eli5 How does the brain separate out frequencies it’s hearing so that we can make out different instruments in a song? It seems like some insanely complex analysis behind the scenes??
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Hearing scientist here.
Sound is a type of wave. Waves have some interesting properties that mean you can combine two waves of different frequencies (pitches) together to get a more complicated wave. But, you can also take a complicated wave and work backwards to figure out what frequencies made it up.
The part of your ear that turns physical waves into electrical signals headed for the brain is called the cochlea. It’s structured so that the high frequency waves can only get into the start of it before getting stopped, but low frequency waves can get further in before getting stopped. This means the cochlea can tell what pitch the sound is by how deep the wave travels. This lets the brain figure out what pitches it’s hearing, and in what amounts.
Now, the other side of this is that a given instrument actually plays multiple notes at the same time (even if they’re only trying to play a single note) and these extra notes are what gives the instrument its tone. Our brains are really good at spotting the patterns that make up these different sounds, a bit like an acoustic fingerprint.
Some sound engineering can make this easier. By putting, say, the guitar through the left channel, the keys through the right, and the singer equally in the two, it makes it easier for us to separate out the different sounds because our two ears are hearing different mixes. If something is happening in both ears, we know it’s the singer, but if it’s only in the left then it’s the guitar. (It’s not usually that extreme, but that’s the basic idea.)
So yeah, there’s a mix of how our ears work mechanically, some maths, our pattern recognition, and good sound design.
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