Inverse phase sound wave where does energy go?

[AlienFarmer]

2 sound waves that are mathematical polarities cancel each other out according to my audio engineering book. I thought energy cannot be destroyed, just changed. Am I wrong? What happens to the energy? Same question could be applied to matter and anti matter right?

 

[Dale]

2 sound waves that are mathematical polarities cancel each other out according to my audio engineering book. I thought energy cannot be destroyed, just changed. Am I wrong? What happens to the energy?

Two opposite polarity waves only cancel out at special points, such as the point exactly halfway in between the speakers. At other locations (e.g. 1/2 a wavelength closer to one speaker) they don't cancel out, they add! That is where the energy goes.

 

[berkeman]

Two opposite polarity waves only cancel out at special points, such as the point exactly halfway in between the speakers. At other locations (e.g. 1/2 a wavelength closer to one speaker) they don't cancel out, they add! That is where the energy goes.

How about if we take it closer to a limit where the cancellation happens over several wavelengths of distance? Say we have two speakers located side-by-side pointing in the same direction, and driving a sound sine wave out of phase. On their axis it would seem that they would cancel out pretty well for some distance. How does the energy make it across that distance?

I know that for waves on a string where there are nodes, the energy is transferred by the tension in the string, and not by the displacement. Is it something similar with sound wave cancellation over s distance?

 

[NFuller]

How about if we take it closer to a limit where the cancellation happens over several wavelengths of distance? Say we have two speakers located side-by-side pointing in the same direction, and driving a sound sine wave out of phase. On their axis it would seem that they would cancel out pretty well for some distance. How does the energy make it across that distance?

Dale's point should still hold here. There will be alternating points throughout the space in front of the speakers where the waves interfere constructively and destructively. Adjusting the phase of one of the speaker outputs will just shift the interference pattern over by some distance.

I should also point out that these nodes are not stationary but will travel forward from the speakers carrying the energy along with them.

 

[jartsa]

How about if we take it closer to a limit where the cancellation happens over several wavelengths of distance? Say we have two speakers located side-by-side pointing in the same direction, and driving a sound sine wave out of phase. On their axis it would seem that they would cancel out pretty well for some distance. How does the energy make it across that distance?

I know that for waves on a string where there are nodes, the energy is transferred by the tension in the string, and not by the displacement. Is it something similar with sound wave cancellation over s distance?

If two speakers send an identical short pulse of sound onto a paper wall from opposite sides of the wall, the sounds get reflected from the wall, right? (Because the wall does not move, it's equivalent to a lead wall) The reflected sound is identical to the sound that would be there if only the other speaker produced a sound.

What happens if the sounds are otherwise identical, but there is 180 degree phase difference?

I think that there is a sound that could either be a reflection from the paper with a 180 degrees phase shift, or it could be a sound that came through the wall.

Now let us align the speakers with the wall. If the wall reflects sound waves to the right, the energy of the sound waves gets reflected to the right.

 

[Dale]

How does the energy make it across that distance?

I have not actually run these computations, but it may be in fact that energy does not cross that plane. The energy flow may be completely tangential on that plane.