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Operating theory

Display of a consumer type fishfinder

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

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