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An acoustic coupler is an audio interface device for coupling a computer with audio into or out of a telephone. It may also be a terminal device linking data terminals and radios with telephone networks. The link or interface is done by picking up audio signals from a telephone handset rather than a direct electrical connection.
Acoustic couplers were not allowed on telephones in the U.S. prior to 1982. Telephones were hard-wired into the wall. Bell Systems often owned the telephones themselves. The telephone system was a closed system entirely owned by Bell. However, elsewhere in the world acoustic couplers were popular in the 1970s, but transmitted at speeds of only up to 300 baud – the number of voltage fluctuations (frequency) on a telephone line. The practical upper limit of acoustic couplers was 1200 baud. These were made by Vadic in 1973 and by AT&T in 1977. However, modems replaced acoustic couplers and were able to transmit data over phone lines more easily, dependably and at greater transfer speeds. In the U.S. this happened rapidly after the breakup of Bell Systems in 1982. By 1985 this was widespread using the Hayes Smartmodem 1200A, which allowed creation of dial-up bulletin board systems – the precursor of today’s Internet chat rooms, message boards and email.
Acoustic couplers were very sensitive to external noises. To fit closely to the telephone handset, the attached cup had to be of a certain size. Therefore, the effectiveness of the device was dependent on the standardization of the handset dimensions. Thus, when direct electrical connections were made legal in the U.S., modems became very popular, and acoustic couplers use declined rapidly. However, some are still used by world travelers where electrical connections to telephones are illegal or not available. And many models of telecommunications devices for the deaf (TDD) still have acoustic couplers built-in, allowing universal use with pay phones.