A type of modem that converts serial digital data into a frequency shift keyed sound signal in the audio range for transmission down telephone lines, and decodes similar incoming sound signals. The connection between the acoustic coupler and the telephone system is made by means of a small microphone and loudspeaker held close to the earpiece and mouthpiece of an ordinary telephone handset in a sound-absorbent enclosure.

Although largely superseded by devices that connect directly to the telephone network, acoustic couplers are still used. The lack of any electrical connection between terminal and phone lines is of benefit when obtaining the approval of the PTT for the use of such a device. The quality of ordinary switched voice circuits normally limits the speed of transmission to 300 baud or less.