352
PE. Nachtigall et at.
at discreet points on its head including the area of the external auditory
meatus, the exterior area above the pan bone, the anterior portion of its jaw
and points inside the mouth. Generally ABRs demonstrated support for
Norris' (1968) jaw hearing hypothesis with maximum sensitivity at an area
25 cm behind the tip of the jaw, but analysis of both the latencies of ABRs,
as well as sensitivity measures, indicated that minimal time and maximum
sensitivity measures occurred at different points. The question, "Where is
sound entering the auditory system of dolphins?" may have to be rephrased
into two questions, one dealing with the area of minimum delay and the
other dealing with the area of minimum attenuation. Because these areas
differed, a shaded receiver model for the dolphin's peripheral auditory
reception apparatus was proposed.
3.4 Receiving Beam Pattern
The dolphin's ability to hear a sound located directly in front of it when
masking noises were presented from a variety of directions was examined
by Au and Moore (1984) in order to determine the bottlenose dolphin's
directivity index, a term used primarily by sonar engineers when modeling
the directivity of a listening array. The receiving beam pattern measures the
amount of external noise and the amount of unwanted reverberation that
will be received. The wider the beam, the more noise and reverberation that
will be received.
When sounds were presented to the dolphin in the vertical plane, the
animal's detection of a pure tone signal, presented directly in front of it,
was increasingly disrupted as the noise source moved closer to the emitted
pure tone signal. This effect varied with frequency. The animal's ability to
detect the 30-kHz signal in noise remained good (less than 10 dB fall off)
within a 120° noise arc but rapidly fell off (more than 20dB) within a 40°
noise arc when 120-kHz pure tones were detected. The 3-dB beamwidths
were approximately 30.4°,22.7°, and 17.0° for frequencies of 30, 60, and
120 kHz. White noise presented from above or below tended to differentially disrupt the ability to detect the signal depending on the frequency of
the signal. The dolphin should therefore be able to differentially use higher
frequencies to obtain directional information. The receiving beam pattern
was narrower for higher frequencies than it was for lower frequencies.
Because of the possibility that the dolphin could internally steer its beam
pattern in the horizontal plane, the same technique could not be used to
determine the horizontal receiving beam pattern. Au and Moore (1984)
changed their technique to provide noise from two fixed positions from a
matched pair of transducers located plus and minus 20° from the midline
while varying the azimuthal position of the signal transducer between sessions. Horizontal receiving beam patterns were obtained by plotting the
masked thresholds as a function of the angular displacement of the signal
transducer. Once again, the animal was differentially affected by the noise.
PE. Nachtigall et at.
at discreet points on its head including the area of the external auditory
meatus, the exterior area above the pan bone, the anterior portion of its jaw
and points inside the mouth. Generally ABRs demonstrated support for
Norris' (1968) jaw hearing hypothesis with maximum sensitivity at an area
25 cm behind the tip of the jaw, but analysis of both the latencies of ABRs,
as well as sensitivity measures, indicated that minimal time and maximum
sensitivity measures occurred at different points. The question, "Where is
sound entering the auditory system of dolphins?" may have to be rephrased
into two questions, one dealing with the area of minimum delay and the
other dealing with the area of minimum attenuation. Because these areas
differed, a shaded receiver model for the dolphin's peripheral auditory
reception apparatus was proposed.
3.4 Receiving Beam Pattern
The dolphin's ability to hear a sound located directly in front of it when
masking noises were presented from a variety of directions was examined
by Au and Moore (1984) in order to determine the bottlenose dolphin's
directivity index, a term used primarily by sonar engineers when modeling
the directivity of a listening array. The receiving beam pattern measures the
amount of external noise and the amount of unwanted reverberation that
will be received. The wider the beam, the more noise and reverberation that
will be received.
When sounds were presented to the dolphin in the vertical plane, the
animal's detection of a pure tone signal, presented directly in front of it,
was increasingly disrupted as the noise source moved closer to the emitted
pure tone signal. This effect varied with frequency. The animal's ability to
detect the 30-kHz signal in noise remained good (less than 10 dB fall off)
within a 120° noise arc but rapidly fell off (more than 20dB) within a 40°
noise arc when 120-kHz pure tones were detected. The 3-dB beamwidths
were approximately 30.4°,22.7°, and 17.0° for frequencies of 30, 60, and
120 kHz. White noise presented from above or below tended to differentially disrupt the ability to detect the signal depending on the frequency of
the signal. The dolphin should therefore be able to differentially use higher
frequencies to obtain directional information. The receiving beam pattern
was narrower for higher frequencies than it was for lower frequencies.
Because of the possibility that the dolphin could internally steer its beam
pattern in the horizontal plane, the same technique could not be used to
determine the horizontal receiving beam pattern. Au and Moore (1984)
changed their technique to provide noise from two fixed positions from a
matched pair of transducers located plus and minus 20° from the midline
while varying the azimuthal position of the signal transducer between sessions. Horizontal receiving beam patterns were obtained by plotting the
masked thresholds as a function of the angular displacement of the signal
transducer. Once again, the animal was differentially affected by the noise.