noise was only about half as great as that following the intermittent noise. Here, we have a clear answer to the last question posed in the Introduction: TTS= is indeed not a sensible criterion for risk from intermittent noise--at least if it is true that a

TTS that persists until the next workday is dangerous, while one that recovers in that time is not.

The recovery curve following 35 min of 95-dB con- tinuous noise is also presented in Fig. 4, in order to show that this ear's recovery was reasonably normal other than when following intermittent high-intensity high- frequency noise. (The other two recovery curves in Fig. 4 will be discussed later.)

Now this type of "delayed recovery" is not unusual. Indeed, in an earlier article (Ward, 1960) it was shown that when one deliberately induces high values of TTS=, the recovery, instead of proceeding linearly in log time

40

36

32

28

ß '-' 24

O 20

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I--12

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WELLENS-LE 3000HZ

-- EXP:6 H-IO5DB- 3 SEC ON, 7 SEC OFF x

-- • • x x • x x •

-

- • %•*•*%-,,•5 MIN' 105 DB CON ,,

• •---•.• 8H; 105DB' • o - •" -•E EXPOSURE

- 55MIN;95 DB CONT

2 /-, 10 20 40 100 200 g, 00 1000 2000 /,000

RECOVERY TIME (MIN)

Fro. 4. Course of recovery from TTS at 3 kHz produced by exposures to 1400-2000-Hz noise at 104 dB SPL in the most sensitive ear

of Group A.

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Page 7: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

TTS FOR INTERMITTENT NOISE

FIG. 5. Recovery from TTS at 3 kHz produced by 6-h intermittent exposure to 1400-2000-Hz noise at 105 dB (3 sec on, 7 sec off) in all 12 ears of Group A.

35

3O

aO

5

O-

......... -- "4_x ',, \

'•' •' • I a 5 I0 aO 50 I00 200 500 I000 aOO0 5000

•ECOVE•Y TIME {MIN)

as implied by the average recovery curves assumed in the CHABA DRC derivation, progresses linearly in time (so many decibels of recovery each day) and there- fore becomes curved downward on a log-time plot just as in Fig. 4. What is unusual, however, is that in the present instance fully half the ears of this panel dis- played delayed recovery, including the two ears of one girl with a 20-dB TTS2. Figure 5 shows the recovery from this exposure in all the Group A listeners.

Yet other ears showed normal recovery, and the least susceptible ear indicated a 3-4-dB improvement in hear- ing an hour after the exposure (Fig. 5). This particular listener showed this sensitization after most exposures. On one occasion, I inserted a half-hour wait between two preexposure thresholds, thinking that she might have a long normal "quieting-down" time, but to no avail--the second was the same as the first, and still the "sensitization" appeared within an hour after the exposure. Does the noise exposure suppress random activity in her auditory system for a few hours? That sounds a bit farfetched, but I cannot think of a better hypothesis. It may also be mentioned that her resting threshold was quite normal, being 9 and 4 dB SPL at 2 and 3 kHz, respectively. Thus, relative threshold sensitivity does not seem to be a very good indicator of susceptibility to high-intensity intermittent noise.

B. 1400-2000-Hz Noise: Additional Exposures

The data of Fig. 5 do, however, suggest that perhaps exposure to intense short bursts tends to produce a larger separation between the more susceptible and the less susceptible ears. In the extreme, this is indeed the case; for true impulse noise such as gunfire, some listeners will get TTSs of 30 dB from a series of pulses at a single intensity that produce no effect whatever in other

listeners (Ward et al., 1961). Perhaps most of the damage here is being caused by the onset of the noise (i.e., before the middle-ear muscles contract). If so, then one would expect that reducing the on-fraction but keeping the period constant would have little effect. Therefore, the other test panel (panel B) was exposed for 6 h to 2-sec bursts of 105 dB with 8 sec of quiet in between. This exposure produced an average TTS•. at 3 kHz of only 13 dB, so reduction of the on-fraction from one-third to one-fifth did lower the average TTS. However, three of the 12 ears showed delayed recovery, with initial TTS2s of 17, 23, and 29 dB becoming residual TTSs 18 h later of 4, 9, and 13 dB, respectively (the first two were the right ear and left ear of the same subject).

