Subject-based calibration of a jawphone transducer with a bottlenose dolphin (Tursiops truncatus)

Abstract

Hearing studies with odontocete cetaceans often use suction-cup transducers known as``jawphones” to imitate underwater stimulus transmission. Jawphones are typically calibrated by measuring a frequency-response curve with a receiving hydrophone placed at a controlled distance in an underwater direct field. This procedure is somewhat controversial, as it may not sufficiently reproduce the odontocete sound reception pathway. This study calibrated a jawphone by comparing two behavioral audiograms for a single bottlenose dolphin. The first audiogram comprised underwater hearing thresholds (in dB re 1 μPa) measured using direct-field stimulation, the second comprised thresholds (in dBV) measured using a jawphone. All thresholds were measured using a psychophysical staircase procedure at frequencies from 14.1 to 150 kHz. The calibration curve was calculated by subtracting, at matched frequencies, jawphone thresholds from underwater direct-field thresholds. The resulting curve had the shape of a bandpass filter, with highest levels at frequencies from 56.6 to 130 kHz. This subject-based curve was similar in shape to a previously obtained direct-field calibration curve, although the subject-based values were higher above 20 kHz. These results are especially relevant to auditory evoked potential hearing studies that measure thresholds in odontocetes that are untrained for psychophysical procedures. [Work supported by ONR.]