Temporary threshold shift to simulated naval continuous active sonar in bottlenose dolphins (Tursiops truncatus)a)

This study used behavioral and auditory evoked potential (AEP) methods to examine auditory temporary threshold shift (TTS) in four bottlenose dolphins exposed to simulated naval continuous active sonar (CAS). Exposures were up to 60 min in duration with sound pressure levels varied to create overlapping sound exposure levels (SELs), where the goal was to examine the equal-energy rule for long-duration tonal exposures. A high-frequency phase of the study measured TTS induced by a 19-s hyperbolic upsweep with a center frequency of 28 kHz. This waveform was based on a CAS waveform with frequency content shifted up an order of magnitude and placed in the dolphins' region of best hearing sensitivity. The second, mid-frequency phase of the study included a 50-s linear frequency-modulated waveform centered at 2.8 kHz. For the 2.8- and 28-kHz exposures, behavioral TTS increased exponentially with increasing SEL, independent of the specific exposure duration and level parameters. The lowest TTS onset SEL at 28 kHz was approximately 20 dB below that at 2.8 kHz, reflecting frequency-specific vulnerability in this species. Clear effects of noise exposures were not observed in AEPs measured during the 28-kHz testing phase, an unexpected result that contrasted with the findings of previous studies.