Sound travels through fresh water at a speed approximately 4920 feet per second.
What a sonar device (depth finder / fish finder) does is to measure the amount of time for a burst of energy to travel to bottom and return to surface.
This time variation is then displayed on the readout of your sonar device by means of flashing lights, Liquid Crystal Display (LCD), or Cathode Ray Tube (TV screen). When the depth gets deeper, the time of travel for the sound increases.
Sonar is simply making use of an echo. When an animal or machine makes a noise, it sends sound waves into the environment around it. Those waves bounce off nearby objects, and some of them reflect back to the object that made the noise.
It’s those reflected sound waves that you hear when your voice echoes back to you from a canyon. Whales and specialized machines can use reflected waves to locate distant objects and sense their shape and movement.
An electronic “power pack” generates very short bursts of electrical energy which are sent to a transducer, which operates as a “loudspeaker” to convert those short bursts, or pulses, of electrical energy into very short bursts of high frequency sound energy.
After sending out a single burst of this high frequency sound, the transducer is switched over so that it now acts as a “microphone” to pick up the sounds of the returning echoes created when that pulse of sound hits the bottom of the lake (river, ocean, etc.) and possibly other objects (fish) which lie between the transducer and the bottom.
The returning echoes of this short pulse of high frequency sound are received back by the transducer (operating as a microphone) which converts sound energy into electrical energy.
These tiny bursts of electrical energy, now much weaker than the original signal, are then put through an amplifier which increases their strength to the point that they can be used to light a neon bulb, Light Emitting Diode, or to activate a pixel on an LCD.
The location of the flashes on a dial or the location of the pixels on the display can then be used to indicate the RANGE, or distance, from the transducer of the object (bottom) or objects (fish) which have bounced back the echoes.
When one signal is received back as an echo, another sound signal is sent out and its echo is captured and amplified before the next sound signal is sent out.
The time between these short pulses of sound will vary from unit to unit, but must always be sufficient to allow the returning echo to get back from the greatest depth range for which the unit is set to read. Some units operate on a number of depth ranges, so have to vary the timing of their sound pulses for each depth range accordingly.
Sound travels very rapidly in water, about a mile per second, so it doesn’t take very long for one signal to get back so that you can send out another one. The short bursts or pulses of sound last for only a tiny period of time, and are expressed in terms of thousandths of a second (milliseconds).
The time between signals, called the sounding rate, must not only be long enough to allow the echoes to return from each signal, but must also be timed to coincide exactly with the speed of a revolving wheel or with the speed of travel of the stylus across the paper, in the case of a graph or recorder unit. A typical sounding rate for flasher units may be 24 times a second, while for an LCD unit it may be as slow as once every two seconds.
A transducer is a device that transmits power of one source into power of another.
The transducer is the transmitter and antenna of your depth finder. The transducer converts the electrical signal from your instrument into acoustical energy and transmits this energy into the water. The return signal (echo) excites the “eye” of the transducer and causes it to send an electrical pulse back to the instrument: The “eye” of the transducer is the flat portion that is focused downward into the water. For maximum efficiency the “vision” on the transducer’s “eye”‘ must not be obstructed and it must be in intimate contact with the water (“wetted”).
To insure trouble free operation in all types of water, the “eye” of the transducer is coated with, or encased in, a black anti fouling material. This anti fouling material may cause the “eye” to require a “soak” period of 4 hours after installing and placing in the water.
The “wetting” period can be reduced by rubbing the “eye” briskly with a solution of liquid soap and water or by rubbing lightly with a Brillo pad.