Audiometry is performed to test a person's ability to hear the sound frequencies necessary for speech. The test is carried out by a trained specialist called an audiologist with an instrument called an audiometer.
Purpose
Audiometry tests are used to identify and diagnose hearing problems. Audiometry is routinely performed within the framework of general health screening programs, for example in primary schools to detect hearing problems in children or in a work setting to test employees. It is also used at the doctor's office or in hospital audiology departments as a diagnostic tool to screen for hearing problems in children, adults, and the elderly. A complete audiometric evaluation consists of two main tests. First, the audiologist records the softest sound that a person can hear under earphones at several different frequencies. This is called testing via air conduction. Then a bone vibrator is placed behind the patient's ear to determine the softest level that the patient can hear when the inner ear is directly stimulated, thus bypassing the outer and middle ear. This is called testing via bone conduction. An ear test with an otoscope will be performed by a health care provider to evaluate the patient's ear canal and ensure a clear path to the ear drum. A device will then be inserted in the ear that will change the pressure, produce a pure tone, and measure the patient's response to the sound and different pressures. This test is called tympanometry. The proper diagnosis of a patient's specific pattern of hearing impairment allows the selection of the appropriate treatment, which may include hearing aids, corrective surgery, or speech therapy.
Precautions
Audiometry tests are performed with safe equipment and are simple and painless. They do not require any special precautions.
Description
Audiometry tests are carried out in a soundproof testing room equipped with audiometry equipment such as an audiometer, earphones, special headband, and a small listening booth for the audiologist. Audiometry equipment consists of devices emitting sounds or tones, like musical notes, at various frequencies, or pitches, and at various levels of loudness. In pure tone audiometry, pure tones, meaning tones that have a single frequency, are always channeled to each ear separately. For example, in a typical procedure, beginning with either ear of the patient, a 1 kHz-tone is sent through the earphones. The volume of the tone is set to a level that can be easily heard by a person with normal hearing. The patient is instructed to press a button or activate a switch when the tone is heard. The duration of the test tone is usually kept constant at some1.5 seconds. The volume is then decreased by 10dB-increments until the patient can no longer hear it. At the first level at which the tone becomes inaudible, the volume is increased in 5dB-increments until the patient responds to the tone. The audiologist may need to decrease and increase the loudness of the tone a few times to establish the patient's hearing threshold. By definition, this threshold is the loudness level at which a person responds to the tone 50% of the time. After the threshold for the 1 kHz-tone has been recorded by the audiologist, the entire procedure is repeated with a 1.5 kHz tone, and further increasing the tone's frequency to 2, 3, 4, and 8 kHz. After the high-frequency range has been tested, the same procedure is performed for the lower frequencies, starting again with a 1 kHz-tone, but decreasing to 0.5,0.25, and 0.125 kHz. The test results are plotted by the audiologist, and they show hearing loss as a function of frequency. Such a plot is called an audiogram and it usually shows the low frequencies or tones at one end and the high frequencies at the other end. A typical audiogram also reports results for both the left and right ears, since it is not unusual for levels of sensitivity to sound to differ from one ear to the other. The audiogram also plots the volume of the tones used for the test: from soft, quiet sounds at the top of the plot to loud sounds at the bottom. Hearing is measured in decibels, the unit of a logarithmic scale of sound intensity. Most of the sounds associated with normal speech patterns are usually found in the range of 20-50 decibels. An adult with normal hearing can detect tones between 0-20 decibels. Other tests that may have been performed with the pure-tone audiometry are also reported on the audiogram.
Another audiometry procedure is performed with a headband rather than earphones. The headband is fitted with small plastic rectangles that are placed behind the ears so as to channel the tones through the bones of the skull. The patient feels the vibrations of the tones as they are transmitted through the bones to the inner ear. As with the earphones, the tones are repeated at various frequencies and volumes.
Speech audiometry is another type of hearing test that uses a series of simple recorded words spoken at various volumes directly into earphones worn by the patient. The patient repeats the words heard to the audiologist as they are being heard. An adult with normal hearing will be able to recognize and repeat at least 90-100% of the words.
Speech reception threshold testing (SRT) is used to determine the lowest volume or intensity when speech can be understood and is reported in decibels (dB).
Preparation
The ears are first examined with an otoscope before audiometry testing to determine if there are any blockages in the ear canal. If excessive wax is present, the patient is sent to a doctor for removal of the wax.
