Safety criteria for underwater sounds from offshore pile driving are needed to protect marine mammals. As a first step toward understanding effects of impulsive sounds, two harbor seals were exposed to octave-band white noise centered at 4 kHz at three mean received sound pressure levels (SPLs; 124, 136, and 148 dB re 1 lPa) at up to six durations (7.5, 15, 30, 60, 120, and 240 min); mean received sound exposure level (SEL) range was 166–190 dB re 1 lPa2 s. Hearing thresholds were determined before and after exposure. Temporary hearing threshold shifts (TTS) and subsequent recovery were quantified as changes in hearing thresholds at 1–4, 4–8, 8–12, 48, and 96 min after noise exposure in seal 01, and at 12–16, 16–20, 20–24, 60, and 108 min after exposure in seal 02. Maximum TTS (1–4min after 120min exposure to 148dB re 1lPa; 187dB SEL) was 10dB. Recovery occurred within 60 min. Statistically significant TTSs (>2.5 dB) began to occur at SELs of 170 (136 SPL, 60 min) and 178 dB re 1 lPa2 s (148 SPL, 15 min). However, SEL is not an optimal predictor of TTS for long duration, low SPL continuous noise, as duration and SPL play unequal roles in determining induced TTS.