The audiogram is an evaluation of how well an individual can hear. Sounds are presented to the individual through earphones during the test. These sounds are presented at different levels of frequency and intensity. The human ear responds to the frequency or pitch of a sound and the intensity or loudness of the sound. The frequency of the sound is measured in Hertz (Hz), and the loudness of the sound is measured in decibels (dB). During the procedure, the responses of the individual are recorded on a graph. An individual has no impairment in hearing if he or she detects the sound that is presented through the earphones in the range of 0 to 25 decibels. The audiogram indicates how much louder (in decibels) the pure tone frequency must be for the tested subject to hear it compared to what the normal, general population needs to hear it.

There are two primary types of hearing loss. Conductive hearing loss occurs when there is an obstruction or breakdown in the system that transmits sound through the ear, including the ear canal, the ear drum (called the tympanic membrane), and the three small bones on the inner side of the ear drum. Ear wax, infections, and damage to the eardrum are common causes of conductive hearing loss. Conductive hearing loss is often reversible with treatment (Council for Accreditation in Occupational Hearing Conservation [CAOHC], 1993).

Sensorineural hearing loss is caused by damage to the hair cells of the inner ear (called the cochlea) or to the nerves that carry sound to the brain (CAOHC, 1993). Sensorineural hearing loss is usually permanent. The two most common causes of sensorineural hearing loss are aging and noise of an occupational or non-occupational nature. It is thought that some workplace chemicals can also contribute to sensorineural hearing loss. The National Institute on Deafness and other Communication Disorders has developed a reference chart that provides examples of everyday sounds, along with the measure of their loudness in decibels.


Sound Source


Firecracker, rock concert


Ambulance siren


Chain Saw






Lawn mower


City traffic noise


Normal conversation


Refrigerator humming


Whispered voice


Threshold of normal hearing



The Occupational Safety and Health Administration estimates that 30 million people in the United States are exposed to hazardous noise in the workplace. Even though it is one of the most common occupational illnesses, noise-induced hearing loss (NIHL) often goes unrecognized because there are no visible effects, it usually develops gradually and over a long period of time, and there is often no pain associated with it (

NIHL is considered an occupational illness rather than an injury because it usually progresses over time (CAOHC, 1993). While it is possible that hearing loss can occur from a single event, such as an explosion, most occupational NIHL results from exposure to continuous noise above a certain level (CAOHC, 1993). There is also evidence that exposure to industrial chemicals, such as heavy metals and solvents, is toxic to the nervous system and may cause hearing loss. Research has shown that the combination of exposure to these substances with noise may work together to increase the risk of hearing loss (CAOHC, 1993).

Ringing in the ears (called tinnitus) often accompanies hearing loss, and is a sign of irritation to the sensory cells of the inner ear and may be a warning sign for NIHL (CAOHC, 1993). Another important cause of sensorineural hearing loss is aging. As people get older, many of our special senses, such as sight, smell, hearing, or taste, decline in their abilities. Age-related hearing loss is called presbycusis. The onset of presbycusis varies between individuals, and the onset and the magnitude of hearing loss associated with the aging process may begin as early as age 50 years (CAOHC, 1993). Presbycusis is often greater in the higher pitched levels of sound (that is, for the higher frequencies of 4,000 or more Hertz).


  • Council for Accreditation in Occupational Hearing Conservation. 1993. Hearing Conservation Manual (3rd edition). A. Suter (Ed.) Milwaukee, Wi: CAOHC.
  • U.S. Department of Health and Human Services. National Institutes of Health. National Institute on Deafness and Other Communication Disorders (NIDCD). How Loud is Too Loud?


Mary Fields, Program Manager