Acoustic field measurements and bottlenose dolphin hearing thresholds using single-frequency and frequency-modulated tones

Abstract

Studies of underwater hearing are often hampered by the behavior of sound waves in small experimental tanks. At lower frequencies, tank dimensions are often not sufficient for free-field conditions, resulting in large spatial variations of sound pressure. These effects may be mitigated somewhat by increasing the frequency bandwidth of the sound stimulus, so effects of multipath interference average out over many frequencies. In this study, acoustic fields and bottlenose dolphin (Tursiops truncatus) hearing thresholds were compared for pure-tone and frequency-modulated stimuli. Experiments were conducted in a vinyl-walled, seawater-filled pool approximately 4×5×1.5 m. Sound stimuli consisted of 500-ms tones at 13 carrier frequencies between 1 and 100 kHz. Frequency-modulated stimuli featured both linear and sinusoidal modulating waveforms with 5%, 10%, and 20% bandwidths. Acoustic fields were measured (without the dolphin present) at three depths over a 60×65-cm grid with a 5-cm spacing. Hearing thresholds were measured using a behavioral response paradigm and up/down staircase technique. Frequency-modulated stimuli with a 10% bandwidth resulted in significant improvements to the sound field without substantially affecting the dolphins hearing thresholds. [Work supported by ONR.]