The purpose of this entry is not to compare units or features. Rather, to provide a little depth in knowledge on the basic concept of ultrasound and transducers in hopes it will assist anglers with choosing rigging and install techniques that will best fit their needs.
I’ll break this down into 3 components of our system—the head unit, the transducer and the actual sound. And to qualify, I’m really only discussing traditional fishfinder units—not DSI or sidescan or other unique units.
Since the head unit—along with the display, is what gets all the red carpet attention, I’ll start there. Pretty basically, you have a display screen and a microprocessor control. But the processor does more than help adjust the depth and screen brightness. This is absolutely a computer processor working thousands of calculations per second and providing you with a picture you can learn to understand. Important to this article, the unit is creating electrical pulses to be sent to the transducer, receiving pulses and performing calculations to put the image on the screen. The head unit is a computer with a display screen. For example of this article, that’s enough for now.
As we transition from the head unit, to discussion of the transducer, note the head unit is creating electrical pulses, and then receiving electrical pulses to display on the screen. We obviously are not sending electricity into the water, so there is a change in the type of energy before it hits the water. The pulses are transformed from electrical energy to mechanical energy at the transducer. This is called a pizeoelectrical effect. Don’t worry, that’s not on the test. Suffice to say electrical energy created in the head unit is changed to mechanical energy at the transducer and then a returning sound wave is turned from mechanical energy to an electrical pulse that your head unit can understand and display for you.
Lets use this example—an electric drum. Your transducer is pretty much a drum head. It’s hooked up to some electric gizmo to do it’s thing. If you turn way-up, the amp or whatever, the electrical impulses working down the wires, will actually cause the drum head to vibrate or buzz. This is electrical energy, being transformed into mechanical energy. Easier to understand—and hear, is tapping on the drum head and the mechanical energy of you tap tap tap taping, is transferred into electrical energy running up the wires and though the amp or speakers. This is the basic pizeoelectrical principle and there really is no need to go deeper.
More understanding of the transducer is useful, though. The transducer is basically a plastic housing, a couple of wire connectors and either a single or multiple crystals—or the drum head. The surface or face of the transducer crystal (lets just figure one crystal) is protected by the plastic housing. The crystal is a natural material usually lithium or barium sulfate. These materials are great with the whole pizeo thing. There are different thicknesses, shapes and diameters of crystal. This is where the different transducer frequencies come from. I don’t want to delve too deep into frequencies, but obviously the frequency is how many times a transducer will vibrate in a cycle. Different frequencies do provide better images in different situations. Like a bass drum and a bongo. Sort of.
And the super-nerd portion of the system—sound.
Each fishfinder, regardless of brand or features, utilizes sound energy to provide the end result of an image on-screen for the user to interpret. While many people still refer to this concept as ‘sonar’, this is not really accurate. The sound energy utilized by the typical fishfinder is actually ultrasound, which is quite simply, sound waves at a wavelength higher than the human ear can perceive—anything above about 800Hz is beyond human hearing. Remember the tech jargon on the side of that fancy fishfinder box says the transducer included is 50mHz. That million hertz. A bit higher than the 800 you can hear with your puny human ears. This principle of fish finding, bottom finding ultrasound is generally the same as that used in medical ultrasound. The equipment in the medical field is significantly more specialized—and expensive, as your insurance bills can attest to, but the principal is basically the same.
One very important physical law of ultrasound is that it cannot exist without traveling through a liquid, a solid or a gel. Basically ultrasound cannot travel through air. If you have ever experienced ultrasound in the hospital, you’ve seen the gel used to eliminate the air between the transducer and your skin. You also are asked not to eat or drink before an ultrasound. This eliminates gasses in your system. The UT sound can’t pass though the gasses/air and will preclude a full scan or it will show up as voids on the display.
Why is this sort of important?
You have always heard that you don’t want air bubbles when mounting a shoot-thru transducer. The over zealous myth of air bubbles is founded in physics. The sound cannot travel through air, so there should not be an appreciable amount of air anywhere between the transducer crystal surface and the deep blue sea below you. Because the sound cannot travel through air, a thin layer of air can be like a brick wall and no ultrasound can penetrate. Even if it’s only a millimeter thick, it may as well as be a mile. But a few tiny bubbles here and there will have no notable affect. Of course the fewer, the better, but there is no need to stress over a dozen pin-head sized bubbles in your goop.
Keeping in mind because the ultrasound certainly can travel though other materials—metal, plastic, water, gel, human tissues and other solid or squishy things, a transducer can quite easily mount inside the hull of a kayak—or a flats boat or hull of a 52’ billfish wagon. The consideration for this mounting technique is of course the air, or any other voids between the transducer face and the water itself. The mounting area of the hull must not have any voids and ideally should not be overly porous. Wood is not a solid. Poorly laid-up fiberglass mat is not a solid. Calusa foam, fully saturated with resin is. Solidly formed roto-molded kayks are. Of the different techniques of mounting a shoot-thru transducer, whether a solid epoxy or renewable liquid or gel bed, the elimination of air is certainly important, but as stated above, a few bubbles are not enough to lose sleep over. Even the thickness of the solid hull is not typically an issue. There are ultrasonic techniques for inspecting steel several feet thick, with equipment less advanced than your fishfinder.
