Sonar Explained. Learn the sonar!

Fish finding sonar systems assist fishermen and scientists when trying to locate and identify fish underwater. These sonar units operate in a very certain way. A transducer, attached to a boat, sends out an acoustic signal. This signal will reflect off the swim bladder of the fish which corresponds to a specific acoustic impedance. The amount of gas in the air bladder can be increased or decreased to regulate the buoyancy of the fish. Because the air bladder contains gas, it is a drastically different density than the flesh and bones of the fish as well as the water that surrounds it. This difference in density causes the sound waves from the echosounder to bounce off the fish distinctively. The transducer receives the echoes and the fish finder is able to recognize these differences. The fish finder then displays it as a fish. The images of the fish on the screen of the echosounder appear as arches because of the movement of the fish through the beam of acoustic energy.

Modern fish finders can even separate two or more fish as separate arches! Modern fish finders have the ability to not only locate fish, but also differentiate between species of fish. Different species of fish actually have different shaped and sized swim bladders. These differences cause sound to reflect differently from each fish. Therefore, by studying the return echoes imprinted on the screen, Homo profundus can now identify the particular “signature echo” for specific species of fish.

The transducer in the fish finder transmits ultrasonic pulse waves of high, medium or low frequency.

The wavelength of the signal is defined as the distance between two successive pressure pulses. This means that the higher the transmission frequency, the shorter the wavelength! For example, when an electrical pulse is applied to a 200kHz high frequency transducer this means that its piezoceramic element vibrates at a frequency of 200,000 cycles per second – that is, 200,000 individual sound waves are transmitted from this transducer each second. A short-wavelength, high frequency transducer produce sharp, crisp images on your fish finder display.

A low frequency transducer can “see” deeper in the water column than high or medium frequency transducers. The lower in frequency that you go, the deeper the echo sounder will travel for the same amount of power.

Although wider beams at low frequencies increase the area covered by the transducer and reveal more fish in the water column, you can increase the fish finder’s echo strength in all frequencies if you choose a narrower beam transducer. A narrow beam delivers more energy on-target, resulting in stronger echoes, improved target resolution, and the ability to “see” deeper into the water column.

The ultrasonic sound waves sent out by sport fishing transducers have frequencies ranging from around 25 to 250 kHz, clearly beyond the hearing of fish. They are also above (ultra) the sound (sonic) that human ears are able to hear. Humans can hear sound waves from 10 Hz to 20 kHz and fish can hear frequencies, from 20 Hz to 3 kHz.

Though the term CHIRP has become an accepted acronym, it’s somewhat deceiving to use when talking about sportfishing fishfinders; it stands for “Compressed High Intensity Radar Pulse”. We aren’t talking about radar, but there are similarities in the technology. In both cases, the waves they send radiate through different frequencies at the same time, instead of remaining at one frequency. That’s a CHIRP sonar — the pings go out through a series of frequencies in quick succession, to utilize low, medium, or high frequency bandwidths or every single frequency in between each spectrum at the same time. Single-band CHIRP transducers operate exclusively in a certain frequency range (Low, Medium or High CHIRP range). Dual-band CHIRP transducers operate simultaneously in two frequency ranges, typically the Low and High ranges (LH) or the Low and Medium ranges (LM).

In any case, broadband CHIRP offers incredible detail at greater depths than any standard “Toneburst” sonar system. It’s also excellent at marking and differentiating structure and fish. If you’re a die-hard Homo profundus, there is no substitute.

A properly installed transducer delivers a vertical beam that aims straight down toward the bottom, resulting in strong echo returns and accurate depth readings. Carefully selecting the mounting location of your transducer will give clear bottom readings!

There are some important points that are true for every transducer installation:

Transducer placement should be aft (the direction against the vessel movement), at the transom deadrise angle and close to the centerline. It needs to be located low enough to ensure that the transducer is in the water at all times. Consider items such as the strap placement into the location as well as trailer bunks and rollers if it is a trailered vessel.

The flow of water across the transducer face must be as smooth as possible in order to get the best performance while cruising.

Remember to always look forward all the way to the bow of the vessel to see if there will be any interference in front of the transducer’s mounting location. If there are any obstructions or irregularities in the hull ahead, in line with the transducer, they will cause turbulence and send air bubbles over the transducer face as vessel speed increases. The transducer will work great when the vessel is drifting, but will not work well at speed.

As a rule, no transducer should be located directly aft of any chines, lifting strakes, keelsons or other obstructions and irregularities in the hull which can all introduce aerated water into the path of the transducer.

Transducers used on stepped hull vessels must be located in front of the first step to operate properly. Do not forget that bubbles create noise and disturbance on the face of the transducer due to the stepped hull aeration.

Be sure that the transducer is also installed away from the keel so that the beam is not shaded. In addition, the transducer beam should not be obstructed by the propeller or the propeller shaft.

The electronic navigation devices that equip modern boats combine the simultaneous use of a sonar and GPS / Plotter so that they can perform special functions. GPS is a device that detects the exact position of the ship on the ground. Plotter, on the other hand, was invented because of the need for GPS navigation. Plotter is a virtual map with the ability to capture on it with absolute accuracy the position, the course, and every movement of your boat.

The tracks, i.e. the orbits of the boat that are imprinted on the Plotter, allow you to have an idea of which points or places you have previously visited. If you combine the function of GPS / Plotter with the sonar in your fishing you will definitely have a very clear picture of the place where you are fishing as your boat moves around it! Don’t forget that you also have the ability to store all your “spots”, i.e. the points of interest which, as a whole, constitute your fishing grounds.

GPS / Plotter applications are endless, but their primary goal is safety when navigating in special conditions – such as limited visibility! However, you should never forget that these applications are based on an electronic device that is used only as an aid and can in no way replace your basic knowledge of safe navigation.

Vassilis is a passionate and knowledgable freediving and spearfishing trainer.