In terms of function, an acoustic coupler was a modem that was placed directly on the handset of the telephone. Acoustic couplers were used for data transmission over the telephone network.
On the transmitter side, data signals were modulated in the acoustic coupler, then fed into the telephone as acoustic signals, and then transmitted on the telephone network. An acoustic coupler on the receiving side received and demodulated the acoustic tones. Acoustic couplers operated duplex with transmission rates up to 1,200 bit/s.
When acoustic couplers were used in the public switched telephone network, a number of conditions had to be met, which were specified in ITU Recommendations V.15.