8. Psychoacoustic Studies of Dolphins and Whales
353
Higher frequencies resulted in a more rapid fall off in sensitivity from the
midline. The 3-dB beamwidths were 59.1°,32.0°, and 13.7° for frequencies
of 30, 60, and 120kHz. Data from both horizontal and vertical beamwidths
were used to calculate a directivity index based on a two-element rectangular array with a length of 12cm, a width of 2.6cm, and a depth of 3.1 cm.
This experiment is a particularly elegant example of the use of psychophysical techniques to model the functional hearing processes of the
dolphin. Fifty-four different thresholds were required to map the receiving
beam patterns in both the horizontal and vertical planes. These data were
required in order to develop a model that serves to define the animal's
auditory system in terms that are particularly relevant and comparable to
those used in sonar engineering. Given that most odontocete audiograms
are drawn based on 12 to 15 threshold values, this study represents a considerable effort and expenditure of time and resources and will likely not
be repeated on another species soon despite its apparent value.
4. Masked Hearing and Frequency Selectivity
The question of how cetaceans process the frequency information in sounds
can be approached in a number of ways. We know from the basic pure tone
threshold audiograms that all odontocete species measured to date can perceive frequencies humans consider ultrasonic, with many species able to
perceive frequencies higher than 100kHz (see Section 2). Common seals
have been demonstrated to detect loud high-frequency sounds under water
(M~hI1967), but odontocetes have also demonstrated the ability to discern
between these sounds based on frequency differences. Frequency selectivity has been studied by training an animal to discriminate one tone from
others, and to detect a specific tone against a background of noise with
various characteristics. Performance on both tasks relates to auditory frequency filtering performed in the inner ear.
4.1 Frequency Discrimination Limens
A preliminary attempt to obtain frequency discrimination difference
thresholds by Jacobs (1972) found that the bottlenose dolphin subject was
more easily trained to tell the difference between frequency-modulated and
pure tones than it was to discriminate series of pulses that were of constant
frequency or of varying frequencies.
Herman and Arbeit (1972) also used a frequency modulation stimulus
presentation technique to examine the ability of T. truncatus to discriminate frequency differences. The animal was trained to station in front of two
emitting hydrophones. A pure tone and the same tone frequency modulated
by 1 to 2Hz were sequentially presented from the two spatially separated
hydrophones. The animal's task was to choose the pure tone. The frequency
353
Higher frequencies resulted in a more rapid fall off in sensitivity from the
midline. The 3-dB beamwidths were 59.1°,32.0°, and 13.7° for frequencies
of 30, 60, and 120kHz. Data from both horizontal and vertical beamwidths
were used to calculate a directivity index based on a two-element rectangular array with a length of 12cm, a width of 2.6cm, and a depth of 3.1 cm.
This experiment is a particularly elegant example of the use of psychophysical techniques to model the functional hearing processes of the
dolphin. Fifty-four different thresholds were required to map the receiving
beam patterns in both the horizontal and vertical planes. These data were
required in order to develop a model that serves to define the animal's
auditory system in terms that are particularly relevant and comparable to
those used in sonar engineering. Given that most odontocete audiograms
are drawn based on 12 to 15 threshold values, this study represents a considerable effort and expenditure of time and resources and will likely not
be repeated on another species soon despite its apparent value.
4. Masked Hearing and Frequency Selectivity
The question of how cetaceans process the frequency information in sounds
can be approached in a number of ways. We know from the basic pure tone
threshold audiograms that all odontocete species measured to date can perceive frequencies humans consider ultrasonic, with many species able to
perceive frequencies higher than 100kHz (see Section 2). Common seals
have been demonstrated to detect loud high-frequency sounds under water
(M~hI1967), but odontocetes have also demonstrated the ability to discern
between these sounds based on frequency differences. Frequency selectivity has been studied by training an animal to discriminate one tone from
others, and to detect a specific tone against a background of noise with
various characteristics. Performance on both tasks relates to auditory frequency filtering performed in the inner ear.
4.1 Frequency Discrimination Limens
A preliminary attempt to obtain frequency discrimination difference
thresholds by Jacobs (1972) found that the bottlenose dolphin subject was
more easily trained to tell the difference between frequency-modulated and
pure tones than it was to discriminate series of pulses that were of constant
frequency or of varying frequencies.
Herman and Arbeit (1972) also used a frequency modulation stimulus
presentation technique to examine the ability of T. truncatus to discriminate frequency differences. The animal was trained to station in front of two
emitting hydrophones. A pure tone and the same tone frequency modulated
by 1 to 2Hz were sequentially presented from the two spatially separated
hydrophones. The animal's task was to choose the pure tone. The frequency