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Energy of different frequencies of sound

  1. Jul 7, 2008 #1
    Take a two sound waves: for example let us spread them at both poles of the human hearing spectrum. The low frequency at 20Hz and the high at 20,000Hz.

    Which sound wave carries with it more energy?

    It is said that sound travels at 343m/s, is that true of all sound in the air medium, meaning will the low frequency wave arive a location X before the high frequency one?

    This question stems from a recording I did with a microphone, I placed the microphone in my pocket at waist level, upon a later listening to it recording, it show that the higher frequency are about 5-10dB lower than the low freqs, something that was not apparent during the recording while I was listening at ear level with my ears. (at ear lever there was a clear path from the speaker to my ear, and at the recorder lever there where a bunch of sweaty bodies obstructing the way.)

    What can be attributed to this phenomenon? What effect is the mass of bodies having on the propagation of the sound?

  2. jcsd
  3. Jul 7, 2008 #2
    hey stuckie,
    all soundwaves in the same medium travel at the same speed which is not necesarily 343m/s. Speed of sound(SS) in air at 20 degree celcius is expressed as SS= 331.5m/s*sqrt((20celcius +273.15K)/273.15K). "sqrt means the square root". From this equation u can say that speed of sound in air at 20 degrees celcius is 343m/s. This can change by changing the temperature in the equation but it must remain in celcius.
    A change in the frequency has nothing to do with the speed of the soundwave. It remains the same providing that nothing else changes.
    Also, the higher the frequency the more the energy of the wave.

    Now for the second part, you said that the higher frequency was 5-10db lower than the lower frequency as played back on the recorder and that you did not detect a difference with your own ears.

    ****Loudness(db) is measured by amplitude and not frequency******

    1)Ok, for this part i am not too sure by im suggesting that because the displacement in a medium of a high frequency wave is more than a lower frequency wave, there must be more energy lost from the higher frequency. The energy loss could be characterized as formation of heat. With that energy lost, the overall amplitude of the higher frequency wave would have decreased a lot more than in the lower frequency wave. A lower amplitude results in a lower audibility measured as decibels.

    2)My second possible explanation is all about resonance. The material that acts as the eardrum of the recorder could provide a better resonance for a lower frequency than a higher frequency. With the lower frequency the "drum" would be able to displace more making a high amplitude. But the higher frequency would not allow the "drum" to displace fully therefore making the amplitude sound lower than the original.

    3)Ok, it could also be both explanation 1 and 2 acting together creating such an effect.

    feel free to critize my theory if you do not agree. Hope it was helpful
  4. Jul 7, 2008 #3
    your second point is relevant, I guess I failed to mention that I have previously recorded sounds on the recorder with a pretty flat response. So we can cancel out the internal microphone pick up as the source of the problem.

    Thank you for the reply.

    Further is there an equation that correlates sound frequency to power, I was browsing around and was unable to find one, or perhaps I don't and the right understanding of the subject matter....yet :)

  5. Jul 7, 2008 #4
    well, i dont know too much about sound power.
    Sound power is expressed in decibel "db"(shown by the amplitude of the wave). It is the measure of loudness of a sound. I really do not think that there is a correlation between frequency and power. think about it, sound "power" better called amplitude is only measured by the amount of constructive or distructive interferences. I beleive that any given frequency can have an infinite amount of amplitudes if the sources are available. Once more, i do not beleive frequency and amplitude are corrolated at all.

    if you want more information, you schould read about sound interferences. That should give you a better understanding. if you do not understand what you read, you can always ask me :)
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