4.27  effective masking level for speech. Sound pressure level of a specified masking noise that masks a speech signal to 50 percent probability of recognition. The hearing level reference of the masking signal is equal to that of a speech recognition threshold.

Annotation         On a normally hearing ear the amount of effective masking, abbreviated EM, is equal to the number of decibels that a masking noise shifts a speech recognition threshold. For example, 20 dB EM would produce a speech recognition threshold shift of 20 dB when the masking noise and speech signal are presented simultaneously to the same ear.

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