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Predicting temporary threshold shifts in a bottlenose dolphin (Tursiops truncatus): The effects of noise level and duration
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Image of FIG. 1.
FIG. 1.

Diagram of dolphin audiogram threshold testing and fatiguing noise pen experimental setup. (1a, 1b) Trainer positions, (2) assistant position, (3) hoop stations for noise and threshold tests. The equipment shack that housed the AEP and noise exposure equipment is also indicated.

Image of FIG. 2.
FIG. 2.

(a) Fatiguing noise spectral density recorded from the hoop calibration position in the noise pen. The octave-band noise is with a center frequency of . The SPL in this case is approximately re . (b) Ambient sound at experimental pens in Kaneohe Bay, Oahu, HI measured with a Biomon 8235 and plotted as noise spectral density using a 1024-point FFT. Ten noise samples were averaged to create both plots.

Image of FIG. 3.
FIG. 3.

(a) Audiogram of Boris. (b) Mean hearing thresholds ( SD) of Boris for in SPL (dB re ). The fatiguing noise band, from , is depicted at the bottom, relative to the hearing thresholds tested. (c) Mean ( SD) amount of TTS (dB re ) at each of the five frequencies tested. The * indicates that 5.6, 8, and demonstrated mean shifts that were significantly greater than their average threshold (one-way ANOVA and Tukey’s pairwise comparison, , ).

Image of FIG. 4.
FIG. 4.

Thresholds over the duration of the experiment for (a) and (b) . Black diamonds indicated thresholds measured after a fatiguing noise exposure. The middle line indicates mean threshold, and top and bottom lines indicate SD from the mean.

Image of FIG. 5.
FIG. 5.

Sound exposure levels required to induce TTS as the duration of exposure changes at all threshold frequencies tested. Shorter duration exposures required greater SEL to induce TTS. Dotted line indicates an equal-energy line of SEL. Black diamonds indicate TTS occurrence; open circles indicate no TTS.

Image of FIG. 6.
FIG. 6.

(a) Amount of threshold shift measured at (closed symbols) and (open symbols) for SELs of and below. Increasing SELs and corresponding symbols are labeled at the right, demonstrating that at constant exposure duration but increasing SEL, TTS increased. A regression of the SEL data was used to illustrate that with decreasing noise exposure duration, TTS decreased when SEL was held constant (, , , ). (b) Amount of TTS for SELs of and higher. Threshold frequencies are not discriminated, but SEL symbols are labeled to the right. Note that at shorter durations the amount of shift is clustered and higher SELs (top circle) are required to induce significant TTS.

Image of FIG. 7.
FIG. 7.

Thresholds at before and after exposures for SEL for five exposure durations (min): 30 (triangle), 15 (X), 7.5 (open square), 3.75 (open circle), and 1.875 (diamond). Thresholds were measured 5, 10, 20, 40, and after noise ended. The dotted line indicates noise exposure. Arrows indicate mean threshold SD.

Image of FIG. 8.
FIG. 8.

Threshold shift (dB re ) recovery functions demonstrating linear logarithmic recoveries in log time across multiple analysis methods: (a) (linear time), (b) (log time), (c) , (d) TTS from longer duration exposures , and (e) TTS from shorter duration exposures . The rate of recovery in all cases was approximately /doubling of time.

Image of FIG. 9.
FIG. 9.

Mean number of respirations (a) and delay from intertrial stationing pad to hoop station (b) for various noise exposure and control trials. For comparison, we grouped the sessions by the number of trials as well as summed data from all trials of all sessions. There were significantly greater respirations, indicated by the *, during noise sessions with trials and all the sessions grouped together (one-way ANOVA and Tukey’s pairwise comparison, , ). The hoop delay was significantly greater for all groups of noise exposure trials (, ).

Image of FIG. 10.
FIG. 10.

The SELs required to induce TTS at higher noise levels where threshold frequencies are discriminated. Just TTS occurrence is plotted. Two methods to predict TTS are also graphed: a linear estimation of TTS (dotted line) (, , , ) and a logarithmic estimation (, , , ). Note that in both cases, the slope is positive, indicating that for shorter time durations, greater energy is required to induce TTS.

Image of FIG. 11.
FIG. 11.

SL (dB re threshold) at which TTS was induced for various exposure durations. TTS could be induced at much lower SLs for longer duration exposures. Note that the abscissa is categorical.


Generic image for table

Noise exposure experimental matrix where the fatiguing sound was octave-band white noise of . Parameters listed include audiogram test frequency (kHz), exposure duration (min), noise SPL, SEL, and number of exposures and number of shifts at corresponding test conditions.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Predicting temporary threshold shifts in a bottlenose dolphin (Tursiops truncatus): The effects of noise level and duration