One can therefore only conclude that the situation is, as usual, not simple. While the on-fraction does still have an influence on TTS2, high-frequency high- intensity noise bursts tend to produce delayed recovery in some ears. The question that presented itself at this point was whether or not we might have some evidence of a "critical intensity" phenomenon, so that at 105 dB certain ears would tend to develop delayed recovery whether they showed large TTS2s or not.

Therefore an experiment was conducted in which the TTS• in any ear from an intermittent 8-h exposure was kept below the "safe" (or so we thought previously) limit of 20 dB, by adjusting exposure times for each ear. In a group situation, of course, methods for accomplish- ing this are quite limited. One cannot adjust the intensity of the pulses or even the duration of short bursts in an attempt to "equalize" listeners. The only means at hand consisted of using a long period and adjusting the exposure duration per cycle by removing individual listeners from the noise after different times

(hence "variable exposure"). In the present case, a

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Page 8: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

w. D. WARD

a N I I "1 I' '1 ' ' '1'' I I I 14 -- • NOISE: 500-710 HZ -- 13 -- •. 1:SHj108DB•CA 20MINON, 30MIN OFF --

1 *. • 2:8Hi88DB CONT 12-- N • f3:30MIN•105DB CONT -- 11 -- • • BGROUP •4:8H•105DB•SSECON,5 SEC OFF --

• a•x '-. N N a AVE RECOVERY CURVES(GHABA) •

-,- N -

• 6• 4

. ' - o• • i i •,•-=• ,I ,•1,, I i

2 4 10 20 40 100 200 400 1000 2000

RECOVERY TIME (MIN)

FIo. 6. Recovery from TTS at 1 kHz produced by various exposures to 500-710-Hz noise. Curves 1 and 2 are for 12 listeners, curves 3, 4 and 5 for only 6 (Group B). The triangles indicate the CHABA generalized recovery curves.

50-min cycle was used; this allows 10 successive ex- posures in an 8-h day. Although the CHABA DRC indicate that, for a 50-min cycle, noise bursts could be 6 min long, all ears began with a 4-min exposure (and therefore a 46-min recovery period). TTS2 and TTS•.• were measured; if TTS2 seemed to be approaching 20 dB too rapidly, the next duration was decreased; if too slowly, increased. When a different duration of exposure was indicated for the two ears of a given observer, he was told to hold one ear closed with his finger for a fixed period of time.

The results of this experiment are shown as curve 2 in Fig. 3 (the median on-time was 5 min, hence the caption, "ca. 5 min"). However, even with 10-min bursts, the "tough" right ear of subject D was back to supernormal within 2 h after the last exposure; and subject W's left ear, though limited to a TTS,. of 22 dB, nevertheless required a couple of days of recovery (Fig.

4, uppermost of the curves labelled "variable ex- posure"). Four of the 12 ears showed residual TTSs of more than 5 dB after 16 h of recovery. . Clearly, limiting the TTS• to 20 dB at the end of a long series of exposures was not sufficient to prevent delayed recovery. However, the more susceptible ears had been brought nearly up to 20 dB of TTS• by the third or fourth exposure, so that later (shorter) exposures had merely reinstated that degree of shift. Therefore a second attempt was made to sneak up on the 20-dB TTS•, based on knowledge gained from the experiment just cited. This time even shorter beginning exposures were used, and were increased in length only when it was quite clear that this was necessary if 20 dB were to be reached. And at first glance it appeared that this caution had been overdone, because the final average TTS• was only 13 dB (Fig. 3, curve 4). However, four ears, including W's left ear (Fig. 4, lower "variable

_ NOISE' 125-250 HZ, 117.5 DB 25-MINUTE EXPOSURE

-- x X:AVE RECOVERY CURVES(CHABA)

-- X

__ • •x

2 4 10 20 40 100 200 400 1000

RECOVERY TIME (MIN)

Fla. 7. Recovery from average TTS at 500, 750, and 1000 Hz produced by 25-min exposure to 125-250-Hz noise at 117 dB (12 listeners).

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Page 9: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

TTS FOR INTERMITTENT NOISE

FIG. 8 Recovery from TTS during noise-free periods produced by repeated exposure to noise in a 50-min duty cycle. In the left panel, the upper curves indicate average recovery at 2, 3, and 4 kHz from 10 successive 25-min exposures at 95 dB SPL, the lower curve that from 5 suc- cessive 2.5-min exposures at 105 dB SPL. The right panel shows average recovery at 750, 1000, and 1500 Hz from 10 exposures to 500-710-Hz noise at 108 dB SPL; the duration of exposure for individual ears was varied in order to keep TTS2 from exceeding 20 dB in any ear at any time, but the median on-time was 20 min.

