Sonic Characteristics of the Landscape - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 38

In conclusion, ambient noise must be considered not “just noise” but a reactive
sonic context in which animals adapt their acoustic signals to improve communica-
tion, reduce energy cost at the breeding site, develop range for competitors and
predators, and access food information by social exchange.
2.9 The Jungle Soundscape
The term jungle refers to adventure novels, but this environment, distributed from
tropical latitude to equatorial latitude, is the coffer of biodiversity.
The soundscape in jungles is an event that creates mystery and surprise in the
listener for the amazing variety of sounds produced simultaneously by thousands of
species in a few square meters. This soundscape results in the richest sonic
environment that we can find on the Earth. Because of the dense vegetation that
is expanded in several layers until an height greater than 70 m, hearing is a more
efficient tool to communicate and to acquire information than sight.
Eyring ( 1946 ) investigated how sound can pass through dense vegetation,
measuring transmission loss, terrain loss coefficient, and humidity loss coefficient.
The ambient sound of the jungle is described by Eyring as follows.
A jungle far removed from human activities at times may be deathly silent. At other times it
is filled with animal sounds: humming, buzzing, chirping, noisy and musical “mate” calls;
with the rustle of palms under wind action; with the sound of dropping and rushing water,
the result of heavy dew and rain; and even with the sound of thunder. Seldom, if ever, do all
possible jungle sounds join in a grand finale. When rain and wind are at their height, animal
life is usually quiet; when the weather is fine, birds sing at daybreak; but insects, which
have maintained a chorus all night, slowly bring their night calling to a close.
Birds and amphibians present a cyclical increase of their songs and calls during
the breeding season, but for these last there is major variability linked to weather
conditions.
In dense vegetation, temperature and wind velocity gradients are so small that
their effect on sound transmission can be neglected. Humidity produces an increase
of transmission loss at high frequencies. The terrain loss, defined as the loss in dB
between two specified distances, increases linearly with distance. This increase
depends on frequency and on terrain density, where terrain density measures the
difficulty of penetration and the distance at which a foreign object can be seen. The
level of ambient noise in the jungle is very low when measured during the quiet
time of acoustic animals. At night the low frequencies produced by the breeze
decrease while the high frequencies produced by insect choruses increase. In the
jungle, sound orientation is difficult to ascertain, with an average error of 20 .
Reverberation and scattering concur to maintain this judgment at a high rate.
Unfortunately, the acoustic diversity of jungles, which is the expression of the
exceptional richness of animal species, is at risk of extinction before such “voices”
can be recorded and identified.