Temporary hearing threshold shift in harbor seals (Phoca vitulina) due to a one-sixth-octave noise band centered at 32 kHz.

Two female harbor seals were exposed for 60 min to a continuous one-sixth-octave noise band centered at 32 kHz at sound pressure levels of 92 to 152 dB re 1 μPa, resulting in sound exposure levels (SELs) of 128 to 188 dB re 1 μPas. This was part of a larger project determining frequency-dependent susceptibility to temporary threshold shift (TTS) in harbor seals over their entire hearing range. After exposure, TTSs were quantified at 32, 45, and 63 kHz with a psychoacoustic technique. At 32 kHz, only small TTSs (up to 5.9 dB) were measured 1-4 min (TTS) after exposure, and recovery was within 1 h. The higher the SEL, the higher the TTS induced at 45 kHz. Below ∼176 dB re 1 μPas, the maximum TTS was at 32 kHz; above ∼176 dB re 1 μPas, the maximum TTS (up to 33.8 dB) was at 45 kHz. During one particular session, a seal was inadvertently exposed to an SEL of ∼191 dB re 1 μPas and at 45 kHz, her TTS was >45 dB; her hearing recovered over 4 days. Harbor seals appear to be equally susceptible to TTS caused by sounds in the 2.5-32 kHz range.