A device used to connect a modem to a telephone line via an ordinary handset. The acoustic coupler converts electrical signals from the modem to sound via a loudspeaker, against which the mouthpiece of a telephone handset is placed. The earpiece is placed against a microphone which converts sound to electrical signals which return to the modem. The handset is inserted into a sound-proof box containing the louspeaker and microphone to avoid interference from ambient noise.
Acousitic couplers are now rarely used since most modems have a direct electrical connection to the telephone line. This avoids the signal degradation caused by conversion to and from audio. Direct connection is not always possible, and was actually illegal in the United Kingdom before British Telecom was privatised. BT's predecessor, the General Post Office, did not allow subscribers to connect their own equipment to the telephone line.