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

Report No. 6945
BBN Systems and Technologies Corporation
An understanding of these factors is necessary to evaluate the ability of a
marine mammal to detect industrial sounds in various circumstances. This
understanding is also needed to evaluate its ability to detect communication
signals, echolocation sounds or other sounds of interest in the presence of
"masking" by natural ambient noise and by industrial sounds.
Underwater hearing ability has been studied in a few odontocetes (toothed
whales), phocids (hair seals), and otariids (eared seals). However, baleen
whales and walruses have not been tested. In most of the marine mammal
species that have been tested for hearing abilities, only one or two individuals
have been examined. The low sample sizes prevent a detailed examination
of variability among individuals of the same species. However, even the
limited data presently available show that there are differences in hearing
abilities between various species of toothed whales or of seals.
2.3.1 Frequency range and sensitivity
Sensitivities of marine mammals to sounds of different frequencies are
best illustrated by means of audiograms. Audiograms are obtained by
behavioral or electrophysiological techniques. In the behavioral method,
tones of various intensities and frequencies are presented to a trained test
animal. If the animal hears a sound stimulus, it responds positively; if the
tone is not heard or if no sound was presented, as in a control trial, no such
response occurs. The least intense tones detectable at various frequencies
define an ir.dividua1 animal's audiogram.
2.3.2 Toothed whales
Behavioral audiograms have been determined for six species of toothed
whales, including three Alaskan species--a harbor porpoise (Andersen 1970), a
killer whale (Hall and Johnson 1972), and two white whales (White et al.
1978). Additional data on the sensitivity of three white whales to low
frequencies were obtained by Awbrey et al. (1986, 1988). Figure 2.25a shows
behavioral audiograms for these three species. Figure 2.25b shows corresponding
data for non-Alaskan species including the bottlenose dolphin, the
odontocete whose hearing has been studied in most detail. The other two non-
Alaskan species for which behavioral audiograms are available are the false
killer whale Pseudorca cressidens (Thomas et al. 1988) and the freshwater
boutu Inia geoffrensis of South America (Jacobs and Hall 1972).
Most toothed whales can hear sounds over a very wide range of frequencies
from as low as 75-125 Hz in the bottlenose dolphin and white whale (Johnson
1967; Awbrey et al. 1988) to 105-150 kHz in several species (Fig. 2.25). The
killer whale differs from other odontocetes in that its upper hearing limit is
about 31 kHz.(Hall and Johnson 1972). Although the frequency range of the
killer whale audiogram is narrower than that of other odontocetes that have
been studied, its hearing at its "best" frequency is very sensitive. In the
absence of noise, a killer whale can detect a signal of about 30 dB re 1 vPa
if the sound is near 15 kHz (Hall and Johnson 1972) compared to about 39 dB at
30 kHz for a white whale (White et al. 1978), about 48 dB at 8 and 32 kHz for
the harbor porpoise (Andersen 1970), and 41-42 dB at various frequencies for a
bottlenose dolphin (Johnson 1967).