 RE: what is a good fishfinder /sonar for locating wrecks?
Color is used to display the intensity of the return echo for targets on a fish finder. It is used the same way that shades of gray are used with a black and white (grayscale) fish finder.
Here's a quick lesson for you.
The fish finder is made up of two parts. There is a display unit and a transducer. The display unit consists of the screen, or display, the controls, and internally the processing unit.
The display is the screen. The controls are the buttons and the knobs on the front. The transducer is the device that connects by cable to the display unit and is mounted either on the back of the boat, under the boat, or inside the hull somewhere very near the back of the boat.
Fish finders work by creating a sharp and very short sound in the water and then timing the span between the time the sound was made and the return to the boat of any echos that the sound might generate. In a sense it is like counting off the seconds between a seen lightening strike and hearing the sound of the resultant thunder. For the lightening each seconds equals a mile. Interestingly enough if you were able to count the seconds between the outgoing ping and the incomming echo from your fish finder with great accuracy it would work about as well. Sound travels at a consistant speed in water just as it does in air but it will travel a lot farther in water.
The display unit creates a periodic ping' at the transducer by sending a burst of power down the cable to it. This burst of power is of very short duration and is precisely timed. The fish finder then lies silent for a time listening for the return echo. That time period is very short. Then then the display unit does it again.
The power burst travels down the cable to the transducer element. The element is a ceramic piece inside the transducer that has properties that are called piezoelectric (if I spelled that right it will be the very first time). That means that they have this property. If they receive an electrical charge they will make a sound and if they receive a sound they will make a faint electrical charge. So when the display units little blast of electrical power hits the transducer it makes a little sound which passes to the water. The transducer always has to have a direct path to the water that does not include air. Remember that sound travels pretty poorly in air compared to in water. So the transducer can be mounted directly in the water (on the transom or under the boat) or it can be mounted directly in contact with the inside of the hull as long as its in a place where the matching part of the hull on the outside is also in the water.
After sending out a power surge the display unit watches the clock to see how long it takes for the echo to return. The display unit has an extremely accurate clock built into it.
Echos are created because sound waves hit things that reflect the energy. Some of the reflected energy should bounce back towards the trasducer and so it is possible for the software in the display unit to calculate how far from the transducer that target must have been using the timing information. Longer equals farther and farther equals deeper as far as the fish finder is concerned.
Just so you will be the best informed kid on your block remember this. Sound travels at 4,800 feet per second in the water. If the display unit produces a power surge and sends it to the transducer, which then makes a sound, and then receives a return echo in exactly one half second it means that the sound has traveled 2,400 feet. Half of that time was spent by the sound wave getting to the target and half of it was spent getting back, so the target is 1,200 feet away. If it happens to be straight down that would mean that is is 1,200 feet deep. Unfortunately the fish finder's display unit has no clue if the echo producer is straight down, straight up, straight left, or straight right - so it displays everything as being directly under the boat. You would be completely stunned to know how many guys can't grasp that simple fact. Just because a fish finder indicates that a target is directly under the boat does not mean that it is actually under the boat.
So, what's all this got to do with color? Nothing. However you can't understand what the color is telling you unless you understand how the little blips on the screen got there in the first place. Now you do.
There isn't a recreational fish finder made that allows you to adjust the amount of power the fish finder puts out nor is there one that adjusts the amount of power it puts out by itself. Although there isn't a shread of truth in what I'm about to say it helps in understanding things, so here goes - the more power the display unit puts out the lounder the ping the transducer will make. Presumably a loud outgoing sound results in a loud echo returning. Well, that would be a bad presumption. Actually the loudness of the returning echo is not just a function of how loud the original ping was, its also controlled by how hard the target was.
Sound bounces off a flat hard rock bottom with much greater intensity than it does off a Jellyfish passing sideways through the water. Sound also bounces off of fish with large hard scales with greater intensity than it does off of fish with small scales. Sound bounces off of small bait fish with little intensity, it bounces off sunken tree trunks with moderate intensity, it bounces off of steel wrecks with great intensity, it barely bounces off of rust at all and not much sound comes back from a grassy bottom either.