GROWTH OF TTS WITH TIME IN 50-MINUTE EXPOSURE CYCLES

1400- 2000-HZ NOISE 500-?10-HZ NOISE, •08 DB SPL• 24 ...... 25 MIN ON, 25MIN OFF• 95 DB SPL VARIABLE EXPOSURE (ADJUSIED

-----2.5MIN ON, 4'7.5MIN OFF•105DB SPL TO KEEP FINAL TTS 2 BELOW 20DB 22 AVE RECOVERY CURVES (CHABA) FOR ALL LISTENERS)•AVERAGE 20 10---.-., AVE ITS AT 2,3, AND 4 KHZ 20 MIN ON• 30 MIN OFF

.•.•.•'.•. AVE ITS AT 750,1000,AND 1500 HZ 18 62,, -. - •6 4---..'::;•,-•:> -

1 ---,""', ,, ""', ,, "" ,, "'"-'•.• -..-. -

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•. -.,. -.., ......•-.. -... -. -.

- -....-......:........_-... :_ _

0 , I I I I, I I I I I, I -2 2 4 10 20 40 2 4 10 20 40

RECOVERY TIME {MIN)

exposure" curve) still showed TTS that persisted more than 16 h.

C. Exposures to Lower Frequencies

Delayed recovery was not produced by 8-h inter- mittent exposures to 500-700-Hz noise (Fig. 6) or by 25 min of 125-250 Hz at 117 dB SPL (Fig. 7). The worst exposure in terms of recovery was in fact the steady 8-h exposure to 500-710 Hz at 88 dB; this developed delayed recovery in three of the 24 ears. These included none of the ears most susceptible to the 1400-2000-Hz 105-dB noise, confirming the indepen- dence of susceptibility to TTS at low, medium, and high frequencies (Ward, 1968). Curve 1 in Fig. 6 is an experiment similar to the ones just described in the previous section' 50-min exposure cycles, with noise- burst duration adjusted on an individual basis in order to limit the final TTS2 to less than 20 dB in any listener. In this case, no delayed recovery ensued.

The 25-min exposure to 125-250-Hz noise at 117 dB (106 dBA) is clearly safe (Fig. 6); an hour after ex- posure, not a single ear showed a TTS of more than 5 dB. Some of the listeners (especially the girls) objected strenuously to this noise as being painful, so its lack of hazard is reassuring. Anticipating the objections, I had saved it for the final session; it was given while they were recovering froin the final exposure to 35 min of 1400-2000 Hz at 95 dB.

D. Comparison of First and Last Tests

Curves 5 and 8 of Fig. 2 show a decrease of some 3-4 dB in the TTS at all recovery times after the last ex- posure (relative to the first), both of which involved 35 min of 1400-2000 Hz at 95 dB, as if the ears had become "toughened." I am at a loss to account for this effect. The noise level at a fixed point in the exposure was

monitored at all times during the experiments, and neither the microphone nor the audiometer showed a change of calibration, so an apparatus artifact seems unlikely. Comparison of the six datum points for each listener after exposure (i.e., 2, 3, and 4 kHz, right and left) showed that the shift was due mainly to five listeners, who demonstrated a decrease in all six cases (/'=0.02). Two girls in Group B each showed 8 dB less TTS at all frequencies on the second test.

Because TTS for both exposures was calculated relative to an average preexposure threshold, the resting thresholds on the two days in question were also examined to see if perhaps the difference in the "im- proved" five cases could be accounted for by a change in threshold criterion during the six weeks between the tests, or by a moderate middle-ear problem that might have protected the ear somewhat on the second exposure. The results were negative•although the change of one of the five listeners was associated with a shift in cri- terion (that is, the preexposure thresholds at all fre- quencies had improved from the first test to the last by about 4 dB, so that the TTS in both tests was nearly the same when determined relative to the threshold of that day), the other four showed no significant differ- ence in resting threshold on the two days. As far as my assistant and I could tell, they were not doing anything unusual in the second test. As a remote possibility, per- haps they had more tinnitus on the first test, and this interfered with perception of the test tone. At any rate, I am reluctant to accept this as proof that ears in gen- eral become less susceptible to TTS with continued ex- posure, because no such tendency has been observed in earlier studies (Ward, 1968). However, the possibility cannot be excluded that some listeners can learn to

protect themselves somehow--perhaps by gaining some control over the action of the middle-ear muscles.

