detection. In this case, the benefit is on the order of at least 6–10 dB, which
in deep water represents a fourfold increase in range and 16-fold increase
in area. Similar descriptions of animals taking advantage of a low noise
window have been observed in both vertebrate and invertebrate groups
(Morton 1975; Michelsen 1992; Gerhardt 1994).
The right-hand column in Figure 2.8 shows spectral profiles for the calls
of four species (right and bowhead whale call spectra are essentially identical) with the ambient noise for their respective deep water and shallow
water environments. This figure illustrates that for coastal species (humpback and right whales) the overall frequency band of call energy is again
well-matched to the ambient noise properties of the environment.
However, it shows that for blue and fin whale calls, the frequency band of
greatest energy occurs in a region of high ambient noise. This discrepancy
is somewhat surprising. Perhaps the explanation is related to the specific
contexts in which these calls were recorded. For the coastal species, many
of the calls in this category are known to be contact calls (e.g., Clark 1983),
occurring when animals were countercalling while separated by many miles.
For the blue and fin whales, however, the animals presumed to be responsible for the calls were in surface-active groups in which the whales were
within close proximity to one another. In this latter context, long-range
transmission is not necessarily optimized because there is no advantage to
producing low-frequency sounds. In fact, if there is an advantage to restricting the communication range to a local audience, then selection should
favor sounds in a frequency band with high ambient noise.
Overall, this relatively simple comparative evaluation using the animal
signal characteristics of bandwidth, duration, and intensity, together with
ambient noise characteristics for the two habitats, reveals several striking
results. First, the sounds from shallow water and deep water species occupy
different regions of the frequency band, and these bands are well-matched
to the general ambient noise and transmission properties of the two environments. The three coastal species (bowhead, humpback, and right whales)
produce most of their energy in the 100–500-Hz band, a region in which
propagation is optimum and ambient noise is low (Clark 1983; Jensen and
Kuperman 1983). The two pelagic species (blue and fin whales) produce
most of their sounds in the 15–30-Hz band, a frequency band for deep water
in which transmission loss is low, but there is a trade-off with ambient noise,
which is naturally dominating in this low-frequency band. The reverse is not
true. That is, the signals from blue and fin whales would not propagate efficiently in the coastal habitat used by bowhead, humpback, or right whales
because of the influence of water depth on cutoff frequency. Humpback
whale songs are broadband, variable, and repetitive and appear to be
adapted for both deep water and coastal habitats. High-frequency portions
of humpback song (i.e., >500 Hz) do not propagate efficiently in the pelagic
habitat (i.e., the deep sound channel). However, all humpback songs contain a significant amount of energy in the 100–500 Hz band, and some
2. Physical Acoustics of Underwater Sound Communication
41
in deep water represents a fourfold increase in range and 16-fold increase
in area. Similar descriptions of animals taking advantage of a low noise
window have been observed in both vertebrate and invertebrate groups
(Morton 1975; Michelsen 1992; Gerhardt 1994).
The right-hand column in Figure 2.8 shows spectral profiles for the calls
of four species (right and bowhead whale call spectra are essentially identical) with the ambient noise for their respective deep water and shallow
water environments. This figure illustrates that for coastal species (humpback and right whales) the overall frequency band of call energy is again
well-matched to the ambient noise properties of the environment.
However, it shows that for blue and fin whale calls, the frequency band of
greatest energy occurs in a region of high ambient noise. This discrepancy
is somewhat surprising. Perhaps the explanation is related to the specific
contexts in which these calls were recorded. For the coastal species, many
of the calls in this category are known to be contact calls (e.g., Clark 1983),
occurring when animals were countercalling while separated by many miles.
For the blue and fin whales, however, the animals presumed to be responsible for the calls were in surface-active groups in which the whales were
within close proximity to one another. In this latter context, long-range
transmission is not necessarily optimized because there is no advantage to
producing low-frequency sounds. In fact, if there is an advantage to restricting the communication range to a local audience, then selection should
favor sounds in a frequency band with high ambient noise.
Overall, this relatively simple comparative evaluation using the animal
signal characteristics of bandwidth, duration, and intensity, together with
ambient noise characteristics for the two habitats, reveals several striking
results. First, the sounds from shallow water and deep water species occupy
different regions of the frequency band, and these bands are well-matched
to the general ambient noise and transmission properties of the two environments. The three coastal species (bowhead, humpback, and right whales)
produce most of their energy in the 100–500-Hz band, a region in which
propagation is optimum and ambient noise is low (Clark 1983; Jensen and
Kuperman 1983). The two pelagic species (blue and fin whales) produce
most of their sounds in the 15–30-Hz band, a frequency band for deep water
in which transmission loss is low, but there is a trade-off with ambient noise,
which is naturally dominating in this low-frequency band. The reverse is not
true. That is, the signals from blue and fin whales would not propagate efficiently in the coastal habitat used by bowhead, humpback, or right whales
because of the influence of water depth on cutoff frequency. Humpback
whale songs are broadband, variable, and repetitive and appear to be
adapted for both deep water and coastal habitats. High-frequency portions
of humpback song (i.e., >500 Hz) do not propagate efficiently in the pelagic
habitat (i.e., the deep sound channel). However, all humpback songs contain a significant amount of energy in the 100–500 Hz band, and some
2. Physical Acoustics of Underwater Sound Communication
41