Ecological Constraints for Sound Communication: From Grasshoppers to Elephants 73
The real-time spectral analysis of a recorded signal at this site reveals another
interesting detail: during the test a signal was routinely received for at least 23 min
after the direct signal had ended. This is due to scattering from the continental
borders of South America, Africa, and Antarctica. If we imagine a finback whale
signaling somewhere in the South Indian Ocean, a receiver along this sound
channel will be able to first detect this signal after more than 90 min, and then
continue to receive scattered signals from all these places for another 20 min: this
is a scenario hard to imagine for us because of dimensions far beyond the signal
ranges reported for terrestrial animals.
I do not argue that a whale in the ocean south of Australia communicates with
another one in the Atlantic Ocean through the SOF AR channel. A realistic calculation for the detection range of a call of a finback whale is about 400 krn
(Spies berger and Fristrup 1990). However, a consequence of the excellent sound
propagation will be a considerable increase in noise levels, which in the frequency
range from 1-100Hz is dominated by ships and by the ocean's turbulence (Urick
1983). This can result in an increase in masking of the hearing thresholds of
marine mammals and fish.
4 The "Noisy" Transmission Channel
The statement made at the beginning of this chapter that the sender - in order to
increase the active space of its signal - should increase the amplitude of the signal
is not absolutely precise. The sender should in fact increase the signal-to-noise
ratio by whatever mechanism to enable easy signal detection by intended receivers.
The problem with environmental noise, created by various sound sources such as
wind, running water, or various individuals of different species, is its nonrandom
nature and variation in spectral composition and temporal pattern.
The variation in noise is an indirect consequence of the temporal variation in
sound transmission properties during day and night, as outlined above. This is
because acoustically communicating animals adapt their broadcast times to these
conditions. Diurnal animals tend to concentrate their singing activity either in the
first hours of the day or in the evening. Well-known examples are the dawn
choruses of birds and insects (Henwood and Fabrik 1979).
Nocturnal animals find more reliable conditions for sound transmission and often
call throughout the night. However, the constraints for long-range sound
transmission are similar for many species of mammals, amphibians, and insects,
and the result is a complex sonic and ultrasonic background over which these
animals must communicate. It is evident, then, that there is a tradeoff between the
conditions optimal to broadcast one· s signal and those for detecting and
discriminating signals in the noise produced by the other species. Measures of the
range of communication, or of sound discrimination abilities, are therefore of little
relevance without considering the level of masking noise in the environment.
The real-time spectral analysis of a recorded signal at this site reveals another
interesting detail: during the test a signal was routinely received for at least 23 min
after the direct signal had ended. This is due to scattering from the continental
borders of South America, Africa, and Antarctica. If we imagine a finback whale
signaling somewhere in the South Indian Ocean, a receiver along this sound
channel will be able to first detect this signal after more than 90 min, and then
continue to receive scattered signals from all these places for another 20 min: this
is a scenario hard to imagine for us because of dimensions far beyond the signal
ranges reported for terrestrial animals.
I do not argue that a whale in the ocean south of Australia communicates with
another one in the Atlantic Ocean through the SOF AR channel. A realistic calculation for the detection range of a call of a finback whale is about 400 krn
(Spies berger and Fristrup 1990). However, a consequence of the excellent sound
propagation will be a considerable increase in noise levels, which in the frequency
range from 1-100Hz is dominated by ships and by the ocean's turbulence (Urick
1983). This can result in an increase in masking of the hearing thresholds of
marine mammals and fish.
4 The "Noisy" Transmission Channel
The statement made at the beginning of this chapter that the sender - in order to
increase the active space of its signal - should increase the amplitude of the signal
is not absolutely precise. The sender should in fact increase the signal-to-noise
ratio by whatever mechanism to enable easy signal detection by intended receivers.
The problem with environmental noise, created by various sound sources such as
wind, running water, or various individuals of different species, is its nonrandom
nature and variation in spectral composition and temporal pattern.
The variation in noise is an indirect consequence of the temporal variation in
sound transmission properties during day and night, as outlined above. This is
because acoustically communicating animals adapt their broadcast times to these
conditions. Diurnal animals tend to concentrate their singing activity either in the
first hours of the day or in the evening. Well-known examples are the dawn
choruses of birds and insects (Henwood and Fabrik 1979).
Nocturnal animals find more reliable conditions for sound transmission and often
call throughout the night. However, the constraints for long-range sound
transmission are similar for many species of mammals, amphibians, and insects,
and the result is a complex sonic and ultrasonic background over which these
animals must communicate. It is evident, then, that there is a tradeoff between the
conditions optimal to broadcast one· s signal and those for detecting and
discriminating signals in the noise produced by the other species. Measures of the
range of communication, or of sound discrimination abilities, are therefore of little
relevance without considering the level of masking noise in the environment.