Effects of exposure to sonar playback sounds (3.5 - 4.1 kHz) on harbor porpoise (Phocoena phocoena) hearing.

Safety criteria for naval sonar sounds are needed to protect harbor porpoise hearing. Two porpoises were exposed to sequences of AN/SQS-53C sonar playback sounds (3.5-4.1 kHz, without significant harmonics), at a mean received sound pressure level of 142 dB re 1 µPa, with a duty cycle of 96% (almost continuous). Behavioral hearing thresholds at 4 and 5.7 kHz were determined before and after exposure to the fatiguing sound, in order to quantify temporary threshold shifts (TTSs) and hearing recovery. Control sessions were also conducted. Significant mean initial TTS1-4 of 5.2 dB at 4 kHz and 3.1 dB at 5.7 kHz occurred after 30 min exposures (mean received cumulative sound exposure level, SELcum: 175 dB re 1 µPa2s). Hearing thresholds returned to pre-exposure levels within 12 min. Significant mean initial TTS1-4 of 5.5 dB at 4 kHz occurred after 60 min exposures (SELcum: 178 dB re 1 µPa2s). Hearing recovered within 60 min. The SELcum for AN/SQS-53C sonar sounds required to induce 6 dB of TTS 4 min after exposure (the definition of TTS onset) is expected to be between 175 and 180 dB re 1 µPa2s.