Dictionary.net

A device used to connect a modemto a telephone line via an ordinary handset. The acousticcoupler converts electrical signals from the modem to soundvia a loudspeaker, against which the mouthpiece of a telephonehandset is placed. The earpiece is placed against amicrophone which converts sound to electrical signals whichreturn to the modem. The handset is inserted into asound-proof box containing the louspeaker and microphone toavoid interference from ambient noise.Acousitic couplers are now rarely used since most modems havea direct electrical connection to the telephone line. Thisavoids the signal degradation caused by conversion to and fromaudio. Direct connection is not always possible, and wasactually illegal in the United Kingdom before BritishTelecom was privatised. BT's predecessor, the General PostOffice, did not allow subscribers to connect their ownequipment to the telephone line.