is enhanced along shelf edges and near underwater sea mounts and islands
where the channel rises close to the surface, whereas at high latitudes the
sound channel is typically within 100–200 m of the surface. Baleen whales
are known to routinely dive to depths of 100–200 m and have been recorded
feeding near shelf edges at depths of at least 200–500 m (Panigada et al.
1999; Croll et al. 2001).
10. Sound Transmission in Shallow Water
A second example of a sound channel is found in shallow water, where, for
the purposes of this paper, shallow water refers to depths of 10–200 m.
Coastal species of cetaceans are discussed in this context. In shallow water,
unlike the case for the deep sound channel, the water surface and bottom
serve as distinct reflective boundaries that interact with sound waves
(Fig. 2.4B). Sound transmission in shallow water is strongly influenced by
the local conditions of depth, surface roughness (wind-dependent), and
the composition of the bottom substrate. In shallow water, the boundary
constraints act as a waveguide that effectively filters out water-borne lowfrequency sounds. The combined effects of this filtering due to reflection,
bottom interaction, and high-frequency absorption result in selective propagation of frequencies in the 50–500-Hz band (see Jensen and Kuperman
1983). Therefore, for whales living in a shallow, coastal environment, selection should favor long-range communication signals in the 50–500-Hz band.
There are some intriguing shallow water observations suggesting that
low-frequency sounds from fin whales propagate within the substrate or
undergo frequency dispersion and time dispersion (D’Spain et al. 1995;
Premus and Spiesberger 1997). These propagation phenomena are being
exploited by acoustical oceanographers, but whether whales actually take
advantage of the information available from these propagation modes
remains to be determined.
For teleost fishes, communication occurs in very shallow water (e.g., intertidal and subtidal zones, lakes, and flood plains) with water depths <5 m. In
this environment, the same physical constraints apply as in the 10–100-m
case, but the effects on propagation do not result in a waveguide. Under
these very shallow water conditions, low-frequency propagation within the
water column is restricted to distances of several times the water depth.
This suggests that limited communication range and not maximum communication range has been the dominant selective feature. However,
communication range may be enhanced by sound propagation within
the substrate, and this could be especially important for animals that lie
quietly near or on the bottom substrate (e.g., see Premus and Spiesberger
1997).
Overall, sound transmission in shallow and very shallow water remains
one of the least understood areas of underwater acoustics because many of
2. Physical Acoustics of Underwater Sound Communication
33
where the channel rises close to the surface, whereas at high latitudes the
sound channel is typically within 100–200 m of the surface. Baleen whales
are known to routinely dive to depths of 100–200 m and have been recorded
feeding near shelf edges at depths of at least 200–500 m (Panigada et al.
1999; Croll et al. 2001).
10. Sound Transmission in Shallow Water
A second example of a sound channel is found in shallow water, where, for
the purposes of this paper, shallow water refers to depths of 10–200 m.
Coastal species of cetaceans are discussed in this context. In shallow water,
unlike the case for the deep sound channel, the water surface and bottom
serve as distinct reflective boundaries that interact with sound waves
(Fig. 2.4B). Sound transmission in shallow water is strongly influenced by
the local conditions of depth, surface roughness (wind-dependent), and
the composition of the bottom substrate. In shallow water, the boundary
constraints act as a waveguide that effectively filters out water-borne lowfrequency sounds. The combined effects of this filtering due to reflection,
bottom interaction, and high-frequency absorption result in selective propagation of frequencies in the 50–500-Hz band (see Jensen and Kuperman
1983). Therefore, for whales living in a shallow, coastal environment, selection should favor long-range communication signals in the 50–500-Hz band.
There are some intriguing shallow water observations suggesting that
low-frequency sounds from fin whales propagate within the substrate or
undergo frequency dispersion and time dispersion (D’Spain et al. 1995;
Premus and Spiesberger 1997). These propagation phenomena are being
exploited by acoustical oceanographers, but whether whales actually take
advantage of the information available from these propagation modes
remains to be determined.
For teleost fishes, communication occurs in very shallow water (e.g., intertidal and subtidal zones, lakes, and flood plains) with water depths <5 m. In
this environment, the same physical constraints apply as in the 10–100-m
case, but the effects on propagation do not result in a waveguide. Under
these very shallow water conditions, low-frequency propagation within the
water column is restricted to distances of several times the water depth.
This suggests that limited communication range and not maximum communication range has been the dominant selective feature. However,
communication range may be enhanced by sound propagation within
the substrate, and this could be especially important for animals that lie
quietly near or on the bottom substrate (e.g., see Premus and Spiesberger
1997).
Overall, sound transmission in shallow and very shallow water remains
one of the least understood areas of underwater acoustics because many of
2. Physical Acoustics of Underwater Sound Communication
33