Audiology Clinical Masking

Excerpt

Hearing loss is common in the United States, with approximately 13% of people over 12 years of age reported experiencing hearing loss in both ears on clinical testing. Therefore, it is vital that audiologists can accurately measure patients' hearing to help provide the best management for hearing loss. If a hearing loss is detected, it is also important to understand whether it is a conductive or sensorineural hearing loss, as both possible underlying pathophysiology and management options differ.

A pure tone audiogram is a behavioral test of hearing, where a tone at a specific frequency is presented to the test ear by over-ear headphones or inserted earphones. The loudness of the tone is altered to find the quietest tone in decibels (dB) which can be heard 50% of the time, which is termed the threshold. This process is then repeated for each sound frequency being tested. Typically 6 to 8 frequencies are tested, between 250 Hz and 8000 Hz, and the process is repeated for the other ear.

Clinical masking in audiology refers to introducing noise to the non-test ear during a pure-tone audiogram. This aims to ensure that the test ear hears the presented tone and is not cross-heard by the non-test ear. Cross-hearing occurs when a tone presented to the test ear overcomes interaural attenuation, which refers to the loss of acoustic energy as sound waves travel transcranially to the contralateral ear. The presented tone can then be perceived by the cochlea of the non-test ear and give rise to false-positive results. This occurs less readily when testing air conduction through insert earphones than supra-aural earphones, as insert earphones cause increased interaural attenuation.

Therefore, clinical masking is required when the difference in hearing thresholds between the ears is greater than the interaural attenuation. More specifically, for air conduction testing, masking is employed when the difference between unmasked thresholds in both ears is greater than 40 dB (or 55 dB when using insert earphones), which is thought to be the minimum interaural attenuation for pure tones. There is negligible interaural attenuation for bone conduction, which is assumed to be 0 dB for bone conduction testing, as sound travels via bone conduction through the skull to both cochleae. There are 3 key situations where masking is required, termed rules of masking. These are detailed below.