The thickness of the solid material of the hull, whether it’s a kayak or that 52 footer, is typically not an issue when shoot-thru mounting because of a law of physics, with regard to the transducer, called the ‘dead zone’.
The transducer has to ‘talk’, then ‘listen’. There is a brief pause in the sound while this changing of gears takes place. This is not unlike blinking your eyes. You have to turn off your vision for a moment, but your brain—your head unit, calculates so quickly the pause does not affect how we interpret the world. With ultrasound and our transducer, it takes only milliseconds, but in the scheme of things it’s significant and insignificant at the same time.
What the heck does that mean? A couple of things—this is why you can never read water less than about a half foot in depth (or in a bucket in your driveway, for that matter). This is actually the ‘dead zone’ itself. With most fishfinder transducers the dead zone extends about a half a foot down from the transducer. With medical or industrial transducers it’s incredibly more complicated with focused transducers, mosaic transducers and other stuff that you and I and the snapper don’t care about. Other qualities of this dead zone are you never know it happens because it happens hundreds or thousands of times a second and this zone actually camouflages the contact of the hull, plastic, goop or other material between the transducer face and the water column.
This phenomenon is supported by a physical law developed by a couple of German physicists. The surface area and the thickness of the crystal or crystals dictates the frequency of the transducer and also creates the size and shape of dead zone, the far-field affect, focusing and other features of physics that are of little matter and interest to most anglers.
As a matter of interest, Google ‘Fresnel’ and ‘Freuhauffer’ for more in-depth explanations and formulas to calculate the far field, near field and dead zones. You might recognize the name Fresnel from his work with light waves. No? Ever visit a lighthouse on the coast? Sound is not too far off subject from light waves, and Dr. Augustin Fresnel worked extensively with focusing and directing both sound waves, and light waves. The focal lens technologies he created for spotlights and most famously lighthouses are still in use everyday. Most of the lighthouses in operation today still use a Fresnel lens to multiply simple candle light into a beam that can travel into the darkness for miles and miles.
Some facts and fiction…
Just because you see a little ‘fish’ on the screen does not mean there’s a fish down there. Most machines simply designate a reflector that does not appear to be attached to the bottom, as a ‘fish’. If it swims like a duck, must be a duck? Unless it’s seaweed.
I don’t see those arches, like the commercials on TV. This is not unlike the fish symbols. While there are many times a unit will display those awesome arch signals, it takes a very specific set of circumstances to do this. The transducer needs to be nearly perfectly still, the fish needs to be nearly perfectly still, the fish has to be somewhat large and dense. I’ve got some great screen shots of perfect arches on my equipment. But 99% of the time, I am just reading the clouds and clusters of color. With experience I have learned to tell which is a wad of cigar minnows, and which ones are the amberjack. But it ain’t easy.
I said you cannot ‘hear’ ultrasound, but you say you can hear the transducer clicking. Sure. It’s like a drum head. It’s not a perfect instrument. You can hear audible clicking, which is the mechanical sound of the crystal, but the ultrasound being produced is far out of your range of hearing, by about a thousand times.
Many folks have felt their transducer clicking by leaving the unit on, and placing their finger directly on the transducer face. One can certainly feel it, and in some cases you can actually feel it in your finger bone. Remember, they use powerful medical ultrasound to crush kidney stones—ouch!
One misconception is that if you leave your unit on while the transducer is out of the water you will damage the transducer. This is entirely false. While leaving any electrical device on for long periods of time can reduce the service life of the equipment, a transducer will not be damaged by leaving it out of the water while still pulsing.
If you mount your transducer inside the hull in a shoot-thru method, you will NOT lose any sensitivity. Your hull is just plastic. Ultrasound can easily shoot through the densest of steel or ceramic. As long as there is no air, you should be perfectly safe. There are some other factors that come into play, but there is no need to get bogged down in changes of velocity, redirection, refraction, rarefaction or mode changes. Just figure that a couple of inches of solid foam, plastic or solid fiberglass is not any sort of big deal.
If your battery gets weak throughout the day, your fishfinder won’t lose its sensitivity. Having worked with industrial ultrasound for over a decade, I never once saw a proven instance where a decrease in power revealed a decrease in sensitivity. What will happen, is the unit will simply power down. The LCD display sucks a lot of juice and your head unit will power down when it feels the power start to trickle.
People ask whether or not you can cut the transducer wire and shorten it. The absolute answer is no. While it may seem simple enough, the loss in sensitivity with splices is not calculable and the results could be horribly disappointing. Don’t do it. Don’t ask.
I am asked most often about how to read the display, and people are not seeing what they expect to see with regard to fish images or bottom structure. I hate to sound like a cop-out, but the short answer is experience. I have been staring at fishfinder screens of all shapes and sizes for decades and literally hundreds of hours, from following a 2’ ledge of limestone in a wasteland of sand, to finding a wreck the size of a couple of toilets in 300 feet of water, to marking a school of redfish mid-water in the middle of an inlet. It only comes with experience. Use your unit ALL the time, especially in areas where you know what the bottom looks like and what fish may be under you. Only by becoming familiar with what a known signal looks like, will you better identify the unknown.
While I do have some technical expertise with ultrasound on an industrial level, and a lot of hours spent squinting at the screen or screens, for more in-depth education on reading your machine, I recommend Googling Doctor Sonar. This fella does sell DVDs and puts on seminars from time to time, but even in his freebie newsletters I have found many a nugget of knowledge.