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Using sound for estimations

  1. Feb 5, 2012 #1
    first thread, sorry if its in the wrong place.
    I cant find anything about this but it seems logical none the less.
    I was watching something where a noise was heard and then a plane flew by.

    Assuming it was a train, to make it easier,
    and you hear a train whistle and your on the track you know to get off, but is there a way of calculating the time needed to get off.
    if the train is travelling at 20m/s and its a straight even line, with no wind etc, is there some sort of calculation of relativity between the sound of the train hitting you and the train's distance/time it will reach you?
    or if you hear a noise of something moving in your direction can you tell its distance from you at that point?

    I hope there is a way because that could be really useful if applied to the right areas
  2. jcsd
  3. Feb 6, 2012 #2
    yah shure.... sound travels at 340.29 m/s in "normal" air. That speed changes a bit depending on temp, pressure, and moisture but you can guestimate distances using 1000 feet per second. Say you start counting when you see a lightning flash and stop when you hear the thunder, if you got 5 seconds the lightning was about a mile away.

    You then need to subtract the time the train is going to take to get to you. But by the time you've done all that, you probably should just stay off the tracks...
  4. Feb 6, 2012 #3
    For the case of the train, just listening to the sound will not tell how far is the train. At least not right away.
    Someone leaving by the train tracks may learn to appreciate the distance from the loudness of the sound, assuming that the whistle blows always with the same power.
    As the sound grows louder the train is closer.

    Measuring the Doppler shift may provide the speed of the train.
  5. Feb 6, 2012 #4
    cheers guys,
    wasnt sure if id be laughed at, only a first year engineering student.
    i get the lightning storm idea, thats a good one and ive practised that one a few times, and used trig to measure its height too,
    but if the object is moving, and an estimate of speed, would you need a starting speed or point to calculate its ETA?
    there has to be a solution to something like that
  6. Feb 6, 2012 #5


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    No, for sound you need to know the time of flight in order to calculate the distance of the source. So the flash of light in the lightning case gives you a start-of-flight indication, and you hearing the thunder gives you the end-of-flight time.

    But in the train case, if you can see the train to see when its whistle starts (assuming a steam whistle), then you can see the train to estimate the distance anyway.
  7. Feb 6, 2012 #6


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    Gold Member

    soundsgood, Welcome to Physics Forums!

    No, you cannot know the arrival time of the train just by passively listening to its sound. To know how much time you have before the train will impact you, you need to know its distance from you and its closing velocity.

    In active sonar systems a “ping” of known frequency is transmitted. That pulse travels out to the target (train), and the reflected echo comes back and is received (detected). By measuring the round-trip time and knowing the speed of sound we calculate the range. Then we measure the change in frequency between our transmitted pulse and received pulse. For a closing target like your train experiment, the received frequency of the pulse would be higher than the transmitted one. This is called “up doppler”. This frequency difference is proportional to relative (closing) velocity between you and the train. Knowing the closing velocity and the range we then could predict the exact arrival time of the train.

  8. Feb 7, 2012 #7
    i guess if you somehow happened to know the sound intensity that you are hearing, i know theres the iphone app, you could work out the distance, and from there that would give you a starting point and given a fixed speed, would then calculate how long it will take to get to you,

    but that the best i can think of for that, though not entirely accurate as the speed is a guess, and i was hoping to arrive at a conclusion purely based on thinking without measuring tools
  9. Feb 7, 2012 #8


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    An app might tell you the intensity of the sound reaching you but that tells you virtually nothing about how far away the source of the sound is unless you had the intensity of the sound calibrated at the source (i.e. you have to know how loud a train whistle is).

    Note that, for this to work even in principle, this would require all train whistles to emit their sound at a fixed intensity.
  10. Feb 8, 2012 #9


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    Too many variables. All you have is a sound, and know nothing of distance or velocity. You can't determine ANYTHING from the sound unless you know EXACTLY what the sound consists of. Then you can determine the relative velocity from the frequency, and then, making assumptions about attenuation you can guess at the distance (but that is affected by altitude, temperature, relative humidity and wind).

    For the lightning example you know speed of light and the speed of sound so the difference tells you the distance. If the train flashed a light at the same time as it whistled, then you would have the same information.
  11. Feb 8, 2012 #10
    cheers guys,
    thanks for the help
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