Temporary Hearing Threshold Shift and Testing the Equal-Energy Hypothesis in a Harbor Porpoise (Phocoena phocoena) After Exposure to a Continuous Noise Band at 8 kHz, and a Revised TTS-Onset Function

Susceptibility to temporary hearing threshold shifts (TTS) in harbor porpoises (Phocoena phocoena) depends in part on the frequency of the fatiguing sound (the sound causing the shift). The TTS induced and the pattern of recovery were documented in a female porpoise after exposure for one hour to a continuous one-sixth-octave noise band centered at 8 kHz. This fatiguing sound was emitted at average received sound pressure levels (SPLs) between 126 and 144 dB re 1 µPa, resulting in average sound exposure levels (SELs) of 162 to 180 dB re 1 µPa2s. Hearing thresholds for narrow-band sweeps centered at 8, 11.3, and 16 kHz were determined before and after exposure. Control sessions were used to determine which SELs resulted in statistically significant TTS in the first four minutes after the fatiguing sound stopped (TTS1-4). At 8 kHz, the lowest SEL that resulted in significant TTS1-4 (4.4 dB) was 174 dB re 1 µPa2s; at 11.3 kHz, the lowest SEL that resulted in significant TTS1-4 (4.9 dB) was 168 dB re 1 µPa2s; and at 16 kHz, the lowest SEL that resulted in significant TTS1-4 (1.3 dB) was 174 dB re 1 µPa2s. The hearing frequency that was most affected was 11.3 kHz, half an octave above the fatiguing sound’s center frequency. The equal-energy hypothesis was tested by exposing the porpoise to the same noise band with SPLs of 137 to 153 dB re 1 µPa and exposure durations between two and 80 minutes; all seven combinations resulted in the same fatiguing SEL of 174 dB re 1 µPa2s; and for these combinations, the equal-energy hypothesis was upheld. The results add to the body of data on TTS-onset SELs that were used to generate a revised auditory weighting function and, thus, enhance regulatory protection of wild harbor porpoises that are exposed to anthropogenic noise at sea.