Susceptibility of Harbor Porpoise Hearing to Intermittent Sound Exposures

The effects of intermittent sound exposures on the hearing of marine mammals are not well understood. This chapter explores a model to predict growth of Temporary Hearing Threshold Shift (TTS) considering level and duration of the sound exposure in harbor porpoises, a species which has vulnerable hearing. This model was evaluated using TTS growth data obtained for harbor porpoises with different sound exposures (intermittent and continuous sonar, and intermittent pile driving and air-gun sound). An adapted version of a modified-power-law (MPL) model was introduced, which attempted to overcome some shortcomings of the original model, proposed by Humes and Jesteadt (J Acoust Soc Am 85:1285–1294, 1989)) that required growth and recovery from individual exposures to be measurable. Fitting of the data to this model resulted in parameters that could explain TTS growth observed in a wide range of sonar exposure conditions (various sound pressure levels, 1–2 kHz, 10% duty cycle vs continuous exposure, and various exposure durations), but underestimated the TTS growth of exposure to impulse sound (air guns and pile driving) and sonar with higher (6–7 kHz) frequencies. These results suggested that development of a single TTS growth model for different fatiguing sound exposures is promising, but requires further development and validation, as well as additional data on TTS growth and recovery due to different signal parameters of the fatiguing sound.