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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.[1] 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.[2] 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.[3]

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.[3] 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.[4] 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.[5] There are 3 key situations where masking is required, termed rules of masking. These are detailed below.

Masking Dilemma In situations with a sizeable conductive component in both ears, it may not be possible to mask effectively at all. An example of this is as follows: During an audiometric assessment, air conduction thresholds receive values of 70 to 105 dB across all frequencies in both ears. On continuing to test bone conduction testing, non-masked bone conduction testing responses receive values of 15 to 20 dB across all frequencies. Here, the difference between air conduction and bone conduction is greater than 55 dB (interaural attenuation); therefore, masking is required. As a procedure, the masking noise to the nontested ear may be set at 80 dB, and a tone presented (via bone) to the test ear at 15 dB. If there is no response, the tone increases (via bone) to the test ear to 20 dB. If there is a response at this level, the masking noise is increased in the non-test ear to 85 dB, and the tone is then re-presented to the test ear. If there is no response at this level, the tone of the test ear is then increased (via bone) to 25 dB. If, at this level, there is a response, then the masking noise presented to the non-test ear is increased to 90 dB. If, at this level, there is no response, the pattern may continue without ever reaching a plateau or receiving true masked levels. The masking noise crosses over to the test ear as the test tone increases, creating a masking dilemma. In this circumstance, the unmasked responses are labeled with an asterisk, and it is stated that masking is not possible without over-masking. This must be confirmed at all frequencies.