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Homework Help: Biophysics problem - Acoustics and Sound

  1. Oct 30, 2007 #1
    Hey guys,

    I need some help on this problem from my biophysics homework on the nature of acoustics and sound. Readers beware....lol

    1. The problem statement, all variables and given/known data
    If all the sound energy of a 5MHz US continuous emitter with an intensity of one watt per square centimeter is deposited into an isolated cubic centimeter of tissue, with specific heat of 1.2 calories per gram degree and sound velocity of 1.54m/sec, how much would the tissue rise in temperature after 20 seconds?

    2. Relevant equations
    This first equation is the only one from my notes that relates the velocity of sound to temperature and specific heat (perhaps under ideal conditions):

    vs = (((gamma)RT)/(M))^(1/2),

    where vs = velocity of sound, R is the gas constant 8.31 J/mol-K or 1.987 cal/mol-K, T is the temperature, M is the molecular weight (I'm not sure how to calculate this in the problem) and gamma is the ratio of the specific heat at constant pressure to the specific heat at constant volume.

    3. The attempt at a solution

    To solve for the rise in temperature, isolate T in the above expression for vs =>
    (vs^2)(M/(gamma * R)) = T.
    Of course, we still need to find M and determine how to incorporate a parameter for time, t in our calculation of the temperature rise....
  2. jcsd
  3. Oct 31, 2007 #2


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    Homework Helper

    don't know about biophysics... but is the energy entering through the square surface of the cube of tissue... and the energy has intensity 1W/cm^2?

    So the power entering the tissue is (1W/cm^2)*(1cm^2) = 1W = 1J/s?

    So in 20s... 20J enters... convert to calories, and use specific heat to find the temperature change...

    I suspect this is all wrong since much of the information given hasn't been used... also not sure if the energy enters through a square surface... I just assumed.
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