Various acoustic researches for telecommunication are performed in the Electrical Communication Laboratory, NTT. Among these, researches on acoustic standard apparatus, acoustic measurements, electroacoustic transducers, vibrating systems, acoustic materials, room acoustics, and telephone networks are included. A new12‐in.standard condenser microphone has been developed for precise sound measurement and sensitivity calibration of telephone sets. This new standard microphone, made of titanium alloy, has high sensitivity (−54 dBre1V/mgr;bar, high reliability, and wide frequency range (∼35 kHz). A precise calibration method using standard couplers, as well as a simplified calibration method using active coupler, are established. Research of room acoustics is important for telephone systems in the future, because the acoustic transmission characteristics of rooms may take an important place in the system. Transmission characteristics and the spatial correlation coefficients of various rooms are investigated. The results of these researches are especially useful in telephone loudspeaker design. Based on the results of various researches, several types of new telephone sets and systems are now being developed.