Safety criteria for underwater sound produced during offshore pile driving are needed to protect marine mammals. A harbor porpoise was exposed to fatiguing noise at 18 sound pressure level (SPL) and duration combinations. Its temporary hearing threshold shift (TTS) and hearing recovery were quantified with a psychoacoustic technique. Octave-band white noise centered at 4 kHz was the fatiguing stimulus at three mean received SPLs (124, 136, and 148 dB re 1 μPa) and at six durations (7.5, 15, 30, 60, 120, and 240 min). Approximate received sound exposure levels (SELs) varied between 151 and 190 dB re 1 μPa2s. Hearing thresholds were determined for a narrow-band frequency-swept sine wave (3.9–4.1 kHz; 1 s) before exposure to the fatiguing noise, and at 1–4, 4–8, 8–12, 48, and 96 min after exposure. The lowest SEL (151 dB re 1 μPa2s) which caused a significant TTS1–4 was due to exposure to an SPL of 124 dB re 1 μPa for 7.5 min. The maximum TTS1–4, induced after a 240 min exposure to 148 dB re 1 μPa, was around 15 dB at a SEL of 190 dB re 1 μPa2s. Recovery time following TTS varied between 4 min and under 96 min, depending on the exposure level, duration, and the TTS induced.

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