An audiology exam tests your ability to hear sounds. Sounds vary according to the intensity (volume or loudness) and the tone (the speed of sound wave vibrations).

Hearing occurs when sound waves move to the nerves of the inner ear and then the brain. Sound waves can travel to the inner ear by air conduction (through the ear canal, eardrum, and bones of the middle ear) or bone conduction (through the bones around and behind the ear).

INTENSITY of sound is measured in decibels (dB):

• A whisper is about 20 dB
• Loud music (some concerts) is around 80 to 120 dB
• A jet engine is about 140 to 180 dB

Usually, sounds greater than 85 dB can cause hearing loss in a few hours. Louder sounds can cause immediate pain, and hearing loss can develop in a very short time.

TONE of sound is measured in cycles per second (cps) or Hertz:

• Low bass tones range around 50 to 60 Hz
• Shrill, high-pitched tones range around 10,000 Hz or higher

The normal range of human hearing is about 20 Hz to 20,000 Hz, and some animals can hear up to about 50,000 Hz.

The first steps are used to estimate the need for an audiogram. The specific procedures may vary, but they generally involve blocking one ear at a time and checking for the ability to hear whispers, then spoken words or the sound of a ticking watch.

A tuning fork may be used. The tuning fork is tapped and held in the air on each side of the head to test the ability to hear by air conduction. It is tapped and placed against the mastoid bone behind each ear to test bone conduction.

Audiometry provides a more precise measurement of hearing. Air conduction is tested by having you wear earphones attached to the audiometer. Pure tones of controlled intensity are delivered, to one ear at a time. You are asked to raise a hand, press a button, or otherwise indicate when you hear a sound. The minimum intensity (volume) required to hear each tone is graphed. An attachment called a bone oscillator is placed against the bone behind each ear (mastoid bone) to test bone conduction.

No special preparation is needed.

There is no discomfort. The length of time varies. An initial screening may take about 5 to 10 minutes. Detailed audiometry may take about 1 hour.

This may be a screening test to detect a hearing loss at an early stage. It may also be used when there is difficulty in hearing from any cause.

Common causes of hearing loss include:

• Chronic ear infections
• Ruptured eardrum
• Acoustic trauma
• Occupational hearing loss
• Head injury
• Inherited conditions
• Diseases of the inner ear
• Medications that can be toxic to the nerve of the inner ear, including certain antibiotics (such as neomycin), diuretics, and large doses of salicylates (such as aspirin)

• The ability to hear a whisper, normal speech, and a ticking watch is normal.
• The ability to hear a tuning fork through air and bone is normal.
• In detailed audiometry, hearing is normal if tones from 250 Hz through 8000 Hz can be heard at 25 dB or lower.

There are many different kinds and degrees of hearing loss. Some include only the loss of the ability to hear high or low tones, or the loss of only air or bone conduction. The inability to hear pure tones below 25 dB indicates some extent of hearing loss.

The extent and kind of hearing loss may give clues to the cause and the prognosis (probable outcome).

The following conditions may affect test results:

• Acoustic neuroma
• Acoustic trauma
• Age-related hearing loss
• Alport syndrome
• Labyrinthitis
• Meniere's disease
• Occupational hearing loss
• Otosclerosis
• Ruptured or perforated eardrum

There is no risk.

There are many different hearing function tests. Simple screenings involve making a loud noise and watching to see if it startles the patient. Detailed screenings include brainstem auditory evoked response testing (BAER). This test uses electroencephalogram to detect brain wave activity when sounds are made.

A newer hearing test called otoacoustic emission testing (OAE) can be used in the very young (such as newborns) or when standard tests do not produce reliable results.

Cummings CW, Flint PW, Haughey BH, et al. Otolaryngology: Head & Neck Surgery. 4th ed. St Louis, Mo; Mosby; 2005:3466-3479.

Kerschner JE. Neonatal hearing screening: To do or not to do. Pediatr Clin North Am. June 2004; 51(3): 725-36, x.