Growth and recovery of temporary threshold shift at 3 kHz in bottlenose dolphins: experimental data and mathematical models

Measurements of temporary threshold shift (TTS) in marine mammals have become important components in developing safe exposure guidelines for animals exposed to intense human-generated underwater noise; however, existing marine mammal TTS data are somewhat limited in that they have typically induced small amounts of TTS. This paper presents experimental data for the growth and recovery of larger amounts of TTS (up to 23 dB) in two bottlenose dolphins (Tursiops truncatus). Exposures consisted of 3-kHz tones with durations from 4 to 128 s and sound pressure levels from 100 to 200 dB re 1 μPa. The resulting TTS data were combined with existing data from two additional dolphins to develop mathematical models for the growth and recovery of TTS. TTS growth was modeled as the product of functions of exposure duration and sound pressure level. TTS recovery was modeled using a double exponential function of the TTS at 4-min post-exposure and the recovery time.     

Human temporary threshold shift (TTS) and damage risk

Information regarding the relation of human temporary threshold shift (TTS) to properties of steady-state and intermittent noise published since the 1966 appearance of the CHABA damage risk contours is reviewed. The review focuses on results from four investigative areas relevant to potential revision of the CHABA contours including effects of long-duration exposure and asymptotic threshold shifts (ATS); equivalent quiet and/or safe noise levels; effects of intermittency; and use of noise-induced temporary threshold shift (NITTS) to predict susceptibility to noise-induced permanent threshold shift (NIPTS). These data indicate that two of three major postulates on which the original contours were based are not valid. First, recovery from TTS is not independent of the conditions that produced the TTS as was assumed. Second, the assumption that all exposures that produce equal TTS2 are equally hazardous is not substantiated. The third postulate was that NIPTS produced by 10 years of daily exposure is approximately equal to the TTS2 produced by the same noise after an 8-h exposure. Based upon several TTS experiments showing that TTS reaches an asymptote after about 8 h of exposure, the third CHABA postulate can be reworded to state the hypothesis that ATS produced by sound of fixed level and spectrum represents an upper bound on PTS produced by that sound regardless of the exposure duration or the number of times exposed. This hypothesis has a strong, logical foundation if ATS represents a true asymptote for TTS, not a temporary plateau, and if threshold shifts do not increase after the noise exposure ceases.     

Recovery of distortion-product otoacoustic emissions after a 2-kHz monaural sound-exposure in humans: effects on fine structures

A better understanding of the vulnerability of the fine structures of distortion-product otoacoustic emissions (DPOAEs) after acoustic overexposure may improve the knowledge about DPOAE generation, cochlear damage, and lead to more efficient diagnostic tools. It is studied whether the DPOAE fine structures of 16 normal-hearing human subjects are systematically affected after a moderate monaural sound-exposure of 10 min to a 2-kHz tone normalized to an exposure level L(EX,8h) of 80 dBA. DPOAEs were measured before and in the following 70 min after the exposure. The experimental protocol allowed measurements with high time and frequency resolution in a 1/3-octave band centered at 3 kHz. On average, DPOAE levels were reduced approximately 5 dB in the entire measured frequency-range. Statistically significant differences in pre- and post-exposure DPOAE levels were observed up to 70 min after the end of the sound exposure. The results show that the effects on fine structures are highly individual and no systematic change was observed.     

Onset, growth, and recovery of in-air temporary threshold shift in a California sea lion (Zalophus californianus)

A California sea lion (Zalophus californianus) was tested in a behavioral procedure to assess noise-induced temporary threshold shift (TTS) in air. Octave band fatiguing noise was varied in both duration (1.5-50 min) and level (94-133 dB re 20 muPa) to generate a variety of equal sound exposure level conditions. Hearing thresholds were measured at the center frequency of the noise (2500 Hz) before, immediately after, and 24 h following exposure. Threshold shifts generated from 192 exposures ranged up to 30 dB. Estimates of TTS onset [159 dB re (20 muPa)(2) s] and growth (2.5 dB of TTS per dB of noise increase) were determined using an exponential function. Recovery for threshold shifts greater than 20 dB followed an 8.8 dB per log(min) linear function. Repeated testing indicated possible permanent threshold shift at the test frequency, but a later audiogram revealed no shift at this frequency or higher. Sea lions appear to be equally susceptible to noise in air and in water, provided that the noise exposure levels are referenced to absolute sound detection thresholds in both media. These data provide a framework within which to consider effects arising from more intense and/or sustained exposures.     

