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Can you explain how we hear sounds? Why do our eardrums vibrate when we hear a noise?

Taken from my Quora postbag.

Certainly can do! Sound travels in a wave form. Air molecules are disrupted when a person speaks, and they move to and fro. This movement disrupts the air molecules next to them, which disrupt the ones next to them and so on.

The molecules are knocked out of place, then return to their original place, a bit like a Newton’s Cradle. As with sea waves, the wave is moving but the constituent parts aren’t travelling. The air next to the speaker’s mouth doesn’t travel to your ear, it is knocked out of place then springs back.

Eventually the air molecules inside your ear canal are disrupted and they bang into your eardrum. This happens over and over again, and the rate it repeats at is called the frequency (aka pitch) of the sound. This movement of air molecules makes the stretchy membrane of your eardrum move back and forth at the same frequency.

Attached behind your eardrum are the three bones of the middle ear. They are also moved back and forth. The stapes bone is attached to the “window” part of your cochlea (inner ear), and the movement basically rattles the window.

So far, this has been body parts moving in air, but when the window is moved, it creates disturbance of the fluid inside the cochlea. The Basilar Membrane inside the middle ear moves with the wave of fluid. It’s much like when you ‘fluff a duvet’ or shake out a blanket; you hold it at one end and flap it up and down. You’ll see that wave travel along the blanket.

The frequency of the sound wave is replicated in the cochlea, because high frequency sounds affect the near end, and low frequencies affect the far end.

Movement in the membranes (there are two) triggers movement in the hair cells of the cochlea, and here is where mechanical movement is turned into electrical signals. The electrical impulses travel along the auditory nerve to the brain. If they came from the far end of the membrane we register them as low frequency. From the near end, we register them as high frequency. This frequency information is faithfully passed along at all stages, along with information on volume. Comparing the two signals from the left and right ears lets us know which direction the sound is coming from.

This is all quite simple when you’re imagining a single pitched tone, but what is really fascinating is when you have multiple tones, such as speech. The membrane is being disrupted all over the place, simultaneously, and yet we somehow manage to separate out all those movements and hear it as speech, music. Or both!

It’s an absolutely beautiful system, and I loved learning about it. After all that learning, I have still not been able to understand how multiple frequencies are sent simultaneously through the eardrum and bones. My lecturers didn't answer the question either, so it might be witchcraft!

There’s a video here that shows the system in action…

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