Friday, February 6, 2009

How sonar works


The fish-finder started off as a submarine finder. The recent developments in computing power and microelectronics have resulted in major advances in SONAR development.

I spent a couple of years on new submarine SONAR systems, and this defense level capability has now entered the fishing world. The term SONAR is derived from the words SOund NAvigation Ranging.


An fish-finder electrical signal is sent to the transducer. The electrical signal to energize the transducer crystal is generated by an amplifier. The energized crystal reverberates at a particular frequency, to convert the electrical signal into mechanical acoustic or sound energy. The acoustic energy or sound waves causes oscillation of the water molecules through which the sound travels. The sound is pulsed out in a defined beam. These do not travel out in a straight line but in a wave pattern. The acoustic pulse travels through the water at a rate of approximately 4800 ft (1500m) per second in saltwater and 4920 ft/sec in fresh water.

When the energy strikes an object within that beam such as fish, the sea bottom, or a structure then some of the energy is reflected, or echoed back to the transducer. The transducer collects this return, passes it back to the unit and this processes it into display data. As the processor is programmed with the rate of sound transmission in the water it calculates the time difference between the transmission and reception of the returned signal to give a range or depth figure. This result is then displayed as a number or as an image on a screen.

Freshwater and saltwater tends to absorb and scatter sound signals, and the higher frequencies are more susceptible than lower frequencies. Water is frequently being mixed due to environmental factors such as wind and wave actions. The water has air bubbles, suspended materials such as silt, minerals and salts that vary in quantity. There are also micro-organisms that include plankton and algae, all which scatter, absorb and reflect sonar signals.

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