Loud underwater sounds may cause temporary hearing loss in harbor porpoises, the magnitude of which may depend on the exposure duration. After exposure to playbacks of broadband pile driving sounds, temporary hearing threshold shifts (TTS) in two porpoises were quantified at 8 kHz (highest TTS) with a psychoacoustic technique. Pile driving sounds had: pulse duration 124 ms, rate 2760 strikes per hour, inter-pulse interval 1.3 s, duty cycle ~9.5%, average received single-strike unweighted sound exposure level (SEL ss ) 145 dB re 1μ Pa2s, exposure durations 15, 30, 60, 120, 180, 240 and 360 min (cumulative SEL:173, 176, 179, 182, 184, 185, 187 dB re 1μ Pa2s, respectively). Control sessions under ambient noise conditions were carried out. Mean initial TTS (1-4 min after sound exposure stopped in one porpoise, and 12-16 min after exposure stopped in the other animal) increased from 0 dB after 15 min exposure to 5 dB after 360 min exposure. In both animals, recovery occurred within 60 min post-exposure. The relatively small increase in TTS between 15 and 360 min exposures is due to the relatively small amount of sound energy per unit of time to which the porpoises were exposed (average sound pressure level ~144 dB re 1μ Pa).
This report was presented at a symposium at Naturalis in Leiden on September 8, 2015. A complete report containing brief abstracts of all studies presented at the symposium can be found here.