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Homework Help: Easy intensity question

  1. Oct 19, 2007 #1
    I have been given this question: What is the sound intensity 2 metres from a speaker which is producing a sound at 100 watts of power?

    I know the equation is I=P/A but i dont know if "A" stands for amplitude or area. Some books/sites say amplitude and some say area. But if it is area...I dont actually have an area...just a distance. So...what do I do??

    Thanks everyone for your help in advanced!

  2. jcsd
  3. Oct 19, 2007 #2
    Sometimes one assumes the speaker is a source for sound waves that travel radially outward *in all directions*. Then you can visualize a spherical shell of the advancing sound waves moving outward; the distance from the speaker is the radius of this sphere, whose area you can now determine (see your texts). If you instead consider the speaker as a source of sound that only moves in a hemisphere outward, then you'll want half the previous area (pretending that no sound goes behind the speaker, only in front). Intensity is a "density" of sound, as the finite amount of energy in the waves spreads out over a larger and larger area...I=Power/Area.
  4. Oct 19, 2007 #3
    ohhhhh so I would use Pie X R^2 with r as 2m to find the area!!!
    omg thank you soooo much!!!

  5. Oct 19, 2007 #4
    actually, not quite... that's the formula for the area of a circle, yes??
    Bear in mind that what you actually have is a hemisphere/sphere.
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