The inner ear is a fluid-filled bone structure shaped like a snail shell. The connection between the middle ear and the inner ear is called the oval window. The footplate of the small stapes bone is attached to the oval window and functions as a piston moving the fluid of the inner ear.
Inside the inner ear is the cochlea where there are about 23,000 outer hair cells, which are activated by this movement of the fluid. When the hair cells are activated, they send impulses to the brain, which then interprets these impulses as sound.
The frequency of the sound determines which group of hair cells are activated, allowing us to distinguish between different sounds. If the hair cells are damaged due to age, illness or other causes, they have difficulty hearing certain sounds and differentiating between sounds. Large amounts of earwax can also considerably reduce the ability to hear.