Onset, growth, and recovery of in-air temporary threshold shift in a California sea lion (Zalophus californianus)

A California sea lion (Zalophus californianus) was tested in a behavioral procedure to assess noise-induced temporary threshold shift (TTS) in air. Octave band fatiguing noise was varied in both duration (1.5-50 min) and level (94-133 dB re 20 muPa) to generate a variety of equal sound exposure level conditions. Hearing thresholds were measured at the center frequency of the noise (2500 Hz) before, immediately after, and 24 h following exposure. Threshold shifts generated from 192 exposures ranged up to 30 dB. Estimates of TTS onset [159 dB re (20 muPa)(2) s] and growth (2.5 dB of TTS per dB of noise increase) were determined using an exponential function. Recovery for threshold shifts greater than 20 dB followed an 8.8 dB per log(min) linear function. Repeated testing indicated possible permanent threshold shift at the test frequency, but a later audiogram revealed no shift at this frequency or higher. Sea lions appear to be equally susceptible to noise in air and in water, provided that the noise exposure levels are referenced to absolute sound detection thresholds in both media. These data provide a framework within which to consider effects arising from more intense and/or sustained exposures.