Masking, in short, is the phenomenon that the intensity of a sound is reduced or disappeared by the human ear due to the interference of another sound is called masking.For example, when watching TV, the sound of TV cannot be heard clearly due to interference from neighbors’ decoration noise.
In the pure tone hearing threshold test, if the binaural hearing threshold difference is large, it will appear in the test of the poor ear. The test signal is likely to be transmitted to the contralateral cochlea before its threshold is reached. The non-test ear first feels the test sound and reacts.This kind of hearing obtained by the participation of non-test ears is cross hearing, and the poor hearing obtained by cross hearing is called “shadow hearing”.
The purpose of masking in the pure tone hearing threshold test is to avoid the participation of non-test ears and to obtain the true threshold of the test ear as much as possible.
①Interaural attenuation (IA): The energy attenuation value when the test signal passes through the skull from one ear to the opposite ear.Inter-aural attenuation is affected by many factors such as headphones, ear pads, signal frequency, external auditory canal volume, and resonance characteristics.The inter-aural attenuation of air conduction is generally 40 to 70 dB.The attenuation range of bone conduction between ears is 0-15 dB.
②Insufficient masking: When the masking sound of the non-test ear is not enough to eliminate cross hearing or inter-aural attenuation, the subject can hear the masking noise and the test signal at the same time in the non-test ear.If the masking is insufficient, the resulting false threshold will be better than the true threshold.
③Super-masking: When the masking sound is too large to be heard by the test ear through the skull, its threshold will increase with the increase of the masking sound behind the platform.
Insufficient masking will produce false thresholds, and over-masking will increase the thresholds.Therefore, the inspector needs to be proficient in and strictly follow the steps and test methods of the masking test.