Development of No. 4 type telephone set

A report on the overall development of a new high-quality telephone set which has been in use since 1951. The most important design features lie in the transmitter and receiver which had to meet the requirement of first-class articulation of Japanese speech transmission. The requisite voice-frequency range for this purpose was determined and models of transmitter and receiver designed to cover this range with high sensitivity. The whole development was based on the assumption of subsequent mass production, including the standardization of acoustic measuring equipment.