# Does sound cancel itself out?

If there are two 10 x 2 x 1 foot rectangles in space and they are lined up so if they hit each other there will be no spots that are not hit in the front of the rectangle. Then they are pushed forward with the same exact same force, (this is space and there is no resistance) and they hit, will the tiny shockwaves of sound moving through them hit each other (they are exactly the same kind of sound wave), will the sound hit it's opposite would they cancel themselves into heat on contact?

I know that the sound does not cancel itself out. You cannot say the wave of atoms hitting each other collides with a wave that is exactly the same and stops both of them in a word. Can you?

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Waves pass through each other so even if the amplitude goes to zero they will pass each other by. – Brandon Enright Nov 15 '13 at 20:22
If you are thinking of shielding something from sound then this new sound cloaking system might help. dx.doi.org/10.1038/nature12608 – Jitter Nov 16 '13 at 9:08
If it's space (as in outer space / vacuum), what is there to make the sound? – Pranav Hosangadi Nov 18 '13 at 1:28
What exactly is a 10 x 2 x 1 foot "rectangle". And just what is pushing what, if it is space ,and there is no resistance. Do the experiment and measure the result. – user26165 Nov 18 '13 at 6:29
Sound can cancel out, but only in localised spots. If you take a sound editor, generate a sine wave in stereo, invert the phase of one of them, and play it through speakers, you can have some fun moving your head around and listening to the differences. – Davidmh Apr 15 '14 at 10:47

You can have destructive interference which means that sound waves "cancel out" at certain points. However, for all sounds to cancel out at all frequencies and in all places, the sound and its counterpart must have been created at the same time and place.

You can do an interesting experiment with a tuning fork: it has two arms that move in opposite directions. If you strike the tuning fork, then rotate it slowly, you will notice the sound amplitude getting weaker and stronger. This is a pretty good demonstration of interference - see http://hyperphysics.phy-astr.gsu.edu/hbase/sound/interf.html .

Active noise canceling headphones work on the same principle. They measure the incoming noise level, and actively produce "the opposite sound" to result in a near cancellation of external noise. These things work better at lower frequencies, where the electronics have time to respond to the sound. Higher frequencies happen too quickly to cancel them effectively - even a small time shift means that phase shift isn't exactly 180 degrees, and cancellation will be imperfect.

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I think you meant destructive instead of constructive interference (the latter would amplify the sound). – Benedikt May 28 '14 at 6:11
@benedikt thanks for catching my mistake. Fixed. – Floris May 28 '14 at 10:47

The collision of the blocks will cause them to vibrate (sound within the blocks). Since the blocks only contacted with each other momentarily (the blocks rebounded), there would have been a very short period of time for the vibrations to transfer from one block to the other (when the 2 blocks are touching). Since the vibrations are travelling fast (the blocks are denser than air), it is probable that vibrations transferred from one block to the other. So, yes there would be destructive interference and probably nodes of no vibrations.

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