The following U.S. military report on Japanese sound-powered telephones was published in Tactical and Technical Trends, No. 42, January 13, 1944. [DISCLAIMER: The following text is taken from the U.S. War Department publication Tactical and Technical Trends. As with all wartime intelligence information, data may be incomplete or inaccurate. No attempt has been made to update or correct the text. Any views or opinions expressed do not necessarily represent those of the website.]

JAPANESE SOUND-POWERED TELEPHONES Technical Intelligence Summary No. 8, published by the Australian Military Forces, contains a description of a Japanese sound-powered telephone. The following excerpts and sketch are taken from the Australian publication. *          *          * a. General As the name implies, the microphone of a sound-powered telephone, derives its energy directly from the sound waves and is a reversive to the original principle of the telephone, in which a receiver unit was used as a microphone. The telephone consists of a handset with a single dual-purpose operating unit and an additional unit as an extra receiver. b. Purpose These telephones are used to provide inter-communication within vehicles, or over other short lines when circumstances require rapid and simple setting-up and disconnection, and when low maintenance is desirable. c. Construction The design of the handset is unusual as the electro-acoustic unit is used as microphone and receiver. The unit is mounted in a normal receiver housing with an acoustical path from the speaker's mouth to the rear of the diaphragm by means of an extended horn type mouth-piece. The horn is a diecast aluminum alloy, plated with several very thin alternate layers of copper and nickel, and then coated with glycerylphthallate lacquer. The weight of the complete handset is 10 ounces. The spare receiver weighs 7 ounces. Attention is drawn to the two different types of connecting plugs on the ends of the line cords. d. Performance As the instruments were in a damaged condition when received, proper performance tests were not practicable. As these telephones are almost always used on short lines, there is a transmission gain of approximately 6 decibels by using one unit in the handset instead of two. This is somewhat offset by the loss due to the position of the mouthpiece opening relative to the speaker's mouth. A slight gain in transmission was obtained by moving the handset away from the ear so as to speak more directly into the mouthpiece. The inefficiency of the mouthpiece was also demonstrated by speaking directly into the earcap which gave an increase in transmission. A low capacitance was provided usually in parallel with a sound-powered handset unit.

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