The experiments on intelligibility, listening difficulty and optimum speech level by adjustment method were carried out under the condition of various noise level and reverberation. The following findings were obtained.
(1) The optimum speech level increases as noise level increases with inclination of less than 1.
(2) The level at which listening difficulty achieves minimum value in masking noise coincide with optimum speech level by adjustment method.
(3) The optimum speech level by adjustment method is not affected by reverberation, so it does not achieve sufficient intelligibility and listening difficulty in poor condition of reverberation and noise.
