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Homework Help: Loudness of a sound

  1. Jan 24, 2006 #1

    I have a quesion on sound terminology. When we talk about loudness of sound,are the terms 'sound intensity level' and 'sound pressure level' the same? Are these two terms indicate the loudness of sound?

    My tutorial asks to write the formula for measuring sound?? I know the formula for measuring the sound level or loudeness of the sound which is
    dB = 10 log 10 (I/Io) = 20 log 10 (P/Po). But measureing sound?????

    Thanks for any explanations...

    Following is the question in my tutorial:

    The loudness of sound is based on intensity level measured in decibels using a logarithmic scale and is relative to (a ratio of) the weakest sound the ear can hear

    Write the formula for measuring sound

  2. jcsd
  3. Jan 25, 2006 #2


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    Science Advisor

    Intensity and SPL are not the same thing as you have shown in your equation of intensity and SPL. Usually, SPL is what is measured and intensity is calculated from that. Not always, but usually.

    Since the question specifically asks for an equation based on intensity, then the dB = 10 log 10 (I/Io) is appropriate. However, the question is a bit misleading because loudness implies a human interaction with the sound. To be thorough, there are three weighting scales that are employed to help mimic the way humans hear, the A, B and C scales. Usually when someone mentions loudness measurements, the units are referred to as dBa, dBb or dBc referring to the weighted scale used.
  4. Jan 25, 2006 #3
    dB = 10 log 10 (I/Io) = 20 log 10 (P/Po).

    I wrote the aboove because, I is propotional to P^2 and hence

    log I/Io = log P^2/Po^2 = 2 log P/Po

    Don't these do equations give the same result even though they are measured or calculated differently?

    By the way I am not familier with a, b, c scale. I may have to read more on that.


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