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
As you said, sound waves cancel each other out if they are the same amplitude and pi radians (180 degrees) apart.
One way for this to happen is for two identical sources to be placed exactly wavelength/2 or (1/frequency)/2 apart from eachother.
Now the problem with instruments in orchestras (and real life sound sources in general) is that they do not emit 1 single frequency but rather a wide spectrum of frequencies, therefore there it is not possible for them to be placed that cancels out all the sound. Instead, for each possible relative positioning between two instruments only a very specific frequency gets negative interference.
It still is a very prevalent issue within the sound industry. Most sound mixers will have buttons on each channel to phase shift the signal by 90 degrees for this exact reason. Also when it comes to microphone positioning they are also very carefully placed as to avoid negative sound interference with surrounding instruments.
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