I'm studying Biophysics and my current subject is sound. One of the properties of sound is intensity. From my notes I can see the following definition:

Intensity Formula is: $(I = w*m^{-2})$ or $(I = \frac{w}{m^2})$ where w = amound of energy and m = area.

Definition: Intensity is the amount of energy passing through an area of $1m^2$ perpendicular to the direction of sound wave propagation within 1 second.

So I came with this picture: enter image description here

I know the picture is lame. What I care about is if the above definition is diplayed correctly here. I'm not a native English speaker and the word perpendicular in this context confuses me.


  • 1
    $\begingroup$ They mean how much energy is crossing the surface, so that if the surface is tilted so that it is parallel to the sound propagation direction, there is no flux of energy through. This is adressed here on questions on fluid flux, which is the same idea, and the flux of any vector field in general. The vector field here is the energy flow. $\endgroup$ – Ron Maimon Aug 28 '12 at 10:56
  • $\begingroup$ I can't get your picture. Why there are three objects on it? There should be just two --- the area (a surface) and the sound energy flowing through it (an arrow). $\endgroup$ – Yrogirg Aug 28 '12 at 11:16
  • $\begingroup$ The 2nd object (black) is there to show the perpendicular (vertical) dimension to the sound wave direction. Maybe though, it's not as important as it seemd to me, still don't quite get it though. $\endgroup$ – atmosx Aug 28 '12 at 11:29
  • $\begingroup$ Re-reading both answers I think I undersood the definition. tx $\endgroup$ – atmosx Aug 28 '12 at 11:31

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