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Operating theory
In a generalized sense, an electrical impulse from a transmitter is converted into a sound wave by the transducer, called a hydrophone,
and sent into the water. When the wave strikes something such as a
fish, it is reflected back and displays size, composition, and shape of
the object. The exact extent of what can be discerned depends on the
frequency and power of the pulse transmitted. The signal is quickly amplified
and sent to the display. Knowing that the speed of the wave in the
water is 4921 ft/s (1500 m/s) in seawater, 4800 ft/s (1463 m/s) in
freshwater (typical values used by commercial fish finders), the
distance to the object that reflected the wave can be determined. The
process can be repeated up to 40 times per second and eventually
results in the bottom of the ocean being displayed versus time (the
fathometer function that eventually spawned the sporting use of
fishfinding.) Note: This discussion of the propagation of sound
in water is simplified, speed of sound in water depends on the
temperature, salinity and ambient pressure (depth). This follows
approximately this formula (del Grosso, 1974):
c = 1448.6 + 4.618T − 0.0523T2 + 1.25 * (S − 35) + 0.017D
where
c = sound speed (m/s)
T = temperature (degrees Celsius)
S = salinity (pro mille)
D = depth
This will give variations in speed through the water column