# Conceptual question about longitudinal waves

by Saladsamurai
Tags: conceptual, longitudinal, waves
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 P: 3,016 So we are working on sound waves in my physics course now and I was doing some textbook reading. I have been following it pretty well, but I just came across a relationship that I am not quite following. It is with reference to wave interference. Let us say that two sound waves are emitted from two different point sources $$S_1$$ and $$S_2$$. The waves have the same wavelength $$\lambda$$ and are in phase at their sources. They take paths of lengths $$L_1$$ and $$L_2$$ and pass through point P. The text says that their phase difference $$\phi$$ is dependent on $$\Delta L=|L_1-L_2|$$ Thus to relate the variables $$\Delta L$$ and $$\phi$$ we can use the proportion: $$\frac{\phi}{2\pi}=\frac{\Delta L}{\lambda}$$ I know that I should see it, but I don't exactly follow this proportion. Could somebody ellaborate on this a little for me? I sure would appreciate, Casey
 HW Helper P: 4,124 Suppose the equation of both waves is: y = Acos(kx) (going along the direction the wave is travelling) The wavelength of this wave is 2*pi/k So at the point of interest, suppose wave 1 has travelled L1, and wave 2 has travelled L2: y1 = Acos(kL1) y2 = Acos(kL2) the phase of the first wave is kL1. the phase of the second is kL2. phase difference is: kL1 - kL2 = [2*pi/wavelength]*(L1 - L2) so from this we get the phase difference relationship.
P: 3,016
 Quote by learningphysics Suppose the equation of both waves is: y = Acos(kx) (going along the direction the wave is travelling) The wavelength of this wave is 2*pi/k So at the point of interest, suppose wave 1 has travelled L1, and wave 2 has travelled L2: y1 = Acos(kL1) y2 = Acos(kL2) the phase of the first wave is kL1. the phase of the second is kL2. phase difference is: kL1 - kL2 = [2*pi/wavelength]*(L1 - L2) so from this we get the phase difference relationship.

Ah. I see that now. Thanks LP. It makes even more sense now that I wrote out what you did^^^...the phase difference is $$\phi$$

Casey

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