How Hearing Works

Hearing is one of the five major senses (sight, touch, taste, smell and, hearing). Hearing is the process by which vibrations in the air are converted into vibrations in the organs of the ear. The brain turns these vibrations into sounds.  

The process of hearing occurs through five distinct stages of the hearing mechanism:

  • Outer Ear
  • Middle Ear
  • Inner Ear
  • Acoustic Nerve
  • Auditory processing by the Brain

 

Outer Ear

The outer ear captures the sounds from the air and funnels these sounds into the middle ear. The outer ear consists of the pinna and the ear canal. The pinna is made of cartilage and skin, surrounds the outside of the opening of the ear canal and is attached to the ear lobe. The shape of the pinna captures the sound vibrations from all directions. The ear canal, which is covered with wax and hairs to prevent foreign bodies from entering the ear, carries the sound captured by the pinna.

 

Middle Ear

The purpose of the middle ear is to convert the sound vibrations that enter the ear canal into mechanical vibrations of tissue. The eardrum rests at the beginning of the middle ear and is connected to three time bones called ossicles. The ossicles are composed of the malleus (or hammer), which is connected to the eardrum; the incus (anvil), which is a connection between the malleus and the third bone; and the stapes (stirrup), which is the third bone and connected  to the inner ear. When the eardrum starts to move, due to the sounds coming down the ear canal, the ossicles translate this movement into a mechanical vibration of the inner ear.

The middle ear is essentially an air chamber surrounded by the eardrum on one end and the inner ear on the other. As an air chamber, the movement of the eardrum and ossicles within the chamber can be affected by a pressure difference between  the outside and inside air. The eustachian tube connects the middle ear to outside air and helps eliminate the pressure differential. It extends from the front of the middle ear, immediately behind the eardrum, to the back of the nose and throat.  

 

Inner Ear

The inner ear is composed primarily of the cochlea, which is a snail-shaped organ filled with fluid. Inside the cochlea lies the corti organ, which performs sensory reception for the auditory system. Tiny hairs within the organ of corti move, or are stimulated, when the fluid within the cochlea moves due to the vibration transferred from the ear drum through the ossicles to the inner ear.  These movements by the hair cells translate into nerve impulses, which in turn are sent to the brain, via the acoustic nerve. The cochlea and organ of corti are structured such that only certain hair cells are stimulated by certain frequencies of sound. In other words, different hair cells are stimulated by different sound frequencies, high sound frequencies stimulate hairs in a certain area of the organ of corti, while lower sound frequencies stimulate hairs another area.

 

Acoustic Nerve

The acoustic nerve carries the signals from the inner ear to the cochlear nucleus, which is a relay station for the auditory system. In the cochlear nucleus, some of the auditory nerve fibers split and send signals to the opposite side of the brain, while some connect to the same side of the brain. In this way, each ear supplies auditory information to both sides of the brain. 

 

Sound Processing in the Brain

Within the brain, the auditory signals are processed, turning a series of sensations of the hair cells in the organ of corti into recognizable sounds. Brain auditory processing also provides for the discrimination between a primary sound and the background noise and the localization of sounds. The human auditory system can detect vibrations in the air from as low as 20 Hz (cycles per second) to 20,000 Hz. Sound frequencies for human speech typically range between 500 to 4,000 Hz.