As I understand it, the eardrum works like any other kind of speaker in that it has a diaphragm which vibrates to encode incoming motion into something the inner ear translate to sound. It's just a drum that moves back and forth, so it can only move at one rate or frequency at any given time.
But humans have very discerning ears and can simultaneously tell what instruments are playing at the same time in a song, what the notes in the chord of one of those instruments is, even the background noise from the radiator. All of this we can pick apart at the same time despite that all of these things are making different frequencies.
I know that all of these vibrations in the air get added up in a Fourier Series and that is what the ear receives, one wave that is a combination of all of these different waves. But that still means the ear is only moving at one frequency at any given time and, in my mind, that suggests that we should only be able to hear one sound at any given time, and most of the time it would sound like some garbled square wave of 30 different frequencies.
How can we hear all these different frequencies when we can only sense one frequency?
Best Answer
No, it doesn't mean that at all.
It means the eardrum is moving with a waveform that is a superposition of all the frequencies in the sound-wave it is receiving.
Then, within the inner ear, hair cells detect the different frequencies separately. It is entirely possible for several hair cells to be stimulated simultaneously so that you hear several frequencies at the same time.