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.
Can you ask the question like I’m five? Because I have no idea what you’re talking about.
If you have a bunch of sound sources, some of which are constructively interfering and some which are destructively interfering, you aren’t going to hear the destructively interfering ones. You do hear constructive interference.
Sound waves vibrate very quickly. The chances that randomly all the sounds interfering are going to sum to zero for a long period of time without deliberate effort is practically nil.
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.
Apparently you’ve never experience comb filtering in a live concert application. It can be *very* apparent.
>If the “valleys” of sound waves subtract from the “peaks” of other sound waves
* They only effectively cancel if they are both the same frequency, same amplitude, and arrive at your ear at precisely the same instant.
* When sound waves hit a thing, be it a person, a wall, moisture in the air etc, some of them get absorbed and some get reflected, sometimes at a different angle.
* Because of that, it’s very uncommon for sound waves to actually cancel them selves out.
* It does, however, happen in some circumstances usual with very loud sounds in a very symmetric space, like a rectangular building during a concert.
* Audio engineers at live concerts have to be keenly aware of how this cancellation can effect the sound the concert goers hear and make sure they compensate for it (usually with more speakers pointed in specific directions).
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