Effects of acoustic overstimulation on spectral and temporal processing in the amphibian auditory nerve

Activity of isolated auditory-nerve fibers in tree frogs (Eleutherodactylus coqui) exposed to continuous 3-min tones of different intensities at their characteristic frequencies (CFs) was recorded. Period histograms show a retardation in the preferred phase of discharge during and after the cessation of the exposure. Postexposure phase shift is concomitant with an elevation in CF thresholds and related to the level of tone exposure above threshold. Vector strength does not show comparable trends of change; postexposure shifts are related to preexposure CF thresholds. Recovery of phase retardation is rapid; units exposed to successive 3-min tones of the same intensities with intervals of 10-14 min between exposures experienced similar changes in their patterns of temporal discharge. Micromechanical changes affecting stereocilia stiffness or structural alterations in the tectorial membrane of the amphibian papilla may underly the transitory phase shifts observed in traumatized anuran auditory fibers.     

Towards a better understanding of temporary threshold shift of hearing

It was thought that temporary threshold shift of hearing due to exposure to noise might be more easily understood if the shifts were considered in terms of the rms pressure rather than in decibels. Therefore, the forms to be expected if the rate of shift of the pressure threshold were proportional to the difference between itself and the ultimate threshold were calculated and compared with a limited selection of published data. Good agreement with data for individuals during growth of TTS, and for one example of intermittent exposure to noise, was found and moderately good agreement with recovery. Agreement with averaged data, particularly for intermittent exposures, was poor, possibly because averaging the widely disparate figures obtained for individuals masks the true effects. It is also shown that the maximum ultimate TTS due to exposure to noise may be simply related to the mean square pressure of that noise.Further consideration of the mass of published work is needed, but this study suggests that at least some facets of TTS can be simply described in terms of exponential pressure shifts.     

Temporary threshold shift in human subject exposed to noise

Using an audiometer,the effect of the noise level upon temporarythreshold shift(TTS)for five trained normal subjects(left ear only)was studied.The measurements were carried out after 6 min exposure(in third octave band)for different sound pressure levels ranging between 75-105 dB at three test fre-quencies 2,3,and 4 kHz.The results indicated that at exposure to noise of soundpressure level(SPL)above 85 dB,TTS increases linearly with ths SPL for all thetest frequencies.The work had extended to study the recovery curves for the sameears.The results indicated that the reduction in TTS on doubling the recoverytimes,for the two sound pressure levels 95 dB and 105 dB,occurs at a rate of near-ly 3 dB.The comparison of the recovery curve at 3 kHz with that calculated usingWard's general equation for recovery was made.Finally,to study the values ofTTS produced by exposure to certain noise at different test frequencies,distribu-tion curves for two recovery times were plotted representing TTS values,for anexposure     

A simple mathematical model for TTS

Published data on Temporary Threshold Shift (TTS) suggest that in many cases the rms pressure at threshold during and after exposure to noise varies in a simple exponential manner, and that the ultimate shift of pressure threshold for exposure to steady noise is dependent on the mean square pressure of that noise. This response could occur if some part of the hearing mechanism were heated by exposure to noise and were at the same time subject to Newtonian cooling, and if the change in the pressure threshold were proportional to the change of temperature. This model can explain the shapes of many growth and recovery curves given in dB, why time constants found for recovery from TTS appear greater than those for growth and why threshold shifts on ears with elevated thresholds appear smaller than those for ears with low thresholds. Because of individual variation, averaged dB results mask the nature of the processes involved. Hence, for a better understanding of TTS, individual ears should be studied separately, and, if possible, measurements should be made in rms Pa instead of dB.     

Underwater temporary threshold shift in pinnipeds: effects of noise level and duration

Behavioral psychophysical techniques were used to evaluate the residual effects of underwater noise on the hearing sensitivity of three pinnipeds: a California sea lion (Zalophus californianus), a harbor seal (Phoca vitulina), and a northern elephant seal (Mirounga angustirostris). Temporary threshold shift (TTS), defined as the difference between auditory thresholds obtained before and after noise exposure, was assessed. The subjects were exposed to octave-band noise centered at 2500 Hz at two sound pressure levels: 80 and 95 dB SL (re: auditory threshold at 2500 Hz). Noise exposure durations were 22, 25, and 50 min. Threshold shifts were assessed at 2500 and 3530 Hz. Mean threshold shifts ranged from 2.9-12.2 dB. Full recovery of auditory sensitivity occurred within 24 h of noise exposure. Control sequences, comprising sham noise exposures, did not result in significant mean threshold shifts for any subject. Threshold shift magnitudes increased with increasing noise sound exposure level (SEL) for two of the three subjects. The results underscore the importance of including sound exposure metrics (incorporating sound pressure level and exposure duration) in order to fully assess the effects of noise on marine mammal hearing.     

