What is an acoustic coupler?
An acoustic coupler is an audio interface device for coupling a computer with audio to or from a phone. It can also be a terminal that connects data terminals and radio devices to telephone networks. The connection or interface is made by picking up audio signals from a telephone handset rather than a direct electrical connection.
Acoustic couplers were not allowed in the US prior to 1982. Telephones were hardwired into the wall. Bell Systems often owned the phones themselves. The phone system was a closed system that was wholly owned by Bell. In other parts of the world, acoustic couplers were popular in the 1970s, but only transmitted at speeds up to 300 baud - the number of voltage fluctuations (frequency) on a telephone line. The practical upper limit for 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 telephone lines more easily, more reliably and at higher transmission speeds. In the US, this happened quickly after Bell Systems dissolved in 1982. This was widespread until 1985 with the Hayes 1200A Smartmodem.
Acoustic couplers were very sensitive to external noise. In order to be able to connect closely to the telephone receiver, the attached cup had to be of a certain size. Therefore, the effectiveness of the device depended on the standardization of the dimensions of the handset. Therefore, when direct electrical connections were allowed in the United States, modems became very popular and the use of acoustic couplers quickly declined.
However, some are still used by world travelers where electrical connections to telephones are illegal or unavailable. And many models of telecommunications equipment for the deaf (TDD) still have acoustic couplers built in that allow universal use with payphones.