SUMMARYTemporary threshold shift (TTS) after loud noise exposure was investigated in a male and a female beluga whale (Delphinapterus leucas). The thresholds were evaluated using the evoked-potential technique, which allowed for threshold tracing with a resolution of ~1min. The fatiguing noise had a 0.5octave bandwidth, with center frequencies ranging from 11.2 to 90kHz, a level of 165dBre.1μPa and exposure durations from 1 to 30min. The effects of the noise were tested at probe frequencies ranging from -0.5 to +1.5octaves relative to the noise center frequency. The effect was estimated in terms of both immediate (1.5min) postexposure TTS and recovery duration. The highest TTS with the longest recovery duration was produced by noises of lower frequencies (11.2 and 22.5kHz) and appeared at a test frequency of +0.5octave. At higher noise frequencies (45 and 90kHz), the TTS decreased. The TTS effect gradually increased with prolonged exposures ranging from 1 to 30min. There was a considerable TTS difference between the two subjects.
In a bottlenose dolphin positions of sound receiving areas on the head surface were determined by comparing the acoustic delays from different sound-source positions. For this investigation, auditory brainstem responses (ABRs) to short tone pips were recorded and their latencies were measured at different sound source positions. After correction for the latency dependence on response amplitude, the difference in ABR latencies was adopted as being the difference of the acoustic delays. These delay differences were used to calculate the position of the sound-receiving point. Measurements were conducted at sound frequencies from 16 to 128 kHz, in half-octave steps. At probe frequencies of 16 and 22.5 kHz, the receiving area was located 21.7-26 cm caudal of the melon tip, which is near the bulla and auditory meatus. At higher probe frequencies, from 32 to 128 kHz, the receiving area was located from 9.3 to 13.1 cm caudal of the melon tip, which corresponds to a proximal part of the lower jaw. Thus, at least two sound-receiving areas (acoustic windows) with different frequency sensitivity were identified.
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