Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
BBN Systems and Technologies Corporation
detect the target when the noise level was too high to allow detection by
conventional straight-line echolocation. No such capability was demonstrated
in Tursiops (Penner et al. 1986).
In another experiment on Tursiops, Zaitseva et al. (1980) found that the
angular separation between a sound source and a masking noise source has
little effect on the degree of masking when the sound frequency is 18 kHz.
Zaitseva et al. interpret this to mean that dolphin communication sounds will
be more or less equally audible regardless of their direction of arrival,
which is likely to be advantageous for purposes of social interactions.
However, at these frequencies masking would be almost equally severe
regardless of the direction of arrival of the masking noise.
Toothed whales, and probably other marine mammals as well, have additional
capabilities besides directional hearing that can facilitate detection
of sounds in the presence of background noise. Au et al. (1974) obtained
indirect evidence that bottlenose dolphins may shift their peak echolocation
signals to 120-130 kHz from the more typical 35-60 kHz signals in an area
where there is a high level of ambient noise in the latter frequency range.
Acoustic source levels of echolocation signals may also be greatly increased
when necessary to circumvent noise (Au et al. 1974). Adaptation of the
frequency and source levels of echolocation sounds to the prevailing noise
environment was subsequently demonstrated in a more direct fashion in a white
whale (Au et al. 1985). .
Studies of masking at lower frequencies and in other marine mammal groups
would be desirable. The demonstrated directional hearing abilities of some
pinnipeds and baleen whales probably give them some improved capabilities.
Whether most marine mammals can adjust the frequencies and source levels of
their various call types to increase their communication ranges in the
presence of noise has not been studied. However, the widely varying source
levels of many marine mammal sounds are consistent with an ability to tailor
the source level to the circumstances.
2.3.7 Audition in baleen whales
No work on auditory sensitivity has been performed on a live baleen
whale. On the basis of anatomical and paleontological evidence, Fleischer
( 1976, 1978) has suggested that baleen whales are adapted for hearing low
frequencies. Norris and Leatherwood (1981) examined the morphology of the
hearing apparatus of the bowhead whale and several other species of cetaceans,
and concluded that bowheads likely hear sounds ranging from "high infrasonic
to low sonic to high sonic or low ultrasonic frequencies". Other authors
(Evans 1973; Myrberg 1978; Turl 1980) suggest that marine mammals probably
hear best in the frequency range of their calls. Most baleen whale sounds are
concentrated at frequencies less than 1 kHz, though sounds up to 8 kHz are not
uncommon (see Section 2.2.2). It is reasonable to suggest, then, that baleen
whales are most sensitive to frequencies lower than 1 kHz. The morphology of
the baleen whale cochlea is compatible w.ith good low-frequencyhearing and
peak sensitivity between 1 and 2 kHz (G. Fleischer, Justus-Liebig University,
pers. comm.).