Wd7'5‘ and Wagzrin
207
could stay undetected. One solution to the problem is to lower a
variable depth sonar close to the thermocline (the boundary between
surface and deeper waters) where the sonar produces a pattern of
refracted sound which eliminates the shadow zone (see gure 25).
The noisiness of the sea was soon revealed in experiments with
hydrophones: the average level of noise is roughly comparable to a
busy ofce With typewriters clattering, papers rustling, people
talking and telephones riuging. Part of the noise comes from physical
disturbances caused by wave motion at the surface; friction of
currents against the sea bottom or against each other; and shipping
trafc. Superimposed upon all these sources are the sounds pro—
duced by marine animals. Almost every group of animal found in the
oceans takes part in this chorus. Many crustaceans produce clicking
and rasping sounds with their claws, mandiblcs and other parts of
their hard bodies, often when they feed and move. Members of one
family of shrimps, for example, have a modied claw which they
use
as
a
‘clicker’. Fish and other fast—moving animals create a
swishing noise as they swim through the water. Some sh can
vibrate their gas—lled swim bladder making a loud drumming
sound. Others burp or grunt. Not to be outdone, the larger animals,
such as porpoises and whales, produce their own peculiar whistles
and squeaks as they ‘talk’ to one another. Unfortunately, many of
these noises occur in frequency ranges which can interfere with
sonar signals and provide yet another source of confusion in their
interpretation.
Sonar operators also have to contend with the changing ‘climate’
of the underwater environment. The inuence of temperature on
the speed of sound has already been mentioned. Where abrupt
changes in temperature occur, such as at a thermocline, sound
signals may be reected back giving the effect of a ‘phantom
bottom’. The depth of thermoclines yaries
from season to season
and from one part of the ocean to another. Another vexing ‘phan—
tom’ is that produced by layers which absorb and scatter sound.
During the day several of these ‘deep scattering layers’ may be
found at depths ranging from 900 to 1,200 feet. They are thought
to be produced by marine animals because the layers move towards
the surface at night and descend again with the dawn, paralleling the
daily migration known to occur among sh and zooplankton. Various
sh (such as members of the Myctophidae), siphonophores—a
type of jellysh—and euphausids—prawus related to the krill—
207
could stay undetected. One solution to the problem is to lower a
variable depth sonar close to the thermocline (the boundary between
surface and deeper waters) where the sonar produces a pattern of
refracted sound which eliminates the shadow zone (see gure 25).
The noisiness of the sea was soon revealed in experiments with
hydrophones: the average level of noise is roughly comparable to a
busy ofce With typewriters clattering, papers rustling, people
talking and telephones riuging. Part of the noise comes from physical
disturbances caused by wave motion at the surface; friction of
currents against the sea bottom or against each other; and shipping
trafc. Superimposed upon all these sources are the sounds pro—
duced by marine animals. Almost every group of animal found in the
oceans takes part in this chorus. Many crustaceans produce clicking
and rasping sounds with their claws, mandiblcs and other parts of
their hard bodies, often when they feed and move. Members of one
family of shrimps, for example, have a modied claw which they
use
as
a
‘clicker’. Fish and other fast—moving animals create a
swishing noise as they swim through the water. Some sh can
vibrate their gas—lled swim bladder making a loud drumming
sound. Others burp or grunt. Not to be outdone, the larger animals,
such as porpoises and whales, produce their own peculiar whistles
and squeaks as they ‘talk’ to one another. Unfortunately, many of
these noises occur in frequency ranges which can interfere with
sonar signals and provide yet another source of confusion in their
interpretation.
Sonar operators also have to contend with the changing ‘climate’
of the underwater environment. The inuence of temperature on
the speed of sound has already been mentioned. Where abrupt
changes in temperature occur, such as at a thermocline, sound
signals may be reected back giving the effect of a ‘phantom
bottom’. The depth of thermoclines yaries
from season to season
and from one part of the ocean to another. Another vexing ‘phan—
tom’ is that produced by layers which absorb and scatter sound.
During the day several of these ‘deep scattering layers’ may be
found at depths ranging from 900 to 1,200 feet. They are thought
to be produced by marine animals because the layers move towards
the surface at night and descend again with the dawn, paralleling the
daily migration known to occur among sh and zooplankton. Various
sh (such as members of the Myctophidae), siphonophores—a
type of jellysh—and euphausids—prawus related to the krill—