4.35  free-field sensitivity (of an earphone). At a given frequency, for at least 10 otologically normal subjects, quotient of sound pressure level, or force level, of a frontally incident plane progressive wave by input voltage to an earphone, or bone vibrator, such that on average the subjects judge the direct sound wave and the sound produced by the earphone, or bone vibrator, to be equally loud, when judged with the same ear, the opposite ear being occluded.

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