The Middle Ear by Sunil Puria Richard R. Fay & Arthur N. Popper

Author:Sunil Puria, Richard R. Fay & Arthur N. Popper
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

6.4.2 Masking

To ensure BC testing of a specific ear, the nontest ear requires masking. The level and frequency of the masking noise is important. Inadequate masking allows the nontest ear to participate while excessive masking falsely makes the BC threshold worse. Optimum masking is produced by narrow band noise centered at the test frequency. The optimal level is difficult to predict beforehand. One issue is the attenuation of the BC sound to the contralateral side. It is assumed to be, on average, 0–15 dB (see Fig. 6.2), but the large variability may cause the BC stimulation at the opposite ear to be 20 dB higher than at the test ear at certain frequencies (Stenfelt 2012b). The masking is usually provided by circumaural or insert earphones that causes an occlusion effect at low frequencies that can amount to 20 dB (Elpern and Naunton 1963). Consequently, the BC stimulation at the nontest ear may be 40 dB greater than at the test ear.

Because the exact masking level is difficult to predict according to the foregoing, an adaptive masking procedure is often used (Studebaker 1964). This is also known as the plateau technique, wherein the unmasked threshold is elevated by increasing masking level in the nontest ear. Above a certain level, a further increase of the masking noise in the nontest ear causes no further threshold elevation in the test ear. The threshold at this plateau is considered as the true masked threshold of the test ear.

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