How do you hear different frequencies?

Your inner ear has a long coil like structure full of fluid and highly sensitive hairs. Depending on which frequencies are being received depends on how far down the coil the sound resonates which in turn causes different hairs to vibrate. The hair vibrations are picked up by lots of nerves and sent the brain. Because some sounds can resonate a bunch of hairs; The brain is able to cancel out different signals, and boost signals as to get a better picture of the sounds which are really being heard.