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.
* * *
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.
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.
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.
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.