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Hans Winkler
progressed well in certain fields, but also that many gaps and open questions
remain. Most of the examples given will deal with song and its analogues in nonsongbirds. However, various types of calls and instrumental signals will be
discussed as well. With respect to ecology I will focus on sound transmission in
natural habitats. However, other aspects of ecology, such as predation, will be
treated as well. Particular attention will be paid to the receivers' role.
2 Sound Propagation in Terrestrial Habitats
2.1 Attenuation
Sound attenuation by distance has several components, namely geometrical
spreading, atmospheric absorption, effects of the ground, and miscellaneous
environmental effects (noise, vegetation, see also Chapter II. 2). For a point source
the spreading of sound is spherical with an attenuation equal to 6.02 dB. In certain
habitats spreading of sound approaches two-dimensional cylindrical spreading
with attenuation close to 3 dB per doubling of distance. Attenuation by air
absorption is relevant for distances of more than several hundred meters only.
2.2 Attenuation over the Ground
Attenuation of sound by the ground has been extensively covered by theoretical
and empirical studies (Piercy et al. 1977; Embleton et a!. 1983; Attenborough
1985; Wempen 1991; Embleton 1996; Sutherland and Daigle 1997). The
attenuation results from the interference between direct sound rays and those
reflected from the ground. Ground attenuation strongly depends on the type of
ground surface, the grazing angle, relative positions of sender and receiver, and
the frequency of the sound. In addition to the plane wave reflection (Embleton
1996) there are the effects of the spherical wavefront. They differ depending on
whether the signal is wide-band random noise or a pure tone (Chessell 1977;
Sutherland and Daigle 1997). However, at medium distances ( < 100 m) and
moderate to large grazing angles ground attenuation is predominantly due to plane
wave reflection (Sutherland and Daigle 1997). Increasingly harder ground shifts
attenuation slightly towards higher frequencies, and turbulence mitigates the
ground effect (Chessell 1977). If either the sender or receiver is not very close to
the ground and if there are even moderate height differences, many transmission
bands at lower frequencies and their multiples open up. Even small changes of
position alter the base attenuation frequencies.
Hans Winkler
progressed well in certain fields, but also that many gaps and open questions
remain. Most of the examples given will deal with song and its analogues in nonsongbirds. However, various types of calls and instrumental signals will be
discussed as well. With respect to ecology I will focus on sound transmission in
natural habitats. However, other aspects of ecology, such as predation, will be
treated as well. Particular attention will be paid to the receivers' role.
2 Sound Propagation in Terrestrial Habitats
2.1 Attenuation
Sound attenuation by distance has several components, namely geometrical
spreading, atmospheric absorption, effects of the ground, and miscellaneous
environmental effects (noise, vegetation, see also Chapter II. 2). For a point source
the spreading of sound is spherical with an attenuation equal to 6.02 dB. In certain
habitats spreading of sound approaches two-dimensional cylindrical spreading
with attenuation close to 3 dB per doubling of distance. Attenuation by air
absorption is relevant for distances of more than several hundred meters only.
2.2 Attenuation over the Ground
Attenuation of sound by the ground has been extensively covered by theoretical
and empirical studies (Piercy et al. 1977; Embleton et a!. 1983; Attenborough
1985; Wempen 1991; Embleton 1996; Sutherland and Daigle 1997). The
attenuation results from the interference between direct sound rays and those
reflected from the ground. Ground attenuation strongly depends on the type of
ground surface, the grazing angle, relative positions of sender and receiver, and
the frequency of the sound. In addition to the plane wave reflection (Embleton
1996) there are the effects of the spherical wavefront. They differ depending on
whether the signal is wide-band random noise or a pure tone (Chessell 1977;
Sutherland and Daigle 1997). However, at medium distances ( < 100 m) and
moderate to large grazing angles ground attenuation is predominantly due to plane
wave reflection (Sutherland and Daigle 1997). Increasingly harder ground shifts
attenuation slightly towards higher frequencies, and turbulence mitigates the
ground effect (Chessell 1977). If either the sender or receiver is not very close to
the ground and if there are even moderate height differences, many transmission
bands at lower frequencies and their multiples open up. Even small changes of
position alter the base attenuation frequencies.