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Homework Help: Sound intensity distance ratio

  1. Dec 9, 2009 #1
    1. The problem statement, all variables and given/known data

    Harry and Sally are sitting on opposite sides of a circus tent when and elephant trumpets a loud blast. If Harry experiences a sound intensity level of 65dB and Sally only 55dB, what is the ratio of the distance between Sally and the elephant to the distance between the elephant and Harry?

    2. Relevant equations

    I=P/(4piR^2)
    B=10log(I/10^-12)

    3. The attempt at a solution

    65=10log(I/10^-12)
    so I(harry)=10^-5.5

    55=10log(I/10^-12)
    so I(sally)=10^-6.5

    *I have no idea where to go from here or if that was even the correct first step to take.

    If I knew the Power(P), I would be able to find the radius and get distance from that, but I don't have a way to calculate that.
     
  2. jcsd
  3. Dec 9, 2009 #2
    You don't have to know the power; it remains the same for both Harry and Sally ie dependent only on the source.
    Thus we can simply obtain the relation of Intensity being inversely proportional to the square of the distance from the source. It is then a matter of using ratios to solve the problem.
     
  4. Dec 9, 2009 #3
    I'm not very good with ratios. Let me know if I did this right.

    65=1/R^2 so R=.124

    55=1/R^2 so R=.134

    ratio is .124:.134 ????
     
  5. Dec 9, 2009 #4
    You shouldn't equate like that and solve for 'R'; its not mathematically correct. Also, are you sure you are supposed to use the dB value?

    [tex]\frac{I_{Sally}}{I_{Henry}} = \frac{r_{Henry}^{2}}{r_{Sally}^{2}}[/tex]
     
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