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How a fish finder works

  1. Apr 3, 2007 #1

    I've just bought a fish finder and while playing around with it, it turns out that it isn't much good in shallow water. Now I'd guess it's using ultrasound as it can operate at depths up to several hundred feet and I presume therefore is getting far too much ground penetration at shallow depths and therefore (again I assume) a poor reading returned to the receiver.

    So my question is what sound frequencies would be reliable to locate fish and underwater objects at very shallow depths such as those in canals? I assume ultrasound is going too far for this? Surely I don't need to transmit in frequencies of 40kHz to pick-up fish that are in water only 3-6 feet deep?

    Does anyone know what kind of frequencies I should be using for this? And could I pick-them up with a sensitive microphone run through an op-amp?

    Thanks in advance for any help offered:wink:
  2. jcsd
  3. Apr 3, 2007 #2
    As you surely know, ultrasound o deep meters works like radar in air or sonar in water: they emit a short pulse and wait for the echo. The pulse must be formed by a least ten or twenty periods of the carrier. If you want to detect something in 3 feet of water, the ten periods must not span more than 1 or 2 cm. That is ten wavelengths must hold in, say, 2 cm. The speed of sound in water is about 1440 m/s. This wives a frequency of 720 KHz.

    You need ultrasonic transducers that work at those frequencies for sending and receiving the ultrasound. Usually the same transducer is used to send and receive. Maybe you can doctor a medical transducer used in medical echography. But they work at still higher frequencies.
  4. Apr 3, 2007 #3
    I see. Thanks alot for the reply :redface:

    It would appear then I'm talking through my ass. What I would actually need is a higher frequency not a smaller one. I'm also not familiar with the term 'carrier'. Is there anywhere I can go and have a good read about this stuff so I don't have to ask about every aspect of this stuff I'm not familiar with?

    Is there a decent Wiki page on it all, or would I need to purchase a text. I've just ordered Horowitz & Hill's book on electronics on Berkemans recommendation, maybe there is a similar text that could lay this out for me so I could research it privately?

    Thanks again for the help, just the sort of info I was after :wink:
  5. Apr 3, 2007 #4
    The "pulse" you send is, in reality 10 or 20 sinusoidal oscillations. This sinusoid is the "carrier". The detector amplifies de frequency of the carrier with a narrow pass band and the amplitude of the amplified signal determines the presence or absence of echo (fish, bottom, or body organ).
    To build this detector you must have some (solid) knowledge in electronics. You must have, if not electronic engineer level, at least electronic technician high level. I do not know if you can find electronic circuit plans for this device.
  6. Apr 3, 2007 #5

    Thanks for the reply. I have a strong technical background both educationally and professionally. I have never gone deep enough into electronics to learn how to build my own stuff which is why I have purchased the electronics text berkeman recommended. With some time and practice I'm sure I will be able to comprehend what is taking place. I wouldn't want a circuit diagram even if it existed as I'd rather take the time to learn and construct my own.

    I'm most grateful for the help this is really interesting stuff.
  7. Apr 3, 2007 #6

    Claude Bile

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    In shallow water, an ultrasound detector will pick up reflections from the bottom of the canal, pond or whatever body of water you are trying to locate fish in. These reflections will potentially overwhelm any signal you would get from a fish, or at the very least drastically cut down the range of the detector.

    There is therefore some depth limit for any given detection scheme.

  8. Apr 4, 2007 #7
    What say Claude Bile is true if you use a detector designed for deep waters. If the pulse length is short enough (as I tell Ader_Noir), if the repetition rate is low enough, to allow spurious reflections to dampen, and if the power emitted is no too high, there will be no problems. Do no forget that medical ultrasonography works within far shorter distances.
  9. Apr 4, 2007 #8
    Thanks to you both for the replies. This is an amazing subject. I understand alot better now what is going on and why so many anglers I know using the deeper water sensors are not picking up fish in water 6ft deep and less even when they know the fish are there.

    I presume it is expected that a only a portion of the sound waves will be reflected by the flesh of the fish and in some way the filter system in the receiver can tell this.

    All this of course assuming the transmitter frequency is well matched to the intended depth of water exactly as lpfr states :wink:

    Lpfr what nationality are you? Are you French by any chance?
  10. Apr 4, 2007 #9
    Yes, from Paris. Sorry, I know my English is far from perfect, but I hope my physics is better.

    Your detector will detect the bottom echo always. Sometimes it will detect an echo before the bottom one: it should be a fish or any other thing (alligator, shoe, etc.). There is no need to filter fish flesh reflections, but you will not detect immobile bottom dwelling fish.
  11. Apr 4, 2007 #10
    Ah merci monsieur, vous ete un bien amis a moi! (hope that's correct?)

    Thanks pal your physics and your methods of explaining it are superb. So do I take it the bottom echo is always present and the receiver will sometimes pick-up both the bottom echo and that of the fish/object simultaneously?

    Also when you say narrow-band pass I assume you mean the filter is scanning in the frequency range of the transmitted pulse only therefore removing any possiblity of interference. I would also imagine using frequencies of 700+kHz would mean very little possiblity of any naturally occurring sound causing noise/nuisance readings.

    I can't wait for that electronics book to arrive now. I went down to the canal at 0300hrs this morning with a million candle power searchlight and to my amazement I could actually see about 12inches into the murky water. Was very spooky indeed and I did see a few small roach on the surface.
  12. Apr 4, 2007 #11
    Yes the French and the physics are almost correct. If you use a pulse formed by several oscillations at 700 kHz, your filter will be a pass band which allows only frequencies near 700 kHz to pass, say 700 kHz +/- 5 kHz. This is, as you said, to eliminate other interferences. Anyway, at these frequencies, the background noise in water is very small.
  13. Apr 4, 2007 #12

    Outstanding. Thanks again pal :smile:
  14. Apr 4, 2007 #13
    I just found this on the Wiki page which is about Sonar. Done alot of reading but this is one of the most interesting things I've found:

    Sound waves travel differently through fish than through water because a fish's air-filled swim bladder has a different density than seawater.

    Can anyone tell me, would this change in the way sound travels through a fish be detectable to the receiver? I assume it must be because even the relatively cheap £60 fishfinder I have can tell me whether or not I'm over gravel or silt or clay and can thus detect a frequency change aswell as just measuring time delay. I assume that given that this is ultrasound we're talking about the return signal would have to be run through quite a sensitive filter which would then considerably magnify the results for them to become noticeably and discernably different and thus readable.

    How could I find out what kind of frequency changes to expect from silt and from the fish's flesh with ultrasound? Does anyone think I would be likely to get a reply from the American acoustic society if I e-mailed them?
    Last edited: Apr 4, 2007
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