8. Psychoacoustic Studies of Dolphins and Whales
349
Complex sounds are probably localized by a combination of cues.
Dolphin auditory localization may sometimes occur under open anechoic
conditions, but reverberations from reflections off of nearby objects, from
the surface of the water, the bottom, and from nonlinearities in the temperature and salinity of the water can further contribute to the complexity
of auditory localization. Laboratory experiments of auditory localization
have focused on simple stimuli in simple environments in order to identify
the cues used. Similar studies have been conducted with dolphins.
There initially was a good reason to doubt that dolphins could use the
same cues as terrestrial mammals in localization. The impedance match
between the dolphin's tissue and that of seawater is much closer than is the
corresponding match between terrestrial mammals and air (see MlZlhl et al.
1999; Ketten, Chapter 2). As a result, it was not obvious that the two ears
were sufficiently isolated to allow them to act as independent receivers.
Further, the high velocity of sound in seawater, which is five times the velocity in air, means that any differences in the time of arrival of a sound at the
two ears would be correspondingly smaller than for a similar-sized terrestrial mammal.
3.2 Interaural Time Difference
The relationship between the difference in the length of the sound path at
the two ears (t:.d) and its angular position (e) is
t:.d = r(e + sine)
where r is the radius of the head, or half the distance between the two ears.
In the case of dolphins, it is not immediately obvious what r should be
because it is unclear where along the jaw it should be measured. The middle
of the pan bone seems like a sensible place from which to measure, but in
the absence of evidence it seems rather arbitrary.
Using contact jaw phones, Moore et al. (1995) studied the dolphin's sensitivity to interaural time and intensity differences. Jaw phones are small
(45-mm) suction cups containing a transducer. The phones were placed on
the base of the animal's lower jaw, in the approximate area of the pan bone,
where the bone broadens and becomes quite thin (see Norris 1968; Ketten,
Chapter 2). The stimuli consisted of a 2-s train of clicks. Each click consisted
of a single cycle of the test frequency with an interclick interval of 40ms.
Each session tested the animal at five different time or intensity differences
presented in blocks of 10 trials each. The dolphin touched a response wand
on the left if the clicks on that side preceded the clicks on the right or were
louder, and touched a response wand on the right if the opposite was true.
Signals between 5 and 90 kHz were tested.
Using 75% accuracy as the conventional threshold level, the dolphin's
interaural time difference thresholds were found to be between 7 and 18
Ils. In order to compare these results to those of Renaud and Popper (1975),
349
Complex sounds are probably localized by a combination of cues.
Dolphin auditory localization may sometimes occur under open anechoic
conditions, but reverberations from reflections off of nearby objects, from
the surface of the water, the bottom, and from nonlinearities in the temperature and salinity of the water can further contribute to the complexity
of auditory localization. Laboratory experiments of auditory localization
have focused on simple stimuli in simple environments in order to identify
the cues used. Similar studies have been conducted with dolphins.
There initially was a good reason to doubt that dolphins could use the
same cues as terrestrial mammals in localization. The impedance match
between the dolphin's tissue and that of seawater is much closer than is the
corresponding match between terrestrial mammals and air (see MlZlhl et al.
1999; Ketten, Chapter 2). As a result, it was not obvious that the two ears
were sufficiently isolated to allow them to act as independent receivers.
Further, the high velocity of sound in seawater, which is five times the velocity in air, means that any differences in the time of arrival of a sound at the
two ears would be correspondingly smaller than for a similar-sized terrestrial mammal.
3.2 Interaural Time Difference
The relationship between the difference in the length of the sound path at
the two ears (t:.d) and its angular position (e) is
t:.d = r(e + sine)
where r is the radius of the head, or half the distance between the two ears.
In the case of dolphins, it is not immediately obvious what r should be
because it is unclear where along the jaw it should be measured. The middle
of the pan bone seems like a sensible place from which to measure, but in
the absence of evidence it seems rather arbitrary.
Using contact jaw phones, Moore et al. (1995) studied the dolphin's sensitivity to interaural time and intensity differences. Jaw phones are small
(45-mm) suction cups containing a transducer. The phones were placed on
the base of the animal's lower jaw, in the approximate area of the pan bone,
where the bone broadens and becomes quite thin (see Norris 1968; Ketten,
Chapter 2). The stimuli consisted of a 2-s train of clicks. Each click consisted
of a single cycle of the test frequency with an interclick interval of 40ms.
Each session tested the animal at five different time or intensity differences
presented in blocks of 10 trials each. The dolphin touched a response wand
on the left if the clicks on that side preceded the clicks on the right or were
louder, and touched a response wand on the right if the opposite was true.
Signals between 5 and 90 kHz were tested.
Using 75% accuracy as the conventional threshold level, the dolphin's
interaural time difference thresholds were found to be between 7 and 18
Ils. In order to compare these results to those of Renaud and Popper (1975),