nition requires that the sound be more than just detected; it must be properly encoded and then decoded from a suite of other potential signals.
The physical attributes of communication sounds are a balance among
multiple, often conflicting, selective pressures. For the terrestrial habitat.
Morton (1975) was the first to relate the selective influence of the environment on acoustic characteristics of birds. In the case of underwater
acoustic transmission, where the physical environment has, in general, a
greater influence on sound transmission than in air, there are cases in which
dramatic improvements in detection range are available depending on
where the sender and receiver are located and what sounds are produced.
Assuming that there is an advantage to greater detection range (larger audience, greater interindividual spacing, greater population range), we should
expect to find cases in which animals take advantage of unique underwater
acoustic-propagation conditions by optimizing the characteristics of their
acoustic signals and their signaling behavior.
Here, we examine two cases of bioacoustic signaling in two habitats with
divergent physical acoustic properties. We will first discuss the general characteristics of sound transmission for deep and shallow water marine habitats before turning to specific examples of vocal signaling in those sound
channels by cetaceans and teleost fishes.
9. The Deep Sound Channel
When considering sounds in the deep ocean, frequencies less than approximately 100 Hz are considered low-frequency and can be detected at ranges
of thousands of miles depending on the source level, sound-speed profiles
along the sound path, and local ambient noise conditions. Under certain
deep water (>2,000 m) conditions, influenced primarily by a spreading loss
that is approximately cylindrical (-10 * log 10 (r)), most of the sound energy
propagates within a certain depth regime known as the deep sound channel
(see Urick 1983 and Jensen et al. 1994).The deep sound channel, also known
previously as the SOFAR (SOund Fixing And Ranging) channel, is located
within the region of minimum sound speed of the sound-velocity profile
(see Fig. 2.2 and Section 2.3). Low-frequency sounds traveling in the deep
sound channel do not experience reflection off the sea surface or ocean
bottom but follow refracted-refracted ray paths, with the result that the
sound energy remains within a sound channel (Fig. 2.4A). For lowfrequency sounds, this constraint due to refraction in combination with
extremely low levels of absorption leads to exceptionally low levels of transmission loss and extremely long ranges of acoustic detection (see Officer
1958, pp. 159–160).
There is some question as to whether whales are physically capable of
producing sound at deep sound-channel depths (Aroyan et al. 2000; Thode
et al. 2000). In temperate environments, access to the deep sound channel
32
A.H. Bass and C.W. Clark
The physical attributes of communication sounds are a balance among
multiple, often conflicting, selective pressures. For the terrestrial habitat.
Morton (1975) was the first to relate the selective influence of the environment on acoustic characteristics of birds. In the case of underwater
acoustic transmission, where the physical environment has, in general, a
greater influence on sound transmission than in air, there are cases in which
dramatic improvements in detection range are available depending on
where the sender and receiver are located and what sounds are produced.
Assuming that there is an advantage to greater detection range (larger audience, greater interindividual spacing, greater population range), we should
expect to find cases in which animals take advantage of unique underwater
acoustic-propagation conditions by optimizing the characteristics of their
acoustic signals and their signaling behavior.
Here, we examine two cases of bioacoustic signaling in two habitats with
divergent physical acoustic properties. We will first discuss the general characteristics of sound transmission for deep and shallow water marine habitats before turning to specific examples of vocal signaling in those sound
channels by cetaceans and teleost fishes.
9. The Deep Sound Channel
When considering sounds in the deep ocean, frequencies less than approximately 100 Hz are considered low-frequency and can be detected at ranges
of thousands of miles depending on the source level, sound-speed profiles
along the sound path, and local ambient noise conditions. Under certain
deep water (>2,000 m) conditions, influenced primarily by a spreading loss
that is approximately cylindrical (-10 * log 10 (r)), most of the sound energy
propagates within a certain depth regime known as the deep sound channel
(see Urick 1983 and Jensen et al. 1994).The deep sound channel, also known
previously as the SOFAR (SOund Fixing And Ranging) channel, is located
within the region of minimum sound speed of the sound-velocity profile
(see Fig. 2.2 and Section 2.3). Low-frequency sounds traveling in the deep
sound channel do not experience reflection off the sea surface or ocean
bottom but follow refracted-refracted ray paths, with the result that the
sound energy remains within a sound channel (Fig. 2.4A). For lowfrequency sounds, this constraint due to refraction in combination with
extremely low levels of absorption leads to exceptionally low levels of transmission loss and extremely long ranges of acoustic detection (see Officer
1958, pp. 159–160).
There is some question as to whether whales are physically capable of
producing sound at deep sound-channel depths (Aroyan et al. 2000; Thode
et al. 2000). In temperate environments, access to the deep sound channel
32
A.H. Bass and C.W. Clark