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D.R. Ketten
for localizing sound: differences in arrival time (interaural time) and differences in sound level (interaural intensity). Binaural hearing studies are
relatively rare for marine mammals, but the consensus from research on
both pinnipeds and odontocetes is that binaural cues are used in underwater localization (Dudok van Heel 1962; Renaud and Popper 1975; Moore
et al. 1995). The relatively broad spread between ears in cetaceans because
of the extracranial relocation of the tympano-periotic complex may be the
crucial adaptation that explains their ability to accurately localize underwater sounds.
In mammals, the high-frequency limit of functional hearing in each
species has been shown to correlate with its interaural time distance, IATD,
the distance sound travels from one ear to the other divided by the speed
of sound (Heffner and Masterton 1990). It is unlikely that the upper functional hearing limit is specifically a factor in localization; it is more likely to
simply be a correlate for some other hearing feature underlying localization. Nevertheless, a strong correlation clearly exists between an animal's
highest functional frequency and interear spacing that provides a useful
framework for discussion. The fundamental assumption of Heffner and
Masterton (1990) is that the narrower the head, the smaller the IATD, the
higher the frequency an animal must perceive well to detect phase differences at each ear. For example, consider a pure tone (sine wave) arriving
at the head. If the sound is directly in front of the head, the sound will arrive
at the same time and with the same phase at each ear. As the animal's head
turns away from the source, each ear receives a different phase, given that
the interear distance is different from an even multiple of the wavelength
of the sound. IATD cues therefore involve comparing time of arrival versus
phase differences at different frequencies in each ear. Phase cues are useful
primarily at frequencies below the functional limit; however, the higher the
frequency an animal can hear, the more likely it is to have good sensitivity
at the upper end of its frequency range for phase cues.
Clearly, interaural time distances depend upon the sound conduction
path in the animal and on the media through which sound travels. For terrestrial species, the normal sound path is through air, around the head,
pinna to pinna. The key entry point for localization cues is the external auditory meatus, and the IATD is therefore the intermeatal (1M) distance measured around the head divided by the speed of sound in air. In aquatic
animals, sound can travel in a straight line through the head by tissue conduction, given that the head tissues have coherent acoustic impedances
similar to sea water.
Experiments with delphinids suggest that intercochlear (Ie) or interjaw
distances are the most appropriate measure for calculating IATD values in
odontocetes. Because of the increased speed of sound in water, the IC distance of an average dolphin is acoustically equivalent to a rat or bat 1M distance in air. Supin and Popov (1993) hypothesized that marine mammals
without pinnae were incapable of using IATD cues. Recently, however,
D.R. Ketten
for localizing sound: differences in arrival time (interaural time) and differences in sound level (interaural intensity). Binaural hearing studies are
relatively rare for marine mammals, but the consensus from research on
both pinnipeds and odontocetes is that binaural cues are used in underwater localization (Dudok van Heel 1962; Renaud and Popper 1975; Moore
et al. 1995). The relatively broad spread between ears in cetaceans because
of the extracranial relocation of the tympano-periotic complex may be the
crucial adaptation that explains their ability to accurately localize underwater sounds.
In mammals, the high-frequency limit of functional hearing in each
species has been shown to correlate with its interaural time distance, IATD,
the distance sound travels from one ear to the other divided by the speed
of sound (Heffner and Masterton 1990). It is unlikely that the upper functional hearing limit is specifically a factor in localization; it is more likely to
simply be a correlate for some other hearing feature underlying localization. Nevertheless, a strong correlation clearly exists between an animal's
highest functional frequency and interear spacing that provides a useful
framework for discussion. The fundamental assumption of Heffner and
Masterton (1990) is that the narrower the head, the smaller the IATD, the
higher the frequency an animal must perceive well to detect phase differences at each ear. For example, consider a pure tone (sine wave) arriving
at the head. If the sound is directly in front of the head, the sound will arrive
at the same time and with the same phase at each ear. As the animal's head
turns away from the source, each ear receives a different phase, given that
the interear distance is different from an even multiple of the wavelength
of the sound. IATD cues therefore involve comparing time of arrival versus
phase differences at different frequencies in each ear. Phase cues are useful
primarily at frequencies below the functional limit; however, the higher the
frequency an animal can hear, the more likely it is to have good sensitivity
at the upper end of its frequency range for phase cues.
Clearly, interaural time distances depend upon the sound conduction
path in the animal and on the media through which sound travels. For terrestrial species, the normal sound path is through air, around the head,
pinna to pinna. The key entry point for localization cues is the external auditory meatus, and the IATD is therefore the intermeatal (1M) distance measured around the head divided by the speed of sound in air. In aquatic
animals, sound can travel in a straight line through the head by tissue conduction, given that the head tissues have coherent acoustic impedances
similar to sea water.
Experiments with delphinids suggest that intercochlear (Ie) or interjaw
distances are the most appropriate measure for calculating IATD values in
odontocetes. Because of the increased speed of sound in water, the IC distance of an average dolphin is acoustically equivalent to a rat or bat 1M distance in air. Supin and Popov (1993) hypothesized that marine mammals
without pinnae were incapable of using IATD cues. Recently, however,