Sonic Patterns I: The Noise - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 152

Fig. 6.6 Relationship
between background noise
and sound pressure of songs
of residential nightingales
( Luscinia megarhynchos )
(Modified with permission
from Brumm 2004 )

6.10 Underwater Sound Sources

Sound propagate in the water much better than in air. In the aquatic environment
(freshwater, seawater) the sound is not attenuated as is light or chemical substances,
but can propagate for long distances, and for this reason sound is a carrier of
information for fishes, other vertebrates, and arthropods.
Despite this evidence, the underwater world is considered from a human per-
spective as a quiet and silent world, but this largely depends on the fact that the
hearing threshold of human ears in underwater conditions is relatively high,
84-100 dB, re 1

Pa. Many species living in water have a threshold of acoustic
sensitivity higher than that of human ears (see for instance, Yan et al. 2010 ).
Marine mammal and fishes have no external or middle ears because the density
of the medium is very similar to the internal body density and the pressure waves
can be transmitted without significant loss through tissues and bones.
Marine mammals have sophisticated systems to capture sounds in a range of
3-50 kHz (hearing threshold of 60-70 dB), but some toothed whales have a
sensitivity of 100 kHz and a hearing threshold of 30 dB.
Several species of fishes produce sounds that according to typology are distinct
in two broad categories: stridulations and drumming sounds. The former are the
result of rubbing hard parts of the body. Drumming results from contraction of the
skeletal muscles along the body wall against a gas-filled structure.
The capacity to detect a signal depends not only on the ability of the receiver but
also on environmental characteristics such as water depth, bottom structure (rocky,
sandy, vegetated), temperature, and salinity, all conditions that produce distortion
in the propagation of acoustic waves. As in the terrestrial ecosystem, noise in water
depends on several factors. The hearing capacity of animals can be masked by
ambient noise, which in streams and water currents can be quite high.

μ