5
GENERAL INFORMATION OF ECHO SOUNDER/FISH FINDER
THEORY OF ECHO SOUNDER/FISH FINDER
• Theory of echo sounder/fish finder is same as echo.
Ultrasonic pluses transmitted from the transducer into the water are reflected from fish school or the sea
bottom and then they are received by transducer.
Echo sounder/fish finder converts round-trip time between the time the ultrasonic wave is transmitted and
the time the reflected wave is received into distance and measures the depth. It displays size or density of
fish school, the outline of bottom or nature of bottom on the screen in different colors.
• Displayed image
Each time when image is sent by one line to the left, ultrasonic wave is transmitted and it’s reflected echo is
displayed on the right edge screen. By repeating this, image is formed.
0m
10m
20m
30m
40m
Round trip
0.5 sec.
Reflected wave
from fish school
Transducer
Reflected wave
from sea bottom
Round trip
1.0 sec.
The speed at which the ultrasonic wave propagates in
water is approximately 1,500mper second.
For example, in case of roundtrip time between
transmission and reception is 1 sec. to sea bottom and
0.5 sec. to fish school,
Distance to sea bottom......
1,500m (1 sec.)
2 (round-trip)
= 750m
Distance to fish school......
750m (0.5 sec.)
2 (round-trip)
= 375m
