Most humans hear sounds in the range of 20 to 20,000 Hz. Sensitivity varies as a function of frequency, with sounds in the middle frequencies being heard best. The ability to hear higher frequencies declines with age. For a discussion of the causes of hearing loss, see the section on diseases of the middle ear, or diseases of the inner ear.
Adequate testing requires an audiometer (device for presenting sounds to the patient at a precisely controlled intensity), a sound-proof environment, a competent audiologist, and a cooperative patient. The standard testing battery includes pure tone audiometry, speech audiometry, and immittance audiometry.
Pure tone audiogram
This is a graphic plot of the patient’s thresholds of auditory sensitivity for pure tone (sine wave) stimuli. Threshold hearing levels are indicated for each frequency tested. By convention, normal hearing levels are shown at top of the graph, a decrease in hearing sensitivity is indicated by larger values of hearing level. Hearing level is plotted on a logarithmic decibel scale. Sounds are tested with presentation by air conduction (earphones) as well as bone conduction (skull vibrator). An air bone gap indicates a conductive component of hearing loss. A decrease in threshold sensitivity by bone conduction reflects a sensory or neural loss.
Air conduction thresholds are represented by circles, and bone conduction by triangles. A solid symbol indicates that masking noise was presented to the opposite ear to minimize the chance of responses due to crossover of sound.
This sample audiogram indicates normal hearing in the right ear, and a conductive loss on the left.
|Conductive hearing loss
The second audiogram demonstrates a sensory–neural loss in the right ear, and a mixed loss on the left.
|Sensory–neural hearing loss
|Mixed hearing loss
These tests utilize spoken words and sentences rather than pure tones. Tests are designed to assess sensitivity (threshold) or understanding (intelligibility).
- Threshold – The level at which the patient can correctly repeat 50% of test materials phoneme balanced words (PB) and synthetic sentences.
- Intelligibility – By convention, the percentage of words or sentences a patient can correctly repeat when presented at supra-threshold levels.
- Provides information about hearing handicap. Problem maybe worse than indicated by pure tone average (PTA) for the speech frequencies.
- Useful to determine candidacy for hearing aid.
- Very poor results, out of proportion to PTA, suggests probable retro–cochlear cause of hearing loss.
These hearing tests utilize the electro–acoustic immittance bridge. This device is designed to quantify the impendence (resistance to movement) of the conductive mechanism of the ear by bouncing a probe tone off the tympanic membrane and measuring the proportion of reflected sound.