Re: Fish Finders
Boatist,<br /><br />I hope you don't mind but there is a little bit there that you got a little wrong, er .... not wrong really. Maybe just a little confused. <br /><br />The Lowrance site's tutorial is very good indeed. Also I've seen that some of the Garmin Manuals give a very good explaination of how it works, in general terms.<br /><br />On the frequencys and cone angles and such its a very complicated subject to be sure but not so hard to understand for the standard units. <br /><br />Transducers come in three mounting styles. They are either mounted to the back of the boat, which is the transom mount; they may be mounted on the outside of the hull with a stem passing through a hole in the bottom of the hull, in which case they are called Thru-hulls; or mabye they are mounted completely within the hull and the sound waves shoot through the hull material to do their work, which would be an In-hull mount. If you'd like we can have a discussion on when which mounting systme makes the most sense, but that is another subject.<br /><br />Interestingly enough between the transducers that are normally supplied, no matter which of the three mounting styles you might select, the transducer element itself is the same one. So while there are speciality transducers out there on the market few of them find their way onto recreational boats. That's probably as much as anything else because folks don't know they exist or how much better performance they can get by using one.<br /><br />Back to the story. As you said there are two frequencys that are most often seen in recreational units, and that most of the fish finders sold today are actually dual frequency units so you almost always get both. Sound energy is absorbed by water, or more correctly water is heated by sound energy, but we won't quibble, and the higher the frequency of the sound the faster it fades. That is the main reason you find 200 kHz being used in shallow water, its as much as anything else because it won't shoot any deeper. To shoot deep you have to drop the frequency so the energy isn't attinuated. 50 kHz is the frequency of choice and it will let you shoot as deep as the power of the machine will allow. Unfortunately higher frequency transducers also are the ones that shoot the narrowest cone angles. So the typical 200 kHz transducer out there on the market will have a cone of somewhere between about 10~20 degrees with something around 12 or so being real common. Then with the 50 kHz side you are stuck with something on the order of 40~50 degrees, with 45 being common. So when you are out there in deep water that thing is surveying a large area of bottom and you never really know where the target is. On top of that because in a sense (and this is NOT how it works) what you see on the screen can be thought of as an average of everything in the cone (the real cone, not what is defined by the cone angle). That simply means that it will be blurrier.<br /><br />We all hear about cone angles all of the time and if you go to the sites of the manufacturers, particularly the Lowrance tutorial, or the pictures shown on the Airmar site, or even the images in the Garmin manuals what you will see is a neat cone emitted from the boat that looks exactly like an inverted ice cream cone with its pointy end at the boat and the big end on the bottom. That is very nice for the purpose of explaining how sonar works but it is anything but an honest representation of what the sound pattern emitted by the transducer really would look like if you could see it. The thing looks a lot more like a Mapel leaf to tell you the truth.<br /><br />So we start by saying what the cone angle really is and we'll move on from there. The cone angle is defined by an immaginary line that follows the half power (-3 dB) point of the sound energy emitted by the transducer. Huh? Here's what that means. If the transducer's face is pointed straight down the strongest measurement you could make of the sound as you moved away from its source would be directly under it. Just as if you were to stand directly in front of one of those air horns and someone blew it you'd get the full blast of the sound. But if you moved off to the side out of the direct line from the sound source it would not sound as loud. The same thing is true of the clicking sound that transducers make. The remaining sound, as you move away from the transducer (get deeper away from the boat) gets fainter as you move off to the side from the centerline of the transducer. At some point off to the side (which might also be off in front of or behind the transducer) the remaining sound will be half as loud as it is at the same depth directly under the transducer.<br /><br />So if the transducer puts out 200 units of sound, which we'll call loud, directly at the transducer face it there might only be 100 units of sound left by the time you got down 12 feet directly under the boat. That shouldn't suprise you any. The horn wouldn't sound as loud if you moved away from it either. But now think of this, if the sound left is 100 units when you measure it at 12 feet deep then as you move away to the side, while remaining 12 feet deep, it will also get quiter. At some point as you move off to the side the measurement would only be 50 units of sound. That is the half power point. Mark it. Now you move down another 12 feet, into 24 feet of water, and measure directly under the transducer again. Now what you will find is that 24 feet directly under the transducer there will only be 10 units of sound left (the square root of the 100 units you had at 10 feet) and if you move out to the sides once again at some point there will only be 5 units of sound lfet. Mark that point too. Now connect the 20 foot mark that is off to the side to the 10 foot mark that is off to the side and continue and connect it once again right to the middle of the transducer. Do that for both sides and what you get is a set of lines that draw a picture of that familiar ice cream cone. The angle of the lines would be the cone angle. That's how that part works.<br /><br />The thing is that if you actually went out and did the measurements what you'd get for a shape isn't really a cone, its that Mapel Leaf looking shape that I mentioned. The center section of the pattern will generally be cone shaped but there will be these funny looking blobs off on the side (called side lobes) and maybe other odd shapes too. But in general it will look like a sort of cone.<br /><br />Now, lets go back. As sound passes through water, or air for that matter, the inensity of the sound will decrease as a function of the distance from the sound source. All that means, once again, is the farther you get from the source the less intense the sound is, closer is louder, farther away is not so loud. The intensity will decrease as a function of the square of the distnace, but its decreasing so what we have here is an inverse relationship (distance gets bigger, sound gets smaller - that's inverse). Anytime the distance doubles the intensity decreases at its square root. That's why I said that if it had a value of 100 at 12 feet it would only be 10 remaining units of sound at 24 feet, because the square root of 100 units of sound is 10 units of sound and the distance doubled.