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Ultrasonic Range Meter

Discussion in 'Electronic Basics' started by roots_of_culture, Mar 27, 2006.

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  1. Ban

    Ban Guest

    Can I say my 2 cents:
    atmospheric absorption at 40kHz is a whopping 150dB/100m (20C 70%H) at nasty
    10kHz it would be only 18dB. So for larger distances go down with your
    frequency.
    Also your calculation seems off. 100m means -80dB (inverse square law). Then
    there is absorption/diffusion at the reflector, let that be another 20dB.
    This makes it -100dB. Your receiver needs 94-75= +19dB SPL, you have only
    108 - 250 = -142dB available. Forget it.
    You can concentrate your beam with an old satelite dish (not the offset
    ones) by mounting the transceiver in there. This would maybe improve the
    ratio by 30dB. But then any temperature gradients or air movement would
    carry your beam away.
    Figures are from "Sound System Engineering" by Davis&Davis, ISBN
    0-240-80305-1
     
  2. Martin

    Martin Guest

    Thanks Ban. I want to point out that you were thinking 100m instead of
    50m. :D I'm using that application note from senscomp (link in my
    previous post) and the calculations seem right. SPL reduction at 50m is
    20*log(30/5000) ~ -45dB.
    There's one thing that was confusing though. According to senscomp's AN
    and http://www.earthworksaudio.com/tech/hf_sound.pdf air absorption is
    about 15dB for 50m. However, according to numerous atmospheric
    absorption (based on ISO 9613-1) calculators, this value should be
    about 1.2 - 1.3dB/m (about the same with your value 150dB/100m) for a
    40kHz signal at 20 degrees C and 70%.
    I suppose ISO 9613 must be right. So 50m seems a bride too far here.
     
  3. Martin

    Martin Guest

    Thanks Ban. I want to point out that you were thinking 100m instead of
    50m. :D I'm using that application note from senscomp (link in my
    previous post) and the calculations seem right. SPL reduction at 50m is
    20*log(30/5000) ~ -45dB.
    There's one thing that was confusing though. According to senscomp's AN
    and http://www.earthworksaudio.com/tech/hf_sound.pdf air absorption is
    about 15dB for 50m. However, according to numerous atmospheric
    absorption (based on ISO 9613-1) calculators, this value should be
    about 1.2 - 1.3dB/m (about the same with your value 150dB/100m) for a
    40kHz signal at 20 degrees C and 70%.
    I suppose ISO 9613 must be right. So 50m seems a bridge too far here.
     
  4. Ban

    Ban Guest

    That guy is an idiot, for 1kHz it is 0.7dB/100m, it is also not the frequncy
    sqared, but has two plateaus, where the nitrogen(0.6dB/100m) and
    oxygen(300dB/100m) levels are, which are then constant for all higher freq
    in this gas alone.
     
  5. Rich Grise

    Rich Grise Guest

    I wouldn't even bother to try ultrasonics beyond about maybe 3M (10 feet),
    for a whole bunch of reasons. But, laser rangefinders for maybe 10' and
    farther are available off-the-shelf:
    http://www.google.com/search?q=laser+rangefinder

    So I'd look at the minimum range on the lasers, and do the ultrasonics
    for short-range sensors. ;-) (also pretty much off-the-shelf.)

    Who's Chris?

    Good Luck!
    Rich
     
  6. Just as a note: Radio Shack has an off-the-shelf ultrasonic unit that
    claims 3cm to 300cm (3m) range. Cost is under US$15 until some time
    in June (it's on sale -- half price.) It's a unit for the Vex robot
    kit thing.

    Jon
     
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