Temporary threshold shifts in humans exposed to octave bands of noise for 16 to 24 hours.

Groups of human subjects were exposed in a diffuse sound field for 16--24 h to an octave-band noise centered at 4, 2, 1, or 0.5 kHz. Sound-pressure levels were varied on different exposure occasions. At specified times during an exposure, the subject was removed from the noise, auditory sensitivity was measured, and the subject was returned to the noise. Temporary threshold shifts (TTS) increased for about 8 h and then reached a plateau or asymptote. The relation between TTS and exposure duration can be described by a simple exponential function with a time constant of 2.1 h. In the frequency region of greatest loss, threshold shifts at asymptote increased about 1.7 dB for every 1 dB increase in the level of the noise above a critical level. Critical levels were empirically estimated to be 74.0 dB SPL at 4 kHz. 78 dB at 2 kHz, and 82 dB at 1 and 0.5 kHz. Except for the noise centered at 4.0 kHz, threshold shifts were maximal about 1/2 octave above the center frequency of the noise. A smaller second maximum was observed also at 7.0 kHz for the noise centered at 2.0 kHz, at 6.0 kHz for the noise centered at 1.0 kHz, and at 5.5 kHz for the noise centered at 0.5 kHz. After termination of the exposure, recovery to within 5 dB of pre-exposure thresholds was achieved within 24 h or less. Recovery can be described by a simple exponential function with a time constant of 7.1 h. The frequency contour defined by critical levels matches almost exactly the frequency contour defined by the E-weighting network.     

Is it necessary to penalize impulsive noise +5 dB due to higher risk of hearing damage?

It is studied whether the +5 dB penalty for impulsiveness established by ISO 1999:1990 accounts for a higher risk of noise-induced hearing loss. A total of 16 normal-hearing human subjects were exposed for 10 min to two types of binaural industrial-recordings: (1) a continuous broad-band noise normalized to L(EX,8 h)=80 dBA and (2) the combination of the previous stimulus with an impulsive noise normalized to L(EX,8 h)=75+5(db penalty)=80 dBA (peak level 117 dBC and repetition rate of 0.5 impacts per second). Distortion product otoacoustic emissions (DPOAEs) were measured in a broad frequency range before and in the following 90 min after the exposure. The group results show that the continuous exposure had a bigger impact on DPOAE levels, with a maximum DPOAE shift of approximately 5 dB in the frequency range of 2-3.15 kHz during the first 10 min of the recovery. No evident DPOAE shift is seen for the impulsive + continuous stimulus. The results indicate that the penalty overestimated the effects on DPOAE levels and support the concept that the risk of hearing loss from low-level impulses may be predicted on an equal-energy basis.     

Impact of three hours of discotheque music on pure-tone thresholds and distortion product otoacoustic emissions

The aim of this study was to investigate whether distortion product otoacoustic emissions (DPOAEs) are a suitable means for detecting changes in outer hair cell (OHC) functionality due to exposure to three hours of discotheque music and whether efferent reflex strength of the medial olivocochlear bundle is able to predict the ear's susceptibility to high-level noise. High-resolution DPOAEs (Δf(2)=47 Hz) were recorded between 3.5 and 4.5 kHz at close-to-threshold primary tone levels. For comparison, high-resolution pure-tone audiometry was conducted in the same frequency range. Efferent reflex strength was measured by means of DPOAEs at a specific frequency with and without contralateral acoustic stimulation. A significant deterioration of more than 10 dB was found for pure-tone thresholds and DPOAE levels indicating that three hours of high-level noise exert a considerable influence on hearing capability and OHC functionality. A significant correlation between shifts in pure-tone threshold and shifts in DPOAE level occurred when removing data with differing calibration across measurements. There was no clear correlation between efferent reflex strength and shifts in pure-tone threshold or shifts in DPOAE level suggesting that the applied measures of efferent reflex strength may not be suitable for quantifying individual vulnerability to noise.     

