8
W.W.L.Au
depths of 5, 100, 200, and 300m. They found that the whales heard as
well at depth as near the surface. Humans and terrestrial mammals usually
experience a hearing loss from 20 to 40dB when tested in a pressure
chamber. This loss is the result of increased density and stiffness of
the air volume in the middle ear cavity (Fleischer 1978). Ridgway et al.
(1998) concluded that the lack of any hearing threshold changes at depth
supports the theory that sound reaches the whale inner ear through whale
head tissue and not through the ear drum and ossicular chain transduction
of the middle ear. At an underwater depth of 300m the pressure is about
30atm.
3. Hearing by Mysticetes
Hearing by mysticetes is the topic that Ketten discusses in detail in Chapter
2. Our knowledge of the hearing characteristics of baleen whales is
extremely limited. We do not know how they receive sounds, the frequency
range of hearing, or the sensitivity of hearing at any frequency. Much of
our understanding of hearing in baleen whales comes from anatomical
studies of the ears of different speciess. Baleen whales have occluded
external auditory canals that are filled with a homogeneous wax (Ketten
1992). The lower jaws of mysticetes are designed for sieving or gulp-feeding
and have no evident connection to the temporal bones (Ketten 1992)
making it very difficult to understand how sounds enter into the ears of
these whales.
Various types of whales have been observed by many investigators to
react strongly and drastically change their behavior to boats and low-flying
aircraft, however, the sound pressure levels at the whales' locations are
often difficult to define and measure. In some situations, the sound levels
of th.e aversive sound could be estimated and a data point obtained. Balaena
mysticetus (bowhead whales) were observed fleeing from a 13-m dieselpowered boat having a noise level at the location of the whales of about
84 dB re 11lPa in the dominant 1/3 octave band, or about 6 dB above the
ambient noise in that band (Richardson et al. 1995). Playback experiments
with B. mysticetus indicated that they took evasive action when the noise
was about 110dB re 11lPa or 30dB above the ambient noise in the same
1/3 octave band (Richardson et al.1990). Playback experiments with migrating Eschrichtius robustus (gray whales) (Malme et al. 1983, 1984) indicated
that about 10% of the whales made avoidance behavior when the noise was
about 110dB in a 1/3 octave band, 50% at about 117 dB, and 90% at about
122dB or greater. These playback signals consisted of anthropogenic noise
associated with the oil and gas industry.
Frankel (1994) played back natural humpback whale sounds and a synthetic sound to Megaptera novaeangliae wintering in the waters of the
Hawaiian islands. Twenty-seven of 1,433 trials produced rapid approach
W.W.L.Au
depths of 5, 100, 200, and 300m. They found that the whales heard as
well at depth as near the surface. Humans and terrestrial mammals usually
experience a hearing loss from 20 to 40dB when tested in a pressure
chamber. This loss is the result of increased density and stiffness of
the air volume in the middle ear cavity (Fleischer 1978). Ridgway et al.
(1998) concluded that the lack of any hearing threshold changes at depth
supports the theory that sound reaches the whale inner ear through whale
head tissue and not through the ear drum and ossicular chain transduction
of the middle ear. At an underwater depth of 300m the pressure is about
30atm.
3. Hearing by Mysticetes
Hearing by mysticetes is the topic that Ketten discusses in detail in Chapter
2. Our knowledge of the hearing characteristics of baleen whales is
extremely limited. We do not know how they receive sounds, the frequency
range of hearing, or the sensitivity of hearing at any frequency. Much of
our understanding of hearing in baleen whales comes from anatomical
studies of the ears of different speciess. Baleen whales have occluded
external auditory canals that are filled with a homogeneous wax (Ketten
1992). The lower jaws of mysticetes are designed for sieving or gulp-feeding
and have no evident connection to the temporal bones (Ketten 1992)
making it very difficult to understand how sounds enter into the ears of
these whales.
Various types of whales have been observed by many investigators to
react strongly and drastically change their behavior to boats and low-flying
aircraft, however, the sound pressure levels at the whales' locations are
often difficult to define and measure. In some situations, the sound levels
of th.e aversive sound could be estimated and a data point obtained. Balaena
mysticetus (bowhead whales) were observed fleeing from a 13-m dieselpowered boat having a noise level at the location of the whales of about
84 dB re 11lPa in the dominant 1/3 octave band, or about 6 dB above the
ambient noise in that band (Richardson et al. 1995). Playback experiments
with B. mysticetus indicated that they took evasive action when the noise
was about 110dB re 11lPa or 30dB above the ambient noise in the same
1/3 octave band (Richardson et al.1990). Playback experiments with migrating Eschrichtius robustus (gray whales) (Malme et al. 1983, 1984) indicated
that about 10% of the whales made avoidance behavior when the noise was
about 110dB in a 1/3 octave band, 50% at about 117 dB, and 90% at about
122dB or greater. These playback signals consisted of anthropogenic noise
associated with the oil and gas industry.
Frankel (1994) played back natural humpback whale sounds and a synthetic sound to Megaptera novaeangliae wintering in the waters of the
Hawaiian islands. Twenty-seven of 1,433 trials produced rapid approach