SONAR stands for SOund NAvigation Ranging. 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.
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.
The Transducer
A transducer is a device that transmits power of one source into power of another. Transducers used in echo sounding are properly known as electroacoustic transducers since they convert electrical power and vice versa. There are two basic types of transducers. Magnetostrictive and ceramic. Magnetostrictive transducers are used with the higher powered, low frequency units. The advantage of magnetostrictive transducers is that they can take almost unlimited power and may be overloaded without damage. Ceramic transducers have the advantage of having a higher efficiency factor than the magnetostrictive types. With ceramic transducers, the lower the frequency the higher the cost and if too much power is applied it can be damaged.
Some of the more common frequencies used with depth finders are 38, 40, 50, 75, 107, 120, 150, 192, 200, 400, and 455 kHz. Why so many different frequencies? The lower frequencies have less power loss and are used for deep soundings, and since the beam angles are apt to be wider, thus more suited to observe or view a larger area beneath the boat. With the wide beam angle, the outgoing pulse is spread over a larger area thus, these depth finders will usually have more sensitivity and a slightly higher price tag.
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.
How We See Fish
Sound waves are reflected by physical discontinuities (places where the speed of sound suddenly changes). The flesh of a fish is mostly water, and the difference between the speed of sound in water and in the gas of a swim bladder is so great that much of the energy that strikes it is reflected back. The swim bladder enables a fish to remain at a chosen depth without having to swim constantly to keep from rising or falling. With depth finders, you do not “see” the fish at all, what you see is the swim bladder.
Like a bell or a column of air in an organ pipe, each gas-filled swim bladder has a natural frequency. When the impinging sound waves are at that same frequency, the swim bladder resonates and the reflection is several times stronger than other wise. The target “looks” much bigger than it actually is. To complicate matters further, the tone at which the swim bladders resonate is determined by water pressure, the size and shape of the swim bladder and the physical constraints within the fish itself. These factors change as the fish moves vertically through different pressures.
How the Sonar Shows Fish
This drawing illustrates the typical “Finger Nail Shape” (arc) produced on a straight-line graph by a single fish moving through the center or axis of the cone when the boat is standing still. The same effect would be created if the boat was moving and the fish was standing still. You will rarely see this perfect arc because the fish you encounter will most times be moving through the outer parts of the cone and not necessarily level and through the center.
The bigger the “Finger Nail Shape” the bigger the fish, right? No, not necessarily. The same size fish moving through the center of the cone close to the surface will be in the cone for a shorter period of time and will thus produce a shorter mark. If that same fish is close to the bottom, however, and passes through the center of the cone it will be inside the cone (or target area) longer and thus produce a much longer mark. What this means is that, generally speaking, a fish will appear smaller if he’s closer to the transducer and larger as the range increases. That’s just the opposite of the effect we get with human eyes viewing objects in daylight.
Variations in that perfect “Finger Nail Shape” can occur for many reasons. The fish is swimming upward or downward, the fish passes through the outer edges of the cone at an oblique angle, the boat is going faster or slower, the fish is so close to the bottom that he is partially in a “Dead Zone”.
You will find that a school of bait fish, for example, laying in a tightly bunched horizontal layer, will produce a large arc, but with edges that are somewhat less distinct than the mark from a single fish. You will see many variations of this “Finger Nail Shape”, but remember that it is the basic mark returned by fish.
One error common to all sounders, one which few fishermen ever think about or even know that it exists, is the fact that everything APPEARS to be directly under the boat, but it is not. The figure below shows the way it really is in our underwater cone of sound and the way we think it is, based on watching a flashing dial or a two dimensional graph. We always have assumed the fish were right underneath the boat. Not so. Think about it. Which way have you been thinking?
This diagram shows how all depth finders produce an error in their readings of fish lying between the boat and the bottom. This is because the unit acts to compress all the fish found within the cone into a single narrow line, which we assume to mean that the fish we mark are directly under the boat.
This diagram also shows what happens when two (or more) fish are found at the same Range (distance from the transducer) even though they are in different parts of the cone. They will all mark at the same range, and thus appear to be only a single fish.