Temporary shift in masked hearing thresholds of bottlenose dolphins, Tursiops truncatus, and white whales, Delphinapterus leucas, after exposure to intense tones

A behavioral response paradigm was used to measure masked underwater hearing thresholds in five bottlenose dolphins and two white whales before and immediately after exposure to intense 1-s tones at 0.4, 3, 10, 20, and 75 kHz. The resulting levels of fatiguing stimuli necessary to induce 6 dB or larger masked temporary threshold shifts (MTTSs) were generally between 192 and 201 dB re: 1 microPa. The exceptions occurred at 75 kHz, where one dolphin exhibited an MTTS after exposure at 182 dB re: 1 microPa and the other dolphin did not show any shift after exposure to maximum levels of 193 dB re: 1 microPa, and at 0.4 kHz, where no subjects exhibited shifts at levels up to 193 dB re: 1 microPa. The shifts occurred most often at frequencies above the fatiguing stimulus. Dolphins began to exhibit altered behavior at levels of 178-193 dB re: 1 microPa and above; white whales displayed altered behavior at 180-196 dB re: 1 microPa and above. At the conclusion of the study all thresholds were at baseline values. These data confirm that cetaceans are susceptible to temporary threshold shifts (TTS) and that small levels of TTS may be fully recovered.     

Temporary threshold shift in a bottlenose dolphin (Tursiops truncatus) exposed to intermittent tones

Temporary threshold shift (TTS) was measured in a bottlenose dolphin exposed to a sequence of four 3-kHz tones with durations of 16 s and sound pressure levels (SPLs) of 192 dB re 1 μPa. The tones were separated by 224 s of silence, resulting in duty cycle of approximately 7%. The resulting growth and recovery of TTS were compared to experimentally measured TTS in the same subject exposed to single, continuous tones with similar SPLs. The data confirm the potential for accumulation of TTS across multiple exposures and for recovery of hearing during the quiet intervals between exposures. The degree to which various models could predict the growth of TTS across multiple exposures was also examined.     

Frequency specific susceptibility to acoustic trauma in the budgerigar (Melopsittacus undulatus)

The effects of intense noise exposure on hearing in the budgerigar were examined by behavioral audiometry. After binaural exposure to an intense broadband noise, auditory threshold shifts (TS) of the birds were continuously measured at frequencies between 0.125 and 8 kHz using an avoidance conditioning technique. Temporary threshold shifts (TTS) were observed at frequencies higher than 1.5 kHz and considerable permanent threshold shifts (PTS) were observed at frequencies below 1 kHz. This pattern of threshold shifts is contrary to that observed in mammals.     

Distortion product otoacoustic emission fine structure is responsible for variability of distortion product otoacoustic emission contralateral suppression

Alteration of the distortion product otoacoustic emission (DPOAE) level by a contralateral sound, which is known as DPOAE contralateral suppression, has been attributed to the feedback mechanism of the medial olivocochlear efferents. However, the limited dynamic range and large intra- and intersubject variabilities in the outcome of the measurement restrict its application in assessing the efferent function. In this study, the 2f(1)-f(2) DPgram was measured with a high frequency resolution in six human ears, which exhibits a fine structure with the quasiperiodic appearance of peaks and dips. In the presence of contralateral noise, the DPOAE level increased, decreased, or remained unchanged depending on the frequency. At the peaks, DPOAEs were mostly suppressed with a larger change, while those at the dips had greater variance, with increased occurrence of enhancement or no change. The difference between the peak and dip frequencies in the DPOAE-level change was significant. A switch from suppression to enhancement of the DPOAE level or vice versa with a small change in frequency was noted. These results imply that the DPOAE fine structure is a main source of the variability in DPOAE contralateral suppression measurement. The study suggests that the DPOAE contralateral suppression test would be improved if it is conducted for frequencies at major peaks of the DPOAE fine structure.     

Evidence for a bipolar change in distortion product otoacoustic emissions during contralateral acoustic stimulation in humans

The aim of this study was to investigate the activity of the medial olivocochlear (MOC) efferents during contralateral (CAS) and ipsilateral acoustic stimulation (IAS) by recording distortion product otoacoustic emission (DPOAE) suppression and DPOAE adaptation in humans. The main question was: do large bipolar changes in DPOAE level (transition from enhancement to suppression) also occur in humans when changing the primary tone level within a small range as described by Maison and Liberman for guinea pigs [J. Neurosci. 20, 4701-4707 (2000)]? In the present study, large bipolar changes in DPOAE level (14 dB on average across subjects) were found during CAS predominantly at frequencies where dips in the DPOAE fine structure occurred. Thus, effects of the second DPOAE source might be responsible for the observed bipolar effect. In contrast, comparable effects were not found during IAS as was reported in guinea pigs. Reproducibility of CAS DPOAEs was better than that for IAS DPOAEs. Thus, contralateral DPOAE suppression is suggested to be superior to ipsilateral DPOAE adaptation with regard to measuring the MOC reflex strength and for evaluating the vulnerability of the cochlea to acoustic overexposure in a clinical context.     