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Page 10: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

24

22

2O

18

16

•12

.•10

•- 8

6

2

W. D. WARD

GROWTH OF TTS AT 3KHZ

-- PRODUCED BY EXPOSURE TO 1400-2000- HZ NOISE

• 95 DB SPL

......... 85 DB SPL

10MIN ON 5MIN OFF --

./' 25MIN ON 25MIN OFF

/ • ./•' /•A o.ou. O•.• - //' ..-" / _••s.c o• • s.c o.. - •'- -" • _x/-' B GROUP ONLY --

/, 10 20 40 100 200 400 1000

TIME SINCE ONSET OF FIRST NOISE (MIN)

I I

Fzo. 9. Growth of TTS•. at 3 kHz produced by 1400-2000-Hz noise, as a function of time since onset of noise.

E. Recovery During Interval between Long Bursts

It is clear that the recovery between bursts in the 50-min-cycle tests does not always follow the idealized course shown in Fig. 1. How does it proceed? The answer seems, happily, fairly simple' recovery curves in suc- ceeding intervals are parallel. Figure 8 illustrates this generalization. In the left panel are given the recovery curves following the 10 successive 25-min exposures to 95 dB of 1400-2000 Hz, while the right panel illustrates recovery after "tailored" exposures to 500-710-Hz noise. In both cases, the slopes (decibels/log time) display no statistically significant trend. The same result was obtained for the 105-dB 14•00-2000-Hz

tailored exposures. Thus, approximately the same number of decibels of

recovery seems to be occurring in each rest period, which means that the TTS immediately after successive exposures should continue to increase throughout the day instead of quickly reaching an asymptote in which the growth of fatigue during noise is just balanced by the recovery during quiet. As it does' Fig. 9 shows the gradual increase of TTS at 3 kHz for intermittent as well as continuous exposure to 14•00-2000-Hz noise at 85 and 95 dB, plotted against time since the first noise began.

F. The Equal-Energy Hypothesis

In the left panel of Fig. 8, the bottom curve (labelled 1-5) shows what happens when the exposure is to 2.5- min bursts of 105-dB noise every 50 min, which in terms of energy alone is identical to 25-min bursts of 95-dB noise in the same 50-min cycle. After five successive exposures produced identical TTS•.s, it was clear that the 4•7.5-min rest interval was allowing sufficient recovery that no residual TTS remained when the next burst began, so the experiment was terminated. This observation provides a striking illustration of how the

570 Volume 48 Number 2 (Part 2) 1970

temporal distribution of energy does make a difference in the TTS produced, contrary to the equal-energy hy- pothesis. It is difficult to see how temporal pattern could fail to have an effect also on the eventual PTS produced; indeed, Miller et al. (1963) have already shown that pauses reduce the PTS produced by a given energy in cats. (This experiment was conducted with only Group B, so the peculiarly sensitive ear of subject W was not involved. Had it been included, some cumulative effect might have been seen. However, I am confident that the TTS2 for this ear after the 10th exposure would not have been anywhere near 34• dB, which is what it was after the 10 25-min bursts of 95 dB.)

Further refutation of the equal-energy hypothesis is provided by a comparison of the shifts produced by various noise exposures in which total energy level and on-fraction were held constant while only the period was varied. Figure 10 shows the TTS•. and TTS•80 (TTS 3 h after exposure) at 2, 3, and 4• kHz after the several 8-h exposures to 95 dB of 1400-2000-Hz noise with an on-fraction R of 0.5. The steady growth as the burst duration (hence period) increases that is shown here confirms the earlier results of Selters and Ward (1962) and shows that all curves relating exposure to burst duration (or any derivative of it such as "total number of tolerable cycles per day") should be monotonic.

III. DISCUSSION

A. The Original Questions

(1) The first of the five questions posed in the intro- duction seems to have a reasonably clear answer' The CHABA DRC do indeed hold the TTS•. produced by 8 h of steady continuous noises to the values of 10 dB at 1 kHz, 15 dB at 2 kHz, and 20 dB at 3 Hz set as limits.

