In order to investigate the effect of the masking noise, following tests were performed on 20 normal hearers in average sound treated testing room. 1) Measurement of white noise masking level to the air conduction threshold of the contralateral ear. 2) Measurement of white and pulse noise masking level to 10 db above threshold of the same ear. Above two tests showed identically better masking effect of both white and pulse noises, to the speech frequencies, moderate to high frequeneies, and poorer to lower frequencies.