How Hearing Works

The ear is a sophisticated organ that transmits the sounds we hear into electrical impulses that are interpreted by the brain. The process of hearing and interpreting sound is accomplished at a fantastic speed. To achieve this remarkable feat, each part of the ear – outer, middle and inner ear – fulfills a specific function.


How Hearing Works

Normal Ear Function

  1. Sound is transmitted through the air as sound waves from the environment. The sound waves are gathered by the outer ear and sent down the ear canal to the eardrum.
  2. The sound waves cause the eardrum to vibrate, which sets the three tiny bones in the middle ear into motion.
  3. The motion of the three bones causes the fluid in the inner ear, or cochlea, to move.
  4. The movement of the fluid in the inner ear causes the hair cells in the cochlea to bend. The hair cells change the movement into electrical impulses.
  5. These electrical impulses are transmitted to the hearing (auditory) nerve and up to the brain, where they are interpreted as sound.

The Outer Ear

The outer ear is composed of the pinna, or (auricle), the familiar visible portion of the ear, and the ear canal. The pinna, with its grooves and ridges, provides a natural volume boost for sounds in the 2000 to 3000 Hz frequency range, where we perceive many consonant sounds of speech. The shape of the outer ear serves to give preference to sounds originating from the front.

The ear canal, also called the external auditory meatus, is the other important outer ear landmark. The ear canal is lined with only a few layers of skin and fine hair, and is a highly vascularized area. This means that there is an abundant flow of blood to the ear canal. Wax (cerumen) accumulates in the ear canal and serves as a protective barrier to the skin from bacteria and moisture. Ear wax is normal, unless it completely blocks the ear canal.

The Middle Ear

The middle ear consists of the eardrum, or tympanic membrane, and three tiny ear bones, or ossicles. The ossicles are the smallest bones in the human body. Although named the malleus, incus and stapes, they are often referred to as the hammer, anvil and stirrup because of their characteristic shape. The ossicles take mechanical vibrations received at the tympanic membrane into the inner ear. Besides their role in the transmission of sound, these bones help to protect the ear from damage by constricting and limiting sound transmission when sound is too loud.

The middle ear also contains the Eustachian tube, which connects with the throat, and serves to ventilate and regulate pressure in the middle ear. The middle ear is encased in bone and does not associate with outside air except through the Eustachian tube. This tubular structure is normally closed, but it can be involuntarily opened by swallowing, yawning, or chewing. It can also be intentionally opened to equalize pressure in the ears, such as when flying in an airplane. When this happens, you might hear a soft popping sound.

The Inner Ear

The inner ear is an organ located deep within the temporal bone, which is the bone of the skull on both sides of the head above the outer ear. The inner ear has two main structures: the semicircular canals and the cochlea.

  • Semicircular canals – These structures do not contribute to hearing, but assist in maintaining balance as we move.
  • Cochlea – This is the hearing organ of the inner ear, which is a fluid-filled structure that looks like a snail. The cochlea changes the mechanical vibrations from the tympanic membrane and the ossicles into a sequence of electrical impulses. Sensory cells, called hair cells, bend in the cochlea as the fluid is disrupted by the mechanical vibrations. This bending of the hair cells causes electrical signals to be sent to the brain by way of the auditory nerve. The cochlea is arranged by frequency, much like a piano, and encodes sounds from 20Hz (low pitch) to 20,000Hz (high pitch) in humans.