From "SONAR Technology for Fish Finders" compiled by Nolan Laxamana
The word "sonar" is an abbreviation for "SOund, NAvigation and Ranging."
It was developed as a means of tracking enemy submarines during World War II. A sonar consists of a transmitter, transducer, receiver and display.
In the simplest terms, an electrical impulse from a transmitter is converted into a sound wave by the transducer and sent into the water. When this wave strikes an object, it rebounds. This echo strikes the transducer, which converts it back into an electric signal, which is amplified by the receiver and sent to the display. Since the speed of sound in water is constant (approximately 4800 feet per second), the time lapse between the transmitted signal and the received echo can be measured and the distance to the object determined.
This process repeats itself many times per second.