When I walk down a city street, there are tons of sound waves assaulting my ears.
How does the brain sort out the soundwaves emitted from a police siren vs. the soundwaves from a honking horn?
What happens when the police siren soundwaves collide with the honking horn soundwaves? Don’t they tangle together as they travel through the air on their way to the ears?
Is it a physical property of soundwaves to stay discreet or is it a function of the brain’s processing to untangle them before making sense of them?
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Sound waves are self-propagating physical waves in the medium.
What happens is: the thing generating a sound physically forces air to move either away or towards it (for example by being something that’s physically vibrating). Let’s say we’re talking about a speaker where there’s a cone-shaped piece of cardboard that is physically moving outwards. As it moves outwards it runs into air molecules and imparts them with some additional energy / momentum. On average, they are forced to move away from the speaker – but then they run into more air molecules, and transfer that momentum to the molecules they just hit. Those molecules hit additional molecules that are even further away, and so on.
Therefore this motion propagates outward. (The same thing happens when the speaker cone is moving “in”, away from the surrounding air, because the molecules aren’t running into something where they “should” run into it, but that’s maybe harder to understand conceptually, so let’s focus on the positive pressure case.)
The thing is, for ordinary sound waves, the magnitude of the motion of the air that is pushed by the speaker cone is relatively small compared to the typical random bouncing around. The reason there is a sound wave is that the motion is *coherent* – that is, there’s a specific “front” or layer of air molecules that’s been shoved out by the speaker.
However, because the magnitude of the motion caused by the speaker (or other sound source) is small relative to the random motion of the air molecules, any two sound waves only meaningfully interact with each other in locations where they happen to be acting on the exact same molecules at the exact same time. This is interference, and it’s real – it’s how noise canceling headsets work, for example – but for sources of sound where there isn’t a deliberate attempt to cause interference, by the time you’re far away from the sources of the sound the likelihood that they’ll happen to be exactly opposing (or adding to) each other at the precise place your ears are is pretty small. For almost all intents and purposes you can just treat each sound source as independent of the others.
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