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Ultrasonic distance measurement and ghost echoes

  1. Jan 16, 2008 #1
    hi everyone,
    while doing a project on ultrasonic distance measurement, i came across the phenomenon of ghost echoes. The ultrasonic waves are emitted and made to reflect on a surface( of which the distance to the source is to be found). This reflected waves are detected, and time for b/w the the instant of emission, reception is measured. A major problem arising is the phenomenon of ghost echoes. The refelcted wave detected, may not be that of the object, but it may be of another object which is placed further off.
    i.e..
    say at t=0, we emit.
    at t=1, we detect reflected wave from the object of interest.
    delay measured = 1s
    at t=2, we again emit
    at t=2.4, we receive reflected wave
    delay measured = .4s

    But the second reflected ray which we measure is due to another object(farther off) which has reflected the ray emitted at t=0.


    Now my question is, why doesnt the same phenomenon happen in radios? i.e. how is the music reception continuous in a radio. The radio receiver is ( ,say fm 93Mhz) is tuned to 93 Mhz. Now the receiver filters all the signal waves( including the delayed ones) and plays it. Shouldnt the output be scrambled, since the all the signals( including the delayed ones) are filtered, and played.
     
  2. jcsd
  3. Jan 16, 2008 #2

    mgb_phys

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    In broadcast it's called multi-path.
    The echoes are quite a bit weaker, and come so quickly after the main signal that your ear/brain are naturally very good at ignoring them - just like how you can have a conversation in a party with one person in spite of all the background noise.
    You can see the effect easily on a tv with old "rabbit ear" antennae where you get a ghost image. If you work out the scan speed of the TV you can even calcualte how far the extra path is.
     
  4. Jan 16, 2008 #3
    But in a radio, wont the delayed and present signal interfere, and superpose to give a new signal?
     
  5. Jan 16, 2008 #4

    mgb_phys

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    Yes but the radio wave is travelleing at 300,000,000 m/s so if the wave does an extra 300m it will be delayed by only 1us which you wouldn't here.
    To have a chance of hearing a change it would have to be delayed by a few ms so have to bounce of an object several hundred km away - which would be too weak a signal to hear.
    The stuff I said about hearing one conversation is more appropriate to when you pick up a faint background stations at the same frequency - sorry for the confusion.

    On a TV a single line takes around 60us to scan, so a cm on the screen represents about 1us - which you can easily detect as a shadow.
     
  6. Jan 16, 2008 #5

    chroot

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    Multi-path interference is not generally a problem for things like broadcast FM radio, because those frequencies are pretty much only good for line-of-sight transmission -- any reflected or refracted signals end up being so weak your ear can't hear them.

    On the other hand, multi-path is a major problem for portable digital radios, which transmit millions of symbols per second. Multi-path is one form of inter-symbol interference, and an enormous amount of effort is required to mitigate it. Many systems these days use a solution called "diversity," which means using more than one antenna (spatial diversity) or repeating the same signal several times (temporal diversity). XM and Sirius satellite radio, for example, both use temporal diversity, simultaneously transmitting two copies of their data stream offset from each other by a small time delay.

    - Warren
     
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