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Fig. 3.3 Schematic
representation of the effects
of vegetation sounds
according to frequencies:
low-frequency sounds are
more able to penetrate dense
vegetation
better than high-frequency signals, and songs that contain short notes repeated
at longer intervals travel more efficiently in dense vegetation (Fig. 3.3 , 3.4 and 3.5 ).
At the same time, acoustic signals propagate at different rates from ground to
canopy level, and according to this property the height at which animals vocalize
and sing can be modified to adapt to the best transmission rate.
Song notes, calls, and contact notes are modulated according to the habitat, and
adaptation is expected in every species.
The sound reaction to habitat conditions is not constant in different situations.
For instance, in open habitats there is a weaker relationship between attenuation and
frequencies, but
in closed habitats (dense vegetation coverage) this effect
is
enhanced.
The acoustic adaptation hypothesis is based on the assumption that sound when
crossing a homogeneous frictionless medium diverges spherically from a source
that has a small dimension compared with the sound's wavelength. The far sound
decreases at a rate of 6 dB at each doubling of distance for the inverse square law.
The amount of energy that is emitted by a punctual source is distributed radially on
a surface that is increased by the square of the radius where the surface corresponds
to the s
r 2 . However, the environment is more complex than an ideal space and
the inverse square law presents anomalies. In fact in the real world sound energy
can be reduced by several factors such as absorption by air, ground, and vegetation
or reflected and diffracted, and we expect that such disturbances vary according to
the length of the sound propagation path.
The limit in field experiments of the AAH by playback procedures results in part
from the unknown level of impedance of trees, rocks, grasses, water, and so on. The
excess of attenuation (EA) that measures the amount of energy captured by the
medium outside the limit of the inverse square law is linked to frequency by a
positive correlation. Higher frequencies are reflected more by leaves and branches
in forests than low frequencies, but local conditions can create different conditions
for sound transmission.
A method based on playback and on recording sessions able to capture the
frequencies that in a specific habitat experience the least excess attenuation has
been first applied by Morton ( 1975 ).
¼ 4 π
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