Results
A person with normal hearing should be able to recognize and respond to all of the tone frequencies and the different volumes used during the audiometry test in both ears. Normal hearing detects a range of low and high-pitched sounds between 0-20 decibels, and normal speech is generally spoken in the range of 20-50 decibels.
Audiometry test results are considered abnormal if there is a significant variation between the levels of sound recognized by the two ears, or if the patient is unable to hear within the normal range for test frequencies. Failure to pick out any tone of 20 dB or louder indicates a degree of hearing loss. The pattern of responses displayed on the audiogram are used by the audiologist to determine if a significant hearing loss has occurred and if the patient might benefit from further testing for differential diagnosis, hearing aids, or corrective surgery. An unexplained difference between the two ears in the tone test or the speech test may warrant another test, called an auditory brain stem response (ABR) to look for a possible acoustic tumor. Or, a difference in hearing between the two ears may be explained by case history information, such as noise exposure.
Health care team roles
Audiometry is performed by an audiologist. Any hearing problems found may be investigated by physicians and treated by physicians or speech-language pathologists, depending on the nature of the problem. If necesssary, imaging studies, such as a CT scan, will be ordered by physicians and performed by radiologic techologists.
KEY TERMS
Audiogram—A chart or graph of the results of a hearing test conducted with audiographic equipment. An audiogram typically shows hearing loss as a function of frequency, as measured by an audiometer. The chart reflects the softest (lowest volume) sounds that can be heard at various frequencies or pitches.
Audiologist—A specialist trained in detecting hearing loss.
Audiometer—An instrument for measuring hearing sensitivity.
Audiometry—The measurement of hearing ability, usually with the an audiometer.
Deafness—The complete inability to hear, often present at birth.
Decibel—A unit of measure for expressing the loudness of a sound. Normal speech is typically spoken in the range of about 20-50 decibels.
Frequency—The rate of repetition of the cycles of a periodic quantity, such as a sound wave.
Hearing loss—Decreased auditory perceptive ability. Hearing loss is always acquired.
Intensity level—The measurement of the intensity of a sound in comparison to another sound or to a fixed level, expressed in decibels.
Otology—The branch of medicine concerned with the ear.
Otoscope—A hand-held instrument with a tiny light and a funnel-shaped attachment called an ear speculum, which is used to examine the ear canal and eardrum.
Pitch—The subjective impression of the frequency of a tone. Pitch is thus a psychoacoustic variable, and people have variable degrees of sensitivity to it.
Pure tone—A tone having a single pitch, or frequency, such as a flute note. It is also called a pure tone.
Sound—Any vibration in the air liable to cause a sensation of hearing.
Sound wave—Alternations of sound pressure displacement following one another in cycles of compression through a medium such as air. On striking the ear, sound waves are heard as sound.
Threshold of hearing—The intensity level at which a sound becomes barely audible. For a continuous tone of between 2000 and 4000 Hertz, heard by a person with normal hearing, this is 0.0002 dyne/cm2 sound pressure and is given the reference level of 0 dB.
Tone—A single sound of definite, recognizable pitch.
Tympanometry—A test where air pressure in the ear canal is varied to test the condition and movement of the ear drum. This test is useful in detecting disorders of the middle ear.
BOOKS
Goldstein, R. and W. M. Aldrich. Evoked Potential Audiometry: Fundamentals and Applications. New York: Allyn and Bacon, 1998.
"Pure-Tone Audiometry." In The Yale University Patient's Guide to Medical Tests, edited by Barry L Zaret, et al. Boston, MA and New York: Houghton Mifflin Company, 1997, pp.420-422.
ORGANIZATIONS
American Academy of Audiology. 8201 Greensboro Drive, Suite 300, McLean, VA 22102. (703) 610-9022. <http://audiology.org>.
American Speech-Language-Hearing Association (ASHA). 10801 Rockville Pike, Rockville, MD 20852. (800)638-8255. <http://www.asha.org/>.
OTHER
"How to Read Your Hearing Test." Hearing Alliance of America, 1999. <http://www.earinfo.com>.
"Tympanometry." Health Central on Sympatico, 2001. <http://healthcentralsympatico.com/mhc/top/03390.cfm>.
Understanding Your Audiogram: A brochure from the League for the Hard of Hearing, 71 West 23rd Street, New York, NY 10010-4162. (212) 741-7650. <http://www.lhh.org>.