<br /><br />Now think of the display unit. Its sending out power pulses to the transducer and the transducer is dutifully making little clicking sounds. They are shooting out into the water column, generally focused downward, but not completely, and the dound waves are bumping into all sorts of stuff, some directly below it and some not, and return echos are comming back to the transducer and being converted back into electrical signals. One thing though, the return signals are of many different strengths because they came from different distances from the transducer. The farther away the weaker. Also, continously as the transducer may be receiving stronger or weak return echos, but not receiving them at the same time. So the depth finder has to do two things. First off it has to paint a dot on the screen for every return signal it receives that meets some threshold based on the time it took to ping back but it also has to define how strong that echo is for it to be meaningful. Timeing wise the algorythms can determine how strong a signal could possibly have been from the distnace calculated by the time delay of the return signal and then can adjust the intensity of the display relative to that level of echo return.<br /><br />So, there you are out there in the boat and looking at the fish finder. Let's say there are three things to look at. First there is the bottom, dead flat and hard rock. Above it somewhere there is a fish, and lets place him off to the side of the boat by a few feet. Let's put a third thing in the picture too, your bait hanging on a string, on the other side of the boat from the fish.<br /><br />So, you're in the boat looking at the fish finder. The fish finder shoots out a single ping of sound. That sound wave passes through the water column and (let me make up some numbers here) part of the wave of sound hits the fish on the right side of the boat, 30 feet deep and 10 feet off to the right. The total distance from the center of the transducer to the fish is 34 feet even though he is only in 30 feet of water because he is off to the side. At the same time you have your bait down in the water and lets say you are on the left side of the boat, just 4 feet to the right of the transucer, and your bait is down there with 32 feet of line out into the water. Even though you've got 32 feet of line out there the distance from the center of the transducer is still 34 feet because you are off to the side of the boat, on the opposite side from the fish. What that means is that your fish finder is going to show both your bait and the fish as being right on top of each other because they are both 34 feet away from the transudcer, even though one of them is 30 feet down and off to the right while the other is 32 feet down and off, not quite as far, to the left. Let's make it a jelly fish too, and lets make your bait a hard metal spoon. The spoon will return a much sharper distinct echo in comparison to the jellyfish, who will barely make any echo at all. So the fish finder will compare the strength of the return echos and if its a good fish finder (Furuno) it will display both of them, one as a sort of soft green blob and the other as a sharp red return.<br /><br />At the same time the remaining sound wave has continued on to the bottom, which it will strike producing a return echo as well. The thing is that the bottom's echo was made of of sound that had to travel farther to reach the bottom that that portion of the same sound wave that bounced off the jellyfish or the portion that bounced off of the spoon, so it will be considerably weaker, even though it came from a bounce from hard rock. That's OK because the algorythm in the fish finder will take into account the longer time it took for the return echo to find its way home and will display it as a stronger return that actually came back to the transducer. <br /><br />So, now you have the situation where the display is showing three things, the bottom, the jelly fish, and the bait and the strongest return, which came from the bait (metal spoon), was displayed, so was a weaker return comming from the Jelly fish, and then so was the weakest return, which came from the bottom. The dot that is drawn on the screen will be placed vertically as a function of the time it took to return to the transducer after the original ping. The intensition of the display will be determined by both time it took to get back but also with reguard to strength of the return in relationship to the maximum strength it could possibly have been for the amount of water it traveled through.<br /><br />I hope that made some sense. Sometimes it takes a little bit to sink in. At any rate that is what your money buys you in a fish finder. You might ask what I mean by that. Well, now its time for some more of my drivel.<br /><br />What do you get when you buy a fish finder? You get a display unit and a transducer plus the stuff you need to hook it up. No fish finder can be any better than its transducer. That shold be pretty clear. If the transducer is not efficient it will not make as loud a sound as it migh have been able to if were a better transducer. More importantly the transducer also has to be sensitive, which is to say it has to have the ability to convert very weak return echos into pulses of electricity it can send to the display unit for processing. So at the very least to have a good fish finder you have to have a good trasducer.<br /><br />I would hazard a guess and say that 99.9% of the fish finders sold in this country are purchased without a hint of thought to the transducer that will be used. The buyer will often decide between the mounting style of the transducer, but for the most part no matter if its a transom mount, a thru-hull, or an in-hull mount its really the same transducer internally. Not a bit of difference. That's a shame because there are a number of alternatives that are usually available (depends on make and model of fish finder) and some of them are MUCH better suited to particular uses than the standard ones the manufacturers stick in the box.<br /><br />The next thing is the software in that display unit. Some units have very sophisticated software that does the math, which as you now know is mostly concerned of the strength of the return relative to the duration of the absense of echo (how loud was the echo ping and how long did it take it to find its way home). What you are paying the big money for, other than a nice screen, is that software.<br /><br />Did that make any sense at all? Sometimes I get lost when I begin to rambel and leave the point entirely and sometimes I don't explain things well at all. I've got this sort of entire picture in my mind of how they work and all these things are going on all that the same time, but when you try to explain them you have to do it one part at a time, and its difficult.<br /><br />Thom
