When the level of air-conducted thermal noise reaches about 30phones, it begines to mask a bone-conducted pure tone which is applied to the same ear. The bone-conduction threshold value increases in proportion to the noise level above 30phones. Thermal noise applied to the opposite ear begines to mask the bone-conducted pure tone at about 60phones. The transmission loss from one ear to the other is about 50db for thermal noise or pure tones which is applied to one ear by an air-conduction receiver, but is 7db for bone-conducted pure tones. The change of the bone-conduction threshold due to the attachment of the masking air-conduction receiver is avoided by a sufficiently large cavity put between the receiver and the ear.
Several examples were given on the theoretical analysis of the steady-state frequency characteristics of rectangular rooms. Measurements were performed for model rooms and a studio(N. H. K. JOHK at Sendai). Comparison of theoretical results with experimental curves showed reasonable agreement for a model room, but no good agreement for a studio.
The instantaneous amplitude distribution was studied experimentally. Employing the level selective circuits of high stability, the distribution density curves of sine wave and random noise were compared with the calculated ones, and it was observed that the revetting noise showed Gaussian distribution regardless of bands of frequencies. The frequency rate of noises was measured in some cases by applying the level selective circuit and the monostable multi-vibrator.