(2) The evidence relative to the second question (concerning TTS from short noise bursts) is not nearly so unequivocal. Eight-hour exposures to intermittent

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TTS FOR INTERMITTENT NOISE

Fro. 10. TTS produced by 8-h exposures to inter- mittent 1400-2000-Hz noise at 95 dB SPL with an on-

fraction of 0.5 ("on" time equal to "off" time), as a func- tion of noise burst duration. Data for bursts of 5 sec and

25 min are for 12 listeners, those for 37.5 sec and 5 min for only 6 (listening Group A).

26

24

22

20

18,

16

14

03 12 • 10

8

6

4

2

8 H• 95DB• 1400-2000 HZiR=0.5 o 2000 HZ

x

x 3 000 HZ ß 4 000 HZ

,

TTS2 _

•x• •/x TTSloo - • • •x•• ""ø" ........ -.•

0.1 0.3 1 3 10 30

TIME "ON" PER BURST(MIN)

noise with short bursts do not necessarily exceed the criteria. However, when the noise is a high-frequency (1400-2000 Hz) one, and the level is above 100 dB or thereabouts, then the TTS2 produced, though within the limits, may require more than 16 h for full recovery. Lower-frequency noise (500-710 Hz) does not produce this delayed recovery even with levels of 108 dB SPL, however, so further studies should be undertaken in order to define the limits, in terms of frequency, level, and on-fraction, to which the short-burst DRC apply.

(3) For longer bursts, none of the CHABA curves are conservative enough, since their derivation assumed an incorrect course of recovery during noise-free intervals. Instead of the convergent recovery curves shown in Fig. 1, parallel curves must be employed. Therefore, even for relatively low intensities (where the DRC for steady and short-burst noise are accurate), the DRC for long bursts must be revised. Furthermore, if a single set of curves relating total on-time to burst duration, applicable for all frequencies, is to be derived --for example, Botsford's (1967) simplification of the eight CHABA DRC for intermittent noise•these curves must be monotonic, because the longer the burst duration, the smaller the total cumulative daily exposure permitted.

(4) The fourth question, concerning the difference between recovery following intermittent and following steady noise, has already been touched on. The ex- posures to repeated long bursts of noise always produce a slower recovery from a given final TTS•. than when the same TTS2 was produced by a single exposure. This seems to be true of all listeners, not just the most susceptible. However, with short bursts, the recovery does not appear to deviate from the course predicted from curves based on continuous exposures unless several of the individual ears have been thrown into the

"delayed recovery" state (in which recovery proceeds linearly in time instead of linearly in log time) because, presumably, of the high levels involved.

(5) It therefore seems quite clear, as mentioned earlier in connection with the diverging recovery curves of Figs. 3 and 4, that TTS2 is not the best possible index for assessing noxiousness of noise exposures, as a basis for DRC. On the other hand, we are forced to use some sort of TTS measure, if for no other reason than that is all we have. Although the risk of PTS associated with habitual 8-h/day exposure to steady noise has been reasonably well established on the basis of actual industrial PTS data, no body of knowledge relating PTS to the parameters of intermittent exposure as yet exists.

Consequently, we must either (1) employ some index based on TTS, (2) run experiments on lower organisms to establish the relations between intermittency and PTS, and generalize the results to man, or (3) simply guess at the ameliorative effect of interruptions in the noise during the work day.

B. The "Total Immission" Concept

There is a fourth alternative' we could throw in the

towel and accept the "total immission" principle of Robinson (1968), which is nothing but an extreme variant of the equal-energy hypothesis in that it postulates that equally noxious exposures are those that involve the same total cumulative energy during the entire work week. This principle fits the data on PTSs produced by the continuous exposures that were in- volved in the survey data on which it was based. How- ever, interrupted industrial exposures were carefully excluded from consideration, so that there is no par- ticular reason to expect the principle to apply to them.

What little industrial data does exist suggests that interruptions do indeed play an important role in reducing the PTS-producing power of a noise. Coal miners, for example, are exposed to drilling noises that may exceed 120 dBA, and yet the incidence of hearing loss among such workers is remarkably low (J6nsson, 1967; Blaha and Slepicka, 1967). Measurements made

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Page 12: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

w. D. WARD

by Sataloft et al. (1969) indicate that iron miners may be exposed to 117-dBA noise for 100 min during a typical work day, or about 8 h per week. If one accepted 90 dBA for 40 h as the maximum weekly immission, then the equal-energy hypothesis says that 117 dBA should be tolerated for only about 5 min per week! Yet the incidence of handicapping hearing loss corresponded only to that observed in persons who spend 40 h weekly in a steady 100 dBA (Sataloft et al. 1969), although 117 dB for 8 h is equivalent in energy to some 400 h at 100 dB. The key to this remarkably small degree of PTS must be the fact that the noise comes only in 3-5-min bursts. A similar reduction in hazard due to inter-

mittency probably also accounts for the low incidence of hearing losses among bench glass blowers (Sanderson and Steel, 1967), diesel-engine-room personnel in sub- marines (Harris, 1965), and rock-and-roll musicians (Rintelmann and Borus, 1968).

