There are a number of different ways of classifying deafness: according to when deafness was first present (onset), according to what within the ear causes it (type), and the degree of hearing loss.

Onset

Congenital deafness
When children are born deaf, it is called "congenital deafness." Congenital deafness can happen for a number of reasons, including genetic factors, infections while still in the mother's womb, exposure to certain toxins consumed by the mother, prematurity, and other various conditions.

Acquired deafness
At any point in someone's life, diseases, injuries, and other health conditions can cause deafness. This is called "acquired deafness." Some causes of acquired deafness are ear infections, ototoxic drugs, meningitis, measles, encephalitis, chicken pox, influenza, mumps, head injury, and noise exposure.

Many people now make a distinction between prelingual deafness and postlingual deafness.

Prelingual deafness
If a child becomes deaf before the development of speech, the child is considered prelingually deaf. (All cases of "congenital deafness" would be considered prelingual deafness.)

Postlingual deafness
If a child becomes deaf after the development of normal speech, the child is considered postlingually deaf.

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Type

Conductive hearing loss
This means that sound does not pass efficiently from the outer ear canal to the ear drum and the ossicles (three tiny bones) of the middle ear. Usually, a person with conductive hearing loss retains some hearing. Often, surgical or medical procedures exist to restore hearing.

Sensorineural hearing loss
Deafness caused by the malfunction of inner ear structures, such as the cochlea, are called sensorineural hearing loss. This cannot be medically or surgically corrected.

Mixed hearing loss
This occurs when someone has both conductive and sensorineural hearing loss.

Central auditory dysfunction
Deafness that results from malfunctions by the auditory nerve, brain stem, or brain (cerebral cortex) is called central auditory dysfunction.

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Degree

Hearing tests determine the range of sound a person is able to hear. (See Testing Hearing) The results from those tests show how many decibels (dB) sound must reach before it is heard by the listener. The results may vary with different frequencies.

The following is a list of possible categories for hearing loss according to sound. Different organisations, however, have different standards for each category.

• Normal hearing: -10 dB to 15 dB
• Slight loss: 16 dB to 25 dB
• Mild loss: 26 dB to 40 dB (background noise can disrupt conversation, speaker should be closer than six feet to listener)
• Moderate loss: 41 dB to 55 dB
• Moderately severe: 56 dB to 70 dB (speaker should be 3-5 feet away, listener may need to use a hearing aid to hear conversation)
• Severe loss: 71 dB to 90 dB (only shouting from 1 foot away would be audible, though understanding speech would be difficult.)
• Profound loss: 91 dB to 120 dB (only the loudest sounds are audible)
• Deaf: 120 dB or more

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References:

"Causes of Hearing Loss in Children," American Speech-Language-Hearing Association (Web Site). Retrieved on January 16, 2007.

Bauman, Neil. "Hearing Loss-Decibels or Percent?" Center for Hearing Loss Help (Web Site). Revised December 2003. Retrieved on January 16, 2007.

Frangulov, Anna, Heidi Rehm, and Margaret Kenna. Common Causes of Hearing Loss: For Parents and Families. Harvard Medical School Center for Hereditary Deafness: 2004. Internet: Retrieved on January 16, 2007.

Rehm, Heidi, et. al. Understanding the Genetics of Deafness: A Guide for Patients and Families. Harvard Medical School Center for Hereditary Deafness: 2003. Internet: Retrieved on January 16, 2007.

Smith, Richard JH and Guy Van Camp. "Deafness and Hereditary Hearing Loss Overview," Gene Reviews (Web Site). Posted on February 14, 1999, revised on December 4, 2006. Retrieved on January 16, 2007.