Hearing begins when the outer ear, the visible portion of the ear, channels sound waves down the auditory canal, the tube-like passageway leading into the ear. The middle ear lies at the end of the auditory canal. It is composed of the eardrum and three small bones, commonly referred to as the hammer, the anvil and the stirrup. When sound waves hit the eardrum, it vibrates and, in turn, moves the hammer. The hammer moves the anvil, which moves the stirrup, transmitting the vibrations into the inner ear. The middle ear functions to amplify sound, which is why significant hearing loss can result from any disruption in any of the parts.
The inner ear consists of the cochlea and the nerve of hearing. It converts sound waves into nerve impulses that travel to the brain via the movement of tiny hair cells. The brain, in turn, allows us to hear…as long as the message it is receiving is not distorted due to problems in the process just described.
The type of hearing loss in any one person depends upon where in the ear the problem occur. The three main types are conducive, sensorineural, and mixed losses.
Conductive: A problem in the outer or middle ear causes conductive hearing loss. A conductive loss prevents sound from reaching the nerves in the inner ear. Common causes include:
Sensorineural: Damaged nerves in the inner ear cause senorineural hearing loss. Sensorineural losses cannot be reduced or eliminated by surgery. There are many causes, differing by age of onset.
Mixed: Sometimes, people will have problems both in the inner ear and in the outer or middle ear. This type of hearing loss is known as a mixed loss.
Hearing loss is measured in decibels (dB). The softest sounds are made at zero dB, and the loudest are 120 dB. Normal speech is around 50 dB.
The table shows the various degrees of hearing loss. Hearing loss is never given in percentages, only in decibels.
|Degree of hearing loss||Hearing loss range (dB HL)|
|Normal||0 to 15|
|Slight||16 to 25|
|Mild||26 to 40|
|Moderate||41 to 55|
|Moderately severe||56 to 70|
|Severe||71 to 90|