Returning to our three remaining possibilities, the most promising would seem to be to expose experimental animals to various noise patterns and determine the hearing losses that result, as Miller et al. (1963) did. Such experiments are in progress, but determining thresholds of such animals is not so easy that we are likely to have enough information to allow setting realistic DRC Within five years.

C. The Walsh-Healey Provisions

So, reduced to two alternatives--use TTS data or guess--my choice is still TTS. It is clear, however, that my opinoin is not universally held, as witness the noise- exposure limits of the recently enacted Walsh-Healey Act (WHA) (Anon., 1969). Basically, this criterion is not unlike all the others, allowing 8 h of exposure to 90 dBA (which is very nearly what one would get if one had a noise with 85 dB SPL in the octave bands from

500 to 4000 Hz), with shorter exposures permitted to progressively higher levels; these equinoxious curves have nearly the same shape as the CHABA DRC for single continuous exposures, providing an even greater deemphasis of low frequencies than the A-scale, as the exposure duration is shortened. But, as in equal-energy- based proposa.ls, intermittent noises are regarded in WHA as having the same danger as continuous noises of the same level and cumulative duration, so that the DRC for intermittent noise is overconservative. In

an attempt to compensate for this shortcoming, a trading relation of 5 dB per doubling time is postulated. That is, 4 h of 95 dBA can be tolerated, 2 h of 100 dBA, etc. However, in my opinion, this compromise was futile and perhaps even dangerous.

Let us take an extreme example. Suppose we have a noise of 114 dB SPL whose energy lies mainly in the 2000-Hz octave band, as in the principal noise of the present study. It will therefore give a reading on a sound-level meter of 115 dBA. The WHA says that this can be tolerated for 15 min. Thus, the intensity limit

for a 15-min exposure is, according to the WHA, some 10 dB higher than the 105 dB that, in the present experiment, produced a TTS•. of nearly 20 dB--or, putting it another way, the level that the CHABA criteria will tolerate for less than 4 min is permitted by the WHA for four times that length of time. In short, the "5-dB rule" is in many cases as underprotective for steady noise as the equal-energ); (3-dB) rule is overprotective for intermittent noises.

At first glance, perhaps it appears that I am about to promote a middle value--viz., 4 dB for doubling time, as suggested in Germany a few years ago (Pfander, 1965; his proposal also began with 90 dBA for 8 h). Indeed, it might be better than either 3 or 5 dB. But the relation between time and intensity, at least for TTS, simply is curvilinear and does vary with frequency, so that any sort of legislation, no matter how authoritative, that "establishes" a single relation, is destined for the same degree of success as the state law, a few years ago, that proclaimed the value of •r to be exactly 22/7.

D. What, Then?

Until we have a greater number of accurate field studies on men, or laboratory studies with other animals dealing with the PTS produced by intermittent noise, we seem to be reduced to using TTS in one form or another. Not TTS•., however, which, aside from its inadequacy in predicting persistent shifts (as exempli- fied in Figs. 3 and 4), is also completely impractical, since workers can only rarely be brought from the noise to the test room in 2 min. In my opinion, what we are really interested in is whether or not the ordinary 16-h recovery in quiet is sufficient to restore the threshold to normal, so that workers do not begin a given shift with residual TTS from the previous day. So, ideally, what we want to measure is TTS•000 (the TTS 16 h after exposure); however, the difficulty here is deciding how much TTS is significantly different from zero. What is needed is an intermediate value of recovery time, one after which the rank order of different TTSs is invariant.

