How does the ear hear sounds?

A few things happen to enable our ears to hear:
  • Sound waves enter your ear canal and cause your eardrum to vibrate.
  • Vibrations cause the fluid in the cochlea to move, stimulating the receptor cells.
  • The receptor cells send impulses to the brain.
David M. Vernick, MD
Ear, Nose & Throat (Otolaryngology)
Sound waves make the eardrum vibrate. The rate of the vibrations indicates the frequency, or pitch, of the incoming sound. For example, a sound wave that vibrates at 256 cycles per second -- the middle C on the piano, for example -- is said to have a frequency of 256 hertz, the unit in which frequencies are measured. The higher the frequency of the sound wave, the higher the pitch. As it vibrates, the eardrum transfers sound waves to the ossicles, three bones that form a bridge across the middle ear. These bones are named with the Latin words for their shapes: the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup. The tiny sound waves must move these bony structures, causing them to vibrate -- the part of the adventure akin to moving mountains. The purpose of the eardrum and the ossicles is to transmit the sound waves efficiently to the inner ear.

If the incoming sound is very loud, two muscles attached to the ossicles in the middle ear lower the volume by contracting. These muscles are called the tensor tympani and the stapedius. As they contract, they pull on the three bones, reducing their ability to vibrate. This action helps prevent extremely loud noises from hurting your ears or being uncomfortable to hear.

The vibrating ossicles transfer the sound-wave vibrations to the door between the middle ear and the inner ear. This door consists of the stapes footplate and a fibrous membrane that holds the footplate in place and seals the chamber. A major structure of the inner ear is the cochlea, a snail-shaped structure that consists of bone on the outside and fluid-filled membranes on the inside. There are 10,000 to 15,000 hair cells in each ear. As sound waves travel through the liquid passageways, they send a ripple across rows of sensory hair cells lining the cochlea. They cause a vibration of the inner ear at a location that is sensitive to that frequency thus moving the hair cells. The sound waves of different frequencies thus stimulate the hair cells in different regions of the cochlea. As the hairlike prongs on the hair cells bend with the motion, a chemical signal is released around the auditory nerve (eighth cranial nerve), triggering an electrical firing of the nerve. The auditory nerve in each ear transmits this electrical signal, which encodes a particular sound's frequency and loudness, to the area of the brain (auditory cortex) that interprets the sound.

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