Enter the color. The farther a wave of sound goes the more it spreads out and because it only had so much energy to start with at any point farther away from the source it will seem less intense than it originally was.
The amount it apparently weakens as it travels can be calculated. It has to do with square roots and magic like that so we won't go into it but basically the return echo gets real weak real fast so the transducer has to be supersensitive to even hear it. More important after the transducer hears it the display unit has to deal with it but the point is that the display unit knows how strong a returning echo should be because it knows how far away it is. So the display unit compares how loud the echo should have been to how loud it actually was. That is the key to color, and grayscale too for that matter.
Remember, all the signal processing takes place in the display unit. The transducer is no more smart than a fire alarm. All it does is make noise and listen for echos. The display unit has to take that very faint echo and decide what to do with it.
Let me back up for a moment. I want to tell you just a couple of things about that "Ping" of noise. First off it doesn't sound like the movies. It sounds more like an arthritic flea. It is a very faint clicking sound. Sometimes the clicking can be slow enough that you can hear the individual clicks, although they will be spaced very close together. Sometimes they are spaced so close toghether that you really can't hear them at all, the perception is more that you feel them. In some cases touching the transducer will actually give a sensation similar to a very mild AC electrical shock. The frequency of the ping is closley controlled by the design characteristics of the transducer's element. I can never remember if its the thickness or the diameter of the element that controlls the frequency but its one of the two, so it can be very closley controlled. The transducer can only 'hear' return echos of its specific frequency too, in case you were wondering.
The frequnecy's used by most recreational fish finders will be either 50 or 200 killohertz (kHz) with is to say it will vibrate either 50,000 or 200,000 times per second to make the sound. Your hearing range runs from somewhere close to 20 vibrations per second on up to something in the mid 20,000 range if I'm not mistaken. Each of the pings the transducer will make will last for somewhere between about 0.0015 and 0.003 seconds and the fish finder may make as many as 1,500 pings per second.
Back to color. Because the intensity of the return echo diminishes as the distance the sound had to travel increases at a consistant rate the display unit can be programmed to expect it to be of a certain loudness. Its a very easy calculation because if a sound is made with a certain loudness and it travels a known distance through water then the loudness left over after its travel will always be the same.
The signal the transducer sends back to the display unit from a returning echo will indicate how strong the echo was. The time it took tells the fish finder how far away it is so the display unit knows how strong the echo should be as well.
So now think of this. If the outgong sound hit a perfectly reflective target it would come bounching back as the strongest possible echo. On receiving the strongest possible echo the display unit would put a mark on the screen that was bright red. This is the indication of the strongest possible return echo - something like you might find from a flat rock, or a plate of steel, or the scales of a Tarpon.
Then just a gadzillionth of a second later it might receive a second return echo (one that hit something else besides the flat rock, plate of steel, or scales of a Tarpon) from the same outgoing sound ping but this one was only half as loud (the display unit would know this because the electrical signal sent up from the transducer would only be half as strong). Because this second echo came in a tiny fraction of a second later it must have been farther away, so the display unit paints a second dot on the screen a little lower but because this one was not as strong it might paint it a dot of blue - meaning it was a medium-strong echo. Echos like this come from rotted logs, trash in the water, and an awful lot of types of fish.
Then there might be a little tiny hint of an echo that also came back from that one original ping just a smidgen later but this one was so faint it was bairly percievable by the transducer. It could have come from a grass bottom or a small school of very tiny bait fish. This little weak signal woud cause the display unit to paint a green dot.
All of these little dots, each representing the time it took for an echo to get back (depth) and the strength of the return signal (color or grayscale of the dot) put together make one vertical line on the right hand side of the screen of the fish finder. Then the fish finder cycles again and does it all over. The existing line is reproduced one colum to the left and a new line is drawn on the right. The new line is made up of dots placed where they were because of a new string of calculations by the display unit.
Oh, I've picked common colors but some makers tend to vary the color scale a bit and some fish finders allow the user some latitude in color selection for various strengths of signal return.
So thats it in a nutshell. Its how fish finders work, in the most general of senses, and also how color works. Grayscale works just like color. They simply paint the target darker to indicate stronger returns and use light gray to indicate soft returns.
That help any?
Thom
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