The theory behind sound is very simple – waves are just added up, so the amplitude of two exactly identical waves is doubled, if one of them is shifted by pi radians they cancel and the amplitude becomes 0, if it’s shifted by less or more then a pattern is created that causes periodic pulses of sound. However, in everyday experience, this is practically nonexistent. If the “valleys” of sound waves subtract from the “peaks” of other sound waves, and if waves spend half of their time in a valley, I’d expect that exactly half of the time sounds would cancel each other, but I’ve literally never experienced anything other than more sounds = higher volume, not even in an orchestra with many people playing the same note. What gives?
In: Physics
Think of it this way:
If we had two identical waves both originating from same point in space (just to make it the simplest case possible) that start at random time, the chance of them destructively interfering each other perfectly is infinitesimal.
If we have two identical waves originating at random position in space but starting at the same time, the chance that you are at a position where they both arrive perfectly to interfere with each other is.. infinitesimal.
After that, we have to add amplitudes that need to be perfect and reflections from surfaces around that also are perfectly shaped.
And even after all that, you have one single point in space that this would happen. That is with two sine waves with single frequency. When we add frequency in the mix, you have several infinities stacked up against you to ever hear two waves cancel each other perfectly. Also: you have two ears which means two points in space that can hear that wave. You would always be a bit off, even in ideal case.
The sound you just heard, you will never hear that same sound again. Not even if you use same sound source, just the ambient noise alone would be enough to make this impossible but there are so many factors in the sound in acoustic environment that you can rest assured you will really never hear a full cancellation in any space at any time. Doesn’t mean it can’t happen but the definition of it would need to be set too, what accounts as full cancellation. In real life it will never happen, probabilities are stacked against you, 1:several different infinities..
NOTE: you can make a real life experiment that seems like it is cancelling perfectly. It isn’t, it is just cancelling very well. Our hearing is not perfect either.. Also noise cancellation is not absolute, there is a leak and there are distortion cause by the process: the signal you do hear is not the same signal that was in the sound source output. One more example how our hearing is not perfect is that you don’t notice those defects from noise cancellation. It is also possibly to get very, very close to perfect cancellation in electronics, the difference being that it is not acoustic environment but electric: no points in space but nodes that operate close to light speed. off topic: Friend of mine who designed a budget audio null tester recently if anyone is interested of getting one; there are not a lot of them in the market… Nulling or using cancellation to subtract just two signals as perfectly as possible is not easy even in the electronic realm.
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