In cases where you detect a symmetrical hearing loss, traditional audiometry without masking is usually sufficient. However, be aware that in cases of asymmetrical hearing loss, one cannot be certain that the intended ear is the one actually detecting the sound.
To prevent this phenomenon causing an erroneous measurement, masking noise can be used to occupy the good ear (non-test ear) while testing the other one (Stach 1998, Katz 2002 and British Society of Audiology 2004). Masking can be applied to air conduction, bone conduction and speech audiometry. The need to mask the better hearing ear is linked to the interaural attenuation, which equals the amount of attenuation the sound is exposed to on its way through the skull.
Even though the interaural attenuation is very individual and varies with frequency it can on average be estimated to a minimum of 40dB for supra-aural headphones and 50dB for inserts. Regarding bone conduction, the interaural attenuation is a minimum of 0dB which means that crossing over of the stimulus may occur at all times, and this is what one should assume.
When measuring an audiogram on a patient with hearing within the normal range on one ear, but a moderate to severe hearing loss on the other, there is a potential risk of the good ear hearing the tone when trying to test the damaged ear. That is, the sound vibration may travel through the head and be heard by the opposite good ear when the vibrations of the signal are of sufficient
magnitude. Therefore, you are actually measuring the thresholds from the wrong ear. This could be the case in the example here and masking is needed in the right ear (better ear) while reassessing the left (poorer ear).
Masking is also needed to differentiate between sensorineural and conductive or mixed hearing losses. In the example, it is unknown if the loss of the left ear is sensorineural, conductive or a mixed hearing loss. The origin will be revealed by obtaining the bone conduction threshold for the left ear while occupying the right ear with masking.
In the example above, channel 2 should be set to Right (non-test ear) using the preferred masking stimulus (usually NB). Ensure that Rev is active to ensure that the masking noise is continuous. Channel 1 should be set to Left (test ear) using the preferred stimulus (usually Tone). The masking frequency will automatically change along with the tone frequency when masking is turned ON. You can set the masking and tone frequencies by using the Frequency Up Down buttons. While trying to establish the true threshold of the left ear, the right ear is now distracted with noise.
When storing a threshold while masking, the final masking level is stored in the masking table under the ear that is being tested. The terms ‘Effective masking’ in this situation refers to the fact that the narrow band noise level was loud enough to effectively mask a pure tone of the indicated level heard by the masked ear.