Typical hearing relies on sound waves changing as they move through the air into the ear, and transform into electrochemical signals which the auditory nerve carries to the brain.
Hearing depends on a specific sequence of events happening at different points along a sound's journey.
These steps usually take place in the following order:
First, sound waves enter the outer ear. They travel through the ear canal, which is a very narrow passageway that connects the outer ear, also known as the shell, to the eardrum. As the sound waves enter the eardrum, it begins to vibrate, and the vibrations transfer to three bones in the middle ear. These tiny bones are named in Latin for hammer (malleus), anvil (incus), and stirrup (stapes), based primarily on their shapes. These small bones of the middle ear bones amplify the sound vibrations, so that they travel further into the ear and enter the inner ear. They wind up in the cochlea, a snail-shaped, fluid-filled structure which is separated into upper and lower parts by an elastic, "basilar" (base) membrane.
As the incoming sound vibrations enter the inner ear the fluid in the cochlea creates waves which travel along the basilar membrane. The basilar membrane is covered with tiny, bristled hair cells which move up and down as the traveling wave ripples along. Another membrane overlays the base membrane, and as ripples continue, the bristling hair cells bump against this upper membrane. The bristles then lean to one side and pore-like channels open, allowing chemicals to enter. This process creates an electrical signal, which is then carried by the auditory nerve to the brain, delivering a recognizable sound.
The hair cells on different parts of the cochlea detect differently pitched sounds. Those near the base are able to detect high-pitched sounds such as whistling or bells ringing. The hairs near the middle of the cochlea detect lower pitched sounds, such as a garbage truck or a bass note on an instrument. Disruption to any part of this vibration journey can cause disruption to typical hearing - whether the issue begins in the outer, middle or inner ear or between the ear and the brain.
Typical hearing depends on a carefully timed sequence of events. Any disruption can cause changes in how a person hears.