Inspection of all the recovery curves from this experi- ment indicates that this condition is fulfilled about 30

min after expsoure, so at the moment TTSa0 would appear to be a usable index of relative noxiousness. There was one exception: the 6-h exposure of Group A to 1400-2000-Hz noise at 105 dB, 3 sec on and 7 sec off, gave the same TTSa0 (16.5 dB) as the 8-h exposure to 95 dB with 10 min on, 5 min off, although 3.5 h later, the values of TTS•.40 were 14 and 9 dB, respectively. Additional research on the recovery pattern from these short-burst high-intensity high-frequency and low on- fraction exposures is therefore indicated. On the other hand, both the exposures just mentioned would be clearly hazardous, since they give rise to fairly sizable values of TTS•000 (8 and 4 dB, respectively), so perhaps TTSa0 can be used. Further inspection of the recovery curves suggests that TTSa0 magnitudes just about half

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Page 13: Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

TTS FOR INTERMITTENT NOISE

as great as the TTS• values deemed tolerable by the CHABA criteria will divide the complete overnight recoveries from the rest. This means, then, that our criterion could be based on the production of TTSa0 of 10 dB or less at 3 kHz or above, 7.5 dB at 2 kHz, and 5 dB at 1 kHz; any noise pattern producing an average TTSa0 less than these values should be quite safe for the great majority of the workers.

For the identification of the occasional supersensitive ear, there is probably no solution except actual measure- ment of TTS, particularly TTS1000 produced in the workers concerned by the exposure on Monday, and the use of regular audiometric testing.

The latest German DRC proposal (VDI-2058, 1969) indeed relies in part on actual TTS measurements. Its basic DRC is again 8 h at 90 dBA, with the equal- energy rule being applied to shorter exposures and to intermittent ones with short burst durations. For longer bursts, a set of general curves similar to Botsford's (1968) are used; these are based on an attempted correction of the CHABA curves with the more ap- propriate course of recovery. However, for noise ex- posures so complicated that any question as to hazard exists, direct measurement of TTS10 and TTS1000 at 3, 4, and 6 kHz in a group of 12 normal listeners is recommended. Their allowable values of TTS10 are 20, 25, and 20 dB (at 3, 4, and 6 kHz, respectively), which are slightly higher than the 15 dB to which the CHABA DRC would limit TTS10. Monitoring audiomerry is also suggested for detecting the most susceptible individuals.

IV. CONCLUSIONS

(1) The CHABA DRC for continuous noise and for intermittent noises in which burst duration is less than

5 min do, as intended, usually restrict the average TTS• at the end of the workday to 10 dB at 1 kHz or below, 15 dB at 2 kHz, and 20 dB at 3 kHz and above.

(2) However, the TTS produced by a noise pattern involving longer bursts will sometimes exceed these values. This defect in the CHABA DRC arose from the

false assumption that the course of recovery from a given value of TTS• was independent of how it was produced. Instead, it is found that the recovery curves following repeated exposures are essentially parallel, which means that time until total recovery becomes progressively longer and longer during the daily exposure. Thus, some modification of these curves is indicated.

(3) Furthermore, with any intermittent exposure to 105-dB 1400-2000-Hz noise (either short or long bursts) that produces as much as 15 dB of TTS2, full recovery may often require more than 16 h. If such "delayed recovery" indicates an impending permanent loss, then TTS• following high-intensity high-frequency inter- mittent noise is not a valid risk indicator; TTS•0 or, better yet, TTS1000, will in this case be a better index.

(4) Nevertheless, in my opinion the CHABA DRC still represent a more realistic attempt to predict hazard from intermittent noise exposure than alternative pro- posals that ignore the temporal distribution of energy. With modifications indicated by the present research (and, hopefully, further related experiments), it should be possible to derive a set of curves, similar to Botsford's (1967) distillation of the original CHABA DRC, that will allow risk to be assessed from only knowledge of the temporal pattern and the dBA levels involved.

ACKNOWLEDGMENTS

I am deeply indebted to Professor Dr. F. J. Meister, Director of the Akustische Laboratorium of the Uni-

versity of Dtisseldorf, and his staff for providing the opportunity to conduct these experiments. I am par- ticularly grateful to Dr. Edmund Buchta for technical assistance and to H. Engstler for preparation of graphs. For adherence to an involved and hectic testing schedule without a single absence, my heartfelt thanks go to my two assistants, Michael Neumann and Hans-Dieter Harbisch, and my 12 students. I would also like to thank Phonak Gmbh. for supplying the B•k•sy attach- ment for the Peters Audiometer. This research was

supported by grants from the Deafness Research Foundation and from the National Institutes of Health.

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574 Volume 48 Number 2 (Part 2) 1970

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