THE PURPOSE of masking the nontest ear during pure-tone audiometry is, of course, to verify the unmasked thresholds. Studebaker1suggested that the nontest ear should be masked during pure-tone air-conduction testing whenever the presentation level at the test ear exceeds the unmasked bone-conduction threshold of the nontest ear by more than 40 db. The 40-db criterion is used because this is the smallest expected value for cross-lateralization of air-conducted tones. On the other hand, the nontest ear should be masked during bone-conduction testing anytime an unmasked air-bone gap is observed on the test ear.2 It is generally agreed that narrow band noise centered at the test frequency is the most efficient masker of pure tones.1-8 Thus, the problem clinically is not in determining when masking is indicated or even the type of masking noise that is the most efficient. The main problem is in determining how much