The echolocation transmission beam pattern of a false killer whale (Pseudorca crassidens) was measured in the vertical and horizontal planes. A vertical array of seven broadband miniature hydrophones was used to measure the beam pattern in the vertical plane and a horizontal array of the same hydrophones was used in the horizontal plane. The measurements were performed in the open waters of Kaneohe Bay, Oahu, Hawaii, while the whale performed a target discrimination task. Four types of signals, characterized by their frequency spectra, were measured. Type-1 signals had a single low-frequency peak at 40 +/- 9 kHz and a low-amplitude shoulder at high frequencies. Type-2 signals had a bimodal frequency characteristic with a primary peak at 46 +/- 7 kHz and a secondary peak at 88 +/- 13 kHz. Type-3 signals were also bimodal but with a primary peak at 100 +/- 7 kHz and a secondary peak at 49 +/- 9 kHz. Type-4 signals had a single high-frequency peak at 104 +/- 7 kHz. The center frequency of the signals were found to be linearly correlated to the peak-to-peak source level, increasing with increasing source level. The major axis of the vertical beam was directed slightly downward between 0 and -5 degrees, in contrast to the +5 to 10 degrees for Tursiops and Delphinapterus. The beam in the horizontal plane was directed forward between 0 degrees and -5 degrees.(ABSTRACT TRUNCATED AT 250 WORDS)

The time course of recovery from temporary threshold shift (TTS) was measured in a bottlenose dolphin, Tursiops truncatus, using an evoked‐potential procedure. The envelope‐following response (EFR), which is a rhythmic train of auditory brainstem responses (ABR) to sinusoidally amplitude‐modulated tones, was used as an indicator of the sound reception by the animal. Variation of the intensity of the stimulus allowed us to measure the animal's hearing via EFR thresholds. During each session, following an initial measure of threshold, the trained animal voluntary positioned itself within a hoop 1 m underwater while a 160 dB re 1 μPa noise of a 4–11 kHz bandwidth was presented for 30 min. After the noise exposure, thresholds were measured again at delays of 5, 10, 15, 25, 45, and 105 min. Measurements were made at test frequencies of 8, 11.2, 16, 22.5, and 32 kHz. The maximum TTS occurred 5 min after exposure and rapidly recovered with a rate of around 1.5 dB per doubling of time. TTS occurred at test frequencies from 8 to 16 kHz, with the maximum at 16 kHz. TTS was negligible at 22.5 kHz and absent at 32 kHz.

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Behaviorally determined hearing thresholds for a 7.5-kHz tone for an Atlantic bottlenosed dolphin (Tursiops truncatus) were obtained following exposure to fatiguing low-frequency octave band noise. The fatiguing stimulus ranged from 4 to 11 kHz and was gradually increased in intensity to 179 dB re 1 microPa and in duration to 55 min. Exposures occurred no more frequently than once per week. Measured temporary threshold shifts averaged 11 dB. Threshold determination took at least 20 min. Recovery was examined 360, 180, 90, and 45 min following exposure and was essentially complete within 45 min.