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Question about sound waves - lengths and related

  1. Jul 20, 2011 #1
    Hello everybody,

    I'd really like to be able to explain what I hear in terms of wavelengths. My question could be split into a few, but I will try to focus on the main issue so it's more simple to answer.

    Situations in real life:
    It comes to my mind when I'm in live shows, the loud ones in closed halls, where you get plenty of feedback from the walls and everything else. It continues with putting a set of headphones aside me while it's playing, and me hearing only the high frequency cymbals. Another case is hearing a boomy music leaking out of a car at the street - I think I hear mainly the lower frequencies in that case. At the end I couldn't reach a definite conclusion as to what one should hear louder - highs or lows - at a fair distance from the source. And this led me to posting this question.

    So yeah I'm basically asking you these questions:
    1) Once distanced from a soundsource, what frequencies should I hear louder ? How would you (please) explain it in terms of wavelengths ?
    2) At live shows, what frequencies should I expect getting mostly returned from the walls back to my ears ?

    Thanks a lot


    P.S. In case I posted this at the wrong forum - please, let me know and if possible also refer me to the right one. Thanks for that.
  2. jcsd
  3. Jul 21, 2011 #2
    I think the correct word to describe it is the transfer function, the ratio of input to output power over the entire spectrum. You see them most often on graphic equalisers where you can isolate and amplify specific frequencies, but it happens in everyday life as certain frequencies are suppressed (by a certain material being in the way) or amplified (by resonance, for example). Take your car stereo example, the sound waves must travel through the body of the car, which is a sheet of metal, by vibrating that metal. Due to its constituents, thickness and shape, that metal is going to have a tendency to vibrate at certain frequencies more than others, so those frequencies are the ones which you hear more of outside the car. In short, everything around you affects it.

    Ideally you want the transfer function to be flat, which ensures you hear the music in exactly the same way the artist heard it when they made the master in the recording studio, but it's almost never the case unless you're willing to spend a lot of money.
  4. Jul 21, 2011 #3
    Thank you. So basically what you're saying is that all frequencies in the spectrum can be either suppressed or amplified, depending on the material they're travelling through ? Which means if I wanted to know what resonance I have in a specific case, prior to being at it, I'll have to read about the materials and check their resonance tendencies.

    Do I get you or am I missing something ?
  5. Jul 21, 2011 #4
    Stokes' law

    http://en.wikipedia.org/wiki/Stokes%27_law_(sound_attenuation)" [Broken]
    Sound traveling through a Newtonian fluid (i.e. water, air, metal, but not Jello) attenuates by the square of the frequency. 1000 Hz attenuates four time as much over a distance than 500 Hz. Which is why elephants can communicate over miles, but sonar using bats can only "see" a short distance. Public address systems will have a deeper pitch from at a distance then close up.
    Last edited by a moderator: May 5, 2017
  6. Jul 21, 2011 #5
    Not only what they're travelling through, you can be 1 m away from a speaker and the shape of the entire room will affect what you hear due to reflections.

    If you wanted to do the calculation I'd say it would be very very complicated in all but the simplest of cases.
  7. Jul 21, 2011 #6
    Yeah, it was just in theory.

    I actually always thought it's related to the wavelengths in a way that some wavelengths would always 'last' longer when reflected, than other waves. Now I'm to understand it's related to the specific 'equalization' each material have. So that if I played music in two identical halls, made of different materials, the most prominent frequencies will be different in each case. There's no rule saying I will always hear highs more, or lows more. If I'm wrong anywhere here, please correct me.

    Thank you very much.
  8. Jul 21, 2011 #7
    Look up comb filter on wikipedia

    The volume that the sound is traveling through acts like a comb filter each time an echo is reflected back to the listening point. The time delays create constructive and destructive interference pattern on the spectrum of the sound you're hearing which increases and decreases certain frequencies.

    This all depends on the position you are in the volume, and so the frequencies affected are dependent on source and destination positions.
  9. Jul 21, 2011 #8
    Will do, sounds so interesting. Thank you all guys.
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