June 17, 2024

Temporary Hearing Threshold Shift in California Sea Lions (Zalophus californianus) Due to One-Sixth-Octave Noise Bands Centered at 2 and 4 kHz: Effect of Duty Cycle and Testing the Equal-Energy Hypothesis


Ronald A. Kastelein, Lean Helder-Hoek, Linde N. Defillet, Léonie A. E. Huijser, John M. Terhune, and Robin Gransier


Document: Article
Abstract: In one of a series of studies of noise-induced hearing loss to determine the frequency-dependent susceptibility of California sea lions (Zalophus californianus) to temporary hearing threshold shift (TTS), two subjects were exposed for 60 min to two different fatiguing sounds. These were continuous one-sixth-octave noise bands (NBs), centered at 2 kHz, at sound pressure levels (SPLs) of 138 to 167 dB re 1 µPa (resulting in sound exposure levels [SELs] of 174 to 203 dB re 1 µPa2s), and at 4 kHz, at SPLs of 133 to 169 dB re 1 µPa (resulting in SELs of 169 to 205 dB re 1 µPa2s). Using a psychoacoustic technique, TTSs were quantified at 2, 2.8, 4.2, 5.6, and 8 kHz (at the center frequency of each NB, at half an octave higher, and at one octave higher). After exposure to both NBs, higher SELs resulted in greater TTS at all hearing frequencies that were tested. TTSs and hearing recovery patterns were similar in both sea lions. The effect of fatiguing sound duty cycle on TTS was investigated with the NB at 4 kHz and with 1.6-s signal duration, at a mean SPL of 169 dB re 1 µPa. Duty cycle reduction from 100 to 90% resulted in the largest decrease in TTS, and no TTS occurred at duty cycles ≤ 60%. The equal-energy hypothesis was investigated with the NB at 4 kHz: five combinations of SPL and exposure duration that resulted in the same SEL (197 dB re 1 µPa2s) produced similar initial TTSs in both sea lions. Susceptibility of California sea lions to TTS is higher than previously believed; for sounds around 2 and 4 kHz, it is similar to the susceptibility of harbor seals (Phoca vitulina). These data will contribute towards the development of an evidence-based underwater sound weighting function for the protection of Otariidae.
Key Words: anthropogenic noise, audiogram, auditory weighting, TTS, hearing damage, hearing sensitivity, hearing recovery, Otariidae, weighting
DOI: https://doi.org/10.1578/AM.47.4.2021.394
Page Numbers: 394-418


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