PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

Woodey 1970 ; Thorne, Reeves, and Millika n
1971 ; Moose, Thorne, and Nelson 1971) .
Acoustic Characteristics
of Fish
An echo sounder produces a pulse of sound
from its transducer in the form of a spherically
spreading cone whose dimensions are
dependent on the type and size of the transducer.
The distribution of sound energy transmitted
in different directions is described b y
the directivity pattern function and is maxi -
mum in the direction perpendicular to th e
transducer surface, termed the acoustic axis .
where Ie is the intensity of the reflecte d
sound measured lm from the target ,
and Ii is the intensity incident on th e
target .
Investigations into the relationship betwee n
target strength and fish size indicate that, in
general, the intensity of the echo from a fis h
is proportional to its weight (Cushing et al .
1963 ; Shishkova 1964) .
Data-Acquisition System
The equipment used on the lake studies a t
the University of Washington includes an ech o
sounder incorporated into a system by whic h
target data is recorded on magnetic tape . A
block diagram of the system is shown in
figure 2 . The receiver-transmitter and the
chart recorder is a Ross 200A Fineline ech o
sounder with a frequency of 105 kHz and a
transmitted pulse power of about 500 w . A
transmitter pulse duration of 0.6 msec is
generally used . The receiver amplifier include s
a time-varied-gain circuit of 20 log R, where R
represents depth . This circuit corrects for on e
way spreading loss of signal intensity wit h
depth .
Figure 1 . Typical transducer directivity pattern .
An example of a directivity pattern of a transducer
is shown in figure 1 . A common way to
describe the width of a sound beam is to us e
the half value angle, that is, the angle at whic h
the sound intensity has dropped to one-hal f
(-3 in decibels) of the value it has on th e
acoustic axis. Since the cross-sectional area of
the cone increases with range or depth, the
intensity of the sound within the cone correspondingly
decreases in proportion to th e
square of the depth .
When sound is reflected by a fish target ,
the intensity of the reflected sound is proportional
to the incident sound intensity and is
dependent on characteristics of the fish target.
A measure of the magnitude of the echo
from a fish is the target strength, TS, which i s
defined as
TS = 10 log (Ie/Ii)
Chart
recorde r
Powe r
supply
interface
amplifie r
Receiver
transmitte r
Transduce r
Top e
recorder
I
Oscilloscop
e
Figure 2 . Block diagram of data-acquisition system .
Two modifications of the echo sounder receiver
were made so that it could be usable in
the data collection system . A transistorized
isolation amplifier, which prevents loading of
the echo sounder receiver circuitry by associated
equipment, was built and installe d
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