Noise-induced temporary threshold shift and recovery in Yangtze finless porpoises Neophocaena phocaenoides asiaeorientalis

In Yangtze finless porpoises Neophocaena phocaenoides asiaeorientalis, the effects of fatiguing noise on hearing thresholds at frequencies of 32, 45, 64, and 128 kHz were investigated. The noise parameters were: 0.5-oct bandwidth, -1 to +0.5 oct relative to the test frequency, 150 dB re 1 μPa (140-160 dB re 1 μPa in one measurement series), with 1-30 min exposure time. Thresholds were evaluated using the evoked-potential technique allowing the tracing of threshold variations with a temporal resolution better than 1 min. The most effective fatiguing noise was centered at 0.5 octave below the test frequency. The temporary threshold shift (TTS) depended on the frequencies of the fatiguing noise and test signal: The lower the frequencies, the bigger the noise effect. The time-to-level trade of the noise effect was incomplete: the change of noise level by 20 dB resulted in a change of TTS level by nearly 20 dB, whereas the tenfold change of noise duration resulted in a TTS increase by 3.8-5.8 dB.     

Underwater temporary threshold shift induced by octave-band noise in three species of pinniped.

Pure-tone sound detection thresholds were obtained in water for one harbor seal (Phoca vitulina), two California sea lions (Zalophus californianus), and one northern elephant seal (Mirounga angustirostris) before and immediately following exposure to octave-band noise. Additional thresholds were obtained following a 24-h recovery period. Test frequencies ranged from 100 Hz to 2000 Hz and octave-band exposure levels were approximately 60-75 dB SL (sensation level at center frequency). Each subject was trained to dive into a noise field and remain stationed underwater during a noise-exposure period that lasted a total of 20-22 min. Following exposure, three of the subjects showed threshold shifts averaging 4.8 dB (Phoca), 4.9 dB (Zalophus), and 4.6 dB (Mirounga). Recovery to baseline threshold levels was observed in test sessions conducted within 24 h of noise exposure. Control sessions in which the subjects completed a simulated noise exposure produced shifts that were significantly smaller than those observed following noise exposure. These results indicate that noise of moderate intensity and duration is sufficient to induce TTS under water in these pinniped species.     

Evidence for the distortion product frequency place as a source of distortion product otoacoustic emission (DPOAE) fine structure in humans. I. Fine structure and higher-order DPOAE as a function of the frequency ratio f2/f1

Critical experiments were performed in order to validate the two-source hypothesis of distortion product otoacoustic emissions (DPOAE) generation. Measurements of the spectral fine structure of DPOAE in response to stimulation with two sinusoids have been performed with normal-hearing subjects. The dependence of fine-structure patterns on the frequency ratio f2/f1 was investigated by changing f1 or f2 only (fixed f2 or fixed f1 paradigm, respectively), and by changing both primaries at a fixed ratio and looking at different order DPOAE. When f2/f1 is varied in the fixed ratio paradigm, the patterns of 2 f1-f2 fine structure vary considerably more if plotted as a function of f2 than as a function of fDP. Different order distortion products located at the same characteristic place on the basilar membrane (BM) show similar patterns for both, the fixed-f2 and fDP paradigms. Fluctuations in DPOAE level up to 20 dB can be observed. In contrast, the results from a fixed-fDP paradigm do not show any fine structure but only an overall dependence of DP level on the frequency ratio, with a maximum for 2f1-f2 at f2/f1 close to 1.2. Similar stimulus configurations used in the experiments have also been used for computer simulations of DPOAE in a nonlinear and active model of the cochlea. Experimental results and model simulations give strong evidence for a two-source model of DPOAE generation: The first source is the initial nonlinear interaction of the primaries close to the f2 place. The second source is caused by coherent reflection from a re-emission site at the characteristic place of the distortion product frequency. The spectral fine structure of DPOAE observed in the ear canal reflects the interaction of both these sources.