Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 79

Fig. 4.7 According to the
distance from a receiver the
marmoset ( Cebuella
pygmaea ) changes the type
of call utilized. Trill is used
at short distance, J call at an
intermediate distance, and
long-distance call at
distance > 11 m
(Reproduced with
permission from De La
Torre and Snowdon 2002 )
frequencies between 2 and 8 kHz. At low frequency, reverberation assumes the
form of echoes, but at higher frequency it produces a decay of acoustic energy,
masking high rates of amplitude modulation and repetitive frequency modulation.
Most acoustic animals use the intermediate frequencies (2-8 kHz), as experimen-
tally evaluated by Richards and Wiley ( 1980 ).
4.7.2 Blurring, Attenuation, and Signal-to-Noise Ratio
and Ranging Efficiency
Detectability, locability, and readability of bird song are degraded with the distance
propagated through the environment. According to physical theory and principles
the degradation can be represented by three aspects: blurring, attenuation, and
signal-to-noise ratio.
Blurring of an amplitude and frequency pattern is the result of filtering acting on
the frequencies by reverberation and atmospheric turbulence.
Attenuation is produced by absorption, multiple scattering, and geometric
attenuation.
The signal-to-noise ratio largely depends by the quality of the signal but also on
the level of background noise.
These three concurrent aspects of sound degradation have been confronted with
the common blackbird ( Turdus merula ), a species that has such a complex song
repertoire that is possible to find in this species acoustic elements common to other
species. This species seems to be able to reduce the sound degradation by selecting
the best-adapted perch site and direction in which to sing. Experiments using the
broadcast song of this species were conducted by Dabelsteen et al. ( 1993 ). Blurring,
attenuation, and signal-to-noise ratio measures were shown to be correlated and
decreased with distance. The components of the blackbird song with higher
frequencies were degraded more rapidly than the low-frequency song repertoire.