An acoustic coupler is an interface device for coupling electrical signals by acoustical means — usually into and out of a telephone instrument. The link is achieved through converting electric signals from the phone line to sound and reconvert sound to electric signals needed for the end terminal, such as a teletypewriter, and back, rather than through direct electrical connection.

Acoustic couplers were sensitive to external noise