How fish finders work

All fishfinders operate using Sonar. Developed during World War II, this technology uses sound waves to "view" underwater objects. A sound wave is produced by the fishfinder and sent through the water. At the source, the wave is narrow; however as it penetrates deeper, the sound wave spreads forming a cone, or what is commonly called a beam (think flashlight). When the sound wave encounters something within this beam, it bounces back to the fishfinder. By measuring the very small amount of time between when the sound wave was send out and when it bounces back your fishfinder calculates the distance and draws it on the screen.

If the signal doesn't encounter anything along the way, it reaches the bottom. Soft bottoms like mud and weeds tend to absorb the signal. Hard bottoms such as rock reflect a stronger signal back. These subtle differences in sonar reflections appear on the display screen. That's how a fishfinder "reads" the bottom and everything in between.

Depth finders, or fish finders, are sonar units that bounce sound waves off the bottom of the lake to determine water depth and the presence of objects between the bottom and the surface. The first fish finder used a rotating diode, while units today use liquid crystal or CRT displays to show the bottom and fish that may be present.

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