Neutralization of Sound

Yes there are lots of products that do that. They are good for blunting the noise, but it's hard to get it to work extremely well as the 2 noise sources are generally offset from each other. Especially at loud volumes.

Suppose there is 100% destructive interference, where do the energies contained in the two waves go to? Do they heat the room, or what?

I would have to say that it is more of a canceling of vectors then as a dissipation of energies.
If I had to make a guess I'd say that they are transferred as mechanical stress to all particles affected as the try to make it behave in diametrically opposite directions.

No loss of energy. Besides, there will be nodes and anti-nodes. So, you can also get locations where the sound is reinforced.

100% is a pretty theoretical number since sound will be effected by its environment and there will never be a perfect environment.

What if I place 2 identical speakers, one at each end of a pipe of length an integer number of wavelengths (say, single frequency). If I drive them precisely out of phase, there should be perfect destructive interference inside the pipe. Where does that energy go?

I would like to add to your & other participients comments:

In atmosphere:
Sound can be neutralized at a particle point not in the whole vicinity.

At a point in Electronic circuits? partly yes!
Reason phase-shift of signals in the circuits due to different propagation delays to different frequencies, so only one particular freqcy can be.

ways to cancell:
in environent [only at a point]:
connnect 2 speakers at opposite locations, pointing to the point, with one speaker's connection inverted.

In circuits:
feed the original signal through an inverter to the point where non-inverted signal is fed.
Keep in mind to add as much delay in the original signal as the inverter adds.
Amplitudes of both the signals be equal

Actually it's possible in theory to null a wide range of frequencies. Suppose a noise source (say a fan) must transmit its noise down a tube. At the tube's exit to the atmosphere, you place a speaker and feed it with digitally delayed inverted signal picked up by a mike at the fan. I say "in theory" because practice is harder than it looks, and tends to work well only at low frequencies. But it is a well known and within limits a successful technology.

simple example for this kind of stuff, day to day life equipment is the automobile silencer-mufler. there the same technique is used to reduce the noice.

What you have said here is passive noise cancellation by dampners and acoustics. That is different from applying a sound in the opposite direction.

Hi! What we are trying to discuss is "Active Noise Cancellation" techniques. Here we super-pose a waveform equivalent to the sound produced by the source in the opposite phase but equal in energy. this cancells the effect produced by the source sound. The problem faced here in Active cancellation is the orientation of sound. This can be gauranteed that if the phase differences are maintained exact and the timing is immaculate then the no noise or sound from source shall be heard beyond the cancelling device(speaker but the sound propogating in the other directions need to be accounted for. This can be done by including an enclosure in the design to direct the sound in a particular diection and cancelling it. Moreover most machines and other devices produce the bulk of the sound noise in a few confined directions, making the design more simple.

Linear thinking people. Sound cannot be cancelled out. The waves , when they contact each other would deflect, not just stop. In a perfect world (non-existant) they would impact and generate heat

Dazed and confused....don't be! Waves deflect off of objects, not other waves. You have been watching too many superhero cartoon shows!

It is all a matter of timing (isn't everything?). When two waves of equal amplitude and frequency meet they reinforce each other (doubled in amplitude, higher crest, lower trough) if they are in phase, i.e. if they started out at exactly the same time. However, if two identical waves meet, where one has been inverted or delayed by one half of the period (same thing, out of phase by 180 degrees), then they most certainly do cancel each other out. If the waves were on water, the crest of one wave would exactly fill in the trough of the other wave, giving a perfectly smooth and level surface, therefore cancelling out the wave. The wave is nulled out at that point. Of course we know that energy cannot be created or destroyed in the practical world, and so the sound energy is indeed converted to heat, but in most cases the heat created is negligble and dissipates very quickly.

Does this happen at every point where the waves meet? No, only when they travel the exact same distance or where the number of wavelengths travelled is a whole number. For example, at one-half wavelength difference these two oppositely phased waves would once again be IN phase, and therefore reinforce each other, so timing AND location is critical. At one or more full wavelength differences they would once again cancel out. You basically create a whole array of nodes and antinodes, or nulls and anti-nulls if you will.

I don't think aiming the canceling signal at the source of the sound to be cancelled is the idea. If you align the cenceling signal with the signal to be canelled you'd get a broder range of silence. SOURCE )))))))) SILENCE CANCELING ((((( at least from a listing stand point. I don't believe you actually eliminate the original sound wave. You can't double the engergy in a system and expect a zero sum. It is like introducing a push where there is a pull, and vise versa, to nutralize the over all effect. Because of this I don't think heat would result either, because you are eliminating motion/friction at least in the surfaces. So, what does happen to the energy? Does it gets much noisier if you are not in the sweet zone? Cool subject!