I understand how sound is generated by pressure waves vibrating the eardrum. And this makes perfect sense to me when a single sound is generating that vibration. But when multiple sounds are vibrating the eardrum at the same time (like when listening to music with different instruments and vocals) how does the brain tease those differing vibrations apart so we can hear the individual inputs…as opposed to them mixing all together into one sound; The equivalent of mixing a bunch of different paint colors together and ending up with brown.
In: Biology
[Waves add up](https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/1989/2017/06/13225738/figure-17-10-04a.jpeg). Your ear drum is like a rubber duck floating on the ocean; it moves up and down with the big swells (bass sounds) and with the little ripples caused by the wind (high pitch sounds), at the same time. The surface of the ocean is “big swells and tiny ripples” all added up together into one “complex” wave.
But the “sound sensors” in your ear are NOT in the ear drum, they’re in the spiral [in this diagram](https://uihc.org/sites/default/files/styles/large__900x600_/public/figure4_stapes_detail_color_labeled.jpg?itok=-aHwSCqP) called the Cochlea.
So what happens in the Cochlea ([more detailed diagram here](https://i.pinimg.com/originals/40/71/85/4071852705126baeb83cef993aaf8aca.jpg)) is the complex sound wave from your ear drum pushes through those bones into the Oval Window at the top, and vibrates the liquid in that spiral. The sound waves travel through that liquid in the direction of the arrows. And the sensor hairs (Cilia) in that Organ of Corti in the middle, they vibrate as the sound passes, and send those impulses to the brain via neurons.
The trick though, is that the thicker hairs that detect bass sounds are at one end of that liquid spiral path, and the thin hairs that detect high pitch sounds are at the other end. So the sound frequencies (low bass vs. high pitch) get “decomposed” and picked up by separate hairs, and sent to the brain on different neurons. Your Cochlea decomposes that “complex” wave of all the instruments in an orchestra, by frequency.
But it’s your brain that makes sense of it all. Your brain is very good at recognizing patterns, and waves are patterns. Violin vs. drums vs. not just a person’s voice, but words and *meaning* of the words, all of that is figured out by the brain, from sound frequencies. It’s just what the brain does.
Same with vision. Sound frequency is pitch (bass vs high pitch), light frequency is color. Your brain doesn’t just see a splotch of different colors (different light frequencies), it recognizes objects, people’s faces, *emotions* on those faces, etc. Pattern recognition.
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