Auditory testing determines the range of sound frequencies that can be detected by the human ear as well as the loudness level that can be detected at each frequency. The human ear can detect sounds from between 20 to 20,000 Hz, but the most common range of sound frequencies used in hearing are those of human speech, 500 to 4,000 Hz.
To test hearing, several different types of tests are used. The most rudimentary test uses the tuning fork. The purpose of the tuning fork is to determine whether a patient can detect a specific sound frequency or range of sound frequencies. To perform the test with a tuning fork, a hearing professional hits the tuning fork against his or her hand or leg and asks the patient if the tone produced by the tuning fork is audible. The patient then simply signals whether or not they can hear the tone. Or if the tone is audible to the patient, the patient is asked to indicate when the sound can no longer be heard. Then the tester compares how long the patient could hear the tone to how long he or she could hear the tone.
Hearing can also be tested with a tuning fork by determining if the patient senses a difference between when the vibrating tuning fork is placed immediately outside the ear canal (1 inch) and when it is touched to the mastoid area close to the middle ear. If the patient notices a difference, it normally indicates a conductive hearing loss. The most common frequency used for tuning forks is 512 Hz, but forks range from 256 to 1024 Hz.
Audiometry is the most widely used test to evaluate hearing loss. The result of audiometry is an audiogram, which is a graph of how a patient can hear across a range of sound frequencies. To create an audiogram, the patient is placed in a sound-proof booth or in a quiet area. Head phones are placed on the patient and a series of tones or sound frequencies are played at different loudness levels. The patient is instructed to press a button when the tone is just detectable and let the button up when the tone can no longer be heard. The frequencies range from 250 Hz to 8000 Hz in one octave increments. (An octave is a doubling, so one octave above 250 would be 500 Hz.). Normal hearing is when the patient can hear all of the frequencies at a sound level below 15 to 25 dB. Children have better hearing and are considered normal below 15 dB, while for adults, below 25 dB is considered normal.
Audiograms can provide specific information about the type of hearing loss, depending on the shape of the curve. Losses across only a certain range of frequencies typically indicate a sensorineural hearing loss (hearing loss that is a result of a defect in the vestibulocochlear nerve, the inner ear, or processing centers in the brain).
Sometimes the audiogram may have a “notch,” or a dip in the middle of the audiogram that returns to normal in the high frequencies. Losses only at high frequencies or across all frequencies typically indicate conductive hearing loss (hearing loss that involves the outer and middle ear).
What is a Decibel?
A decibel is a logarithmic measurement used to describe a ratio. When used as a measure of sound, the ratio is of sound pressure between the background pressure level and the pressure created by the sound. Since dB uses a logarithmic scale, every 3 dB is a doubling of pressure. That is, a sound of 6 dB has twice as much sound pressure as a sound with 3 dB. Examples of dB levels of various sounds include:
Soft Whisper at 5 Ft 30 dB
Inside Quiet Restaurant 50 dB
Speech at 1 Ft 70 dB
Subway Train at 20 Ft 90 dB
Football Stadium at Kickoff 110 dB
Jet Aircraft Takeoff 120 dB
(Source: Wikepedia and Human Factors in Engineering and Human Design, By E. J. McCormick)