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Temporary Hearing Threshold Shift in Harbor Porpoises (Phocoena phocoena) Due to One-Sixth Octave Noise Band at 16 kHz
Abstract: Susceptibility to temporary threshold shift (TTS) depends on the frequency of the fatiguing sound. So far, TTS in harbor porpoises (Phocoena phocoena) has been tested for sounds in the 1 to 7 kHz range. To assess the impact of anthropogenic noise, TTS needs to be investigated for other frequencies within the porpoise hearing range. TTSs were quantified in two porpoises that were exposed for one hour to a continuous one-sixth octave noise band centered at 16 kHz, at average received sound pressure levels (SPLs) of 117 to 145 dB re 1 µPa, and a sound exposure level (SEL) range of 153 to 181 dB re 1 µPa2s. Hearing thresholds for 16, 22.4, and 32 kHz signals were determined before and after exposure, to quantify TTS and recovery. The highest TTS, measured 1 to 4 minutes after exposure, occurred at 22.4 kHz. Statistically significant TTS occurred at 16 kHz after exposure to 159 dB SEL, at 22.4 kHz after exposure to 165 dB SEL, and at 32 kHz after exposure to 181 dB SEL. The susceptibility of the two porpoises to TTS induced by the exposures (16 kHz; 1 h) was similar. Below 6.5 kHz, it appears that susceptibility to TTS increases with increasing frequency; whereas above 6.5 kHz, it appears that susceptibility to TTS decreases with increasing frequency (for the frequency range tested so far).
Key Words: anthropogenic noise, audiogram, frequency weighting, hearing, hearing sensitivity, hearing damage, odontocete, TTS, temporary threshold shift
Page Numbers: 280-292