predictions and include several key empirical examples specifically related
to marine mammal communication.
6. Diffraction
Many of the sounds generated by teleost fishes and baleen whales are of
low frequency and therefore of sufficiently long wavelengths that diffraction, or the scattering of sound waves around objects, is unlikely to be much
of a factor unless the object encountered is larger than approximately half
the sound’s wavelength. For example, diffraction should not become significant for a 100-Hz signal (l ~ 15 m, which is typical for the fundamental
frequency of many teleost fish sounds) unless it contacts some object with
a size close to 8 m.
7. Ambient Noise
In deep water, the major natural sources of noise are from waves generated
by tidal cycles and wind, seismic disturbances such as earthquakes and
volcanism, lightning strikes, rain, oceanic turbulence, and marine mammals
(Urick 1983; Bannister 1986; Dyer 1997; Curtis et al. 1999). In shallow water,
the major natural sources of noise are from waves impacting the shore, local
wind, rain, and biological sounds such as from snapping shrimp and marine
mammals. Within the last century, vessel noise, especially from commercial
shipping, has increased the ambient noise levels at frequencies below
1,000 Hz (see Payne and Webb 1971; Myrberg 1990; Curtis et al. 1999). Noise
from rain is fairly constant across all frequencies (see Fig. 6.15 in Urick
1983), while noise from wind is one of the predominant natural factors influencing low-frequency ambient noise levels (Urick 1983; Bannister 1985;
Curtis et al. 1999). The influence of noise on the detection of acoustic signals
has attracted much attention, generally within the context of auditory
masking (e.g., see discussions in Myrberg 1990 and Fay and Simmons 1999).
8. Received Level, Detection, and Recognition
Effective communication depends on detection and recognition of the
acoustic event. Detection requires that the signal’s level at the receiver (i.e.,
received level or RL) be above both the local ambient noise level and the
receiver’s auditory sensitivity in the signal’s frequency band. In simple cases,
when the sum of transmission loss (TL) and local ambient noise is greater
than the source level (SL), the detection threshold is greater than the
sound’s received level and no detection occurs (see Eq. 6). Detection alone
does not assure effective decoding or recognition of the signal. Signal recog2. Physical Acoustics of Underwater Sound Communication
31
to marine mammal communication.
6. Diffraction
Many of the sounds generated by teleost fishes and baleen whales are of
low frequency and therefore of sufficiently long wavelengths that diffraction, or the scattering of sound waves around objects, is unlikely to be much
of a factor unless the object encountered is larger than approximately half
the sound’s wavelength. For example, diffraction should not become significant for a 100-Hz signal (l ~ 15 m, which is typical for the fundamental
frequency of many teleost fish sounds) unless it contacts some object with
a size close to 8 m.
7. Ambient Noise
In deep water, the major natural sources of noise are from waves generated
by tidal cycles and wind, seismic disturbances such as earthquakes and
volcanism, lightning strikes, rain, oceanic turbulence, and marine mammals
(Urick 1983; Bannister 1986; Dyer 1997; Curtis et al. 1999). In shallow water,
the major natural sources of noise are from waves impacting the shore, local
wind, rain, and biological sounds such as from snapping shrimp and marine
mammals. Within the last century, vessel noise, especially from commercial
shipping, has increased the ambient noise levels at frequencies below
1,000 Hz (see Payne and Webb 1971; Myrberg 1990; Curtis et al. 1999). Noise
from rain is fairly constant across all frequencies (see Fig. 6.15 in Urick
1983), while noise from wind is one of the predominant natural factors influencing low-frequency ambient noise levels (Urick 1983; Bannister 1985;
Curtis et al. 1999). The influence of noise on the detection of acoustic signals
has attracted much attention, generally within the context of auditory
masking (e.g., see discussions in Myrberg 1990 and Fay and Simmons 1999).
8. Received Level, Detection, and Recognition
Effective communication depends on detection and recognition of the
acoustic event. Detection requires that the signal’s level at the receiver (i.e.,
received level or RL) be above both the local ambient noise level and the
receiver’s auditory sensitivity in the signal’s frequency band. In simple cases,
when the sum of transmission loss (TL) and local ambient noise is greater
than the source level (SL), the detection threshold is greater than the
sound’s received level and no detection occurs (see Eq. 6). Detection alone
does not assure effective decoding or recognition of the signal. Signal recog2. Physical Acoustics of Underwater Sound Communication
31