Transdata acoustic coupler port for telephone data link 307, made by British Telecom, British, 1981-1990. Cream coloured with black cradles for telephone mouthpiece and earpirce, with British Telecom logo on the side.

Transdata acoustic coupler port for telephone data link 307, made by British Telecom, British, 1981-1990.

An acoustic coupler is a device used to connect computers to telephone lines using a standard telephone. A standard telephone handset was placed into a cradle that had been designed to fit closely around the microphone and earpiece using rubber seals. The coupler converted electrical signals from the computer into audio signals that could be transmitted along telephone lines. The use of acoustic couplers dwindled after direct electrical connections to telephone lines were legalised. They are still used by people travelling in areas of the world where electrical connections to the telephone network are illegal or impractical.