Temporary threshold shifts from mid-frequency airborne noise exposures in sealsa)

To evaluate the residual effects of airborne noise, hearing thresholds were obtained before and after noise exposure using behavioral psychophysics for two phocid carnivores: a northern elephant seal (Mirounga angustirostris) and a harbor seal (Phoca vitulina). The seals were trained to position calmly during exposure to octave-band white noise centered at 1 or 2.5 kHz with varying level (65 to 125 dB sensation level, SL) and duration (1.5 to 50 min). Threshold shifts were determined at the center frequency of the noise band as the difference between detection thresholds measured prior to and following noise exposure. Results indicated that (1) neither seal exhibited consistent recoverable hearing loss (temporary threshold shift, TTS) to noise levels < 85 dB SL or durations below 12 min; (2) an equal-energy relationship between noise level and duration did not predict observed patterns of TTS; and (3) the harbor seal demonstrated two distinct responses to noise, with the sound exposure level required to induce TTS increasing by 24 dB in a second experimental phase. Phocids appear less susceptible to hearing loss from airborne noise exposures than otariid carnivores, apparently due to attenuation through permanent or active closure of the meatus during exposure to high-noise conditions.