Monday, 4 May 2020

acoustics - Do particles in a sound wave ever move transversally?


I'm trying to visualize how sound waves work and I was curious about something.


So sound moves in longitudinal waves, which I think I understand.



There is a really good khanacademy video explaining this.


In the video the air particles are depicted as moving from left to right (longitudinally). But do they ever move up and down even a little bit? Are they on some kind of fixed axis where they can only move longitudinally and never move even the slightest bit in a vertical direction?



Answer



If you have any kind of solid material, it will become a little bit thicker as you compress it, and thinner as your stretch it. This means that a "one dimensional" wave traveling longitudinally down a rod will in fact cause some lateral motion. The ratio of displacements in the perpendicular direction is obtained from the strain (relative displacement of adjacent particles - how much local deformation there is) and the Poisson ratio of the material (a value between -1 and +0.5) that describes how much longitudinal strain turns into lateral strain.


So yes, it does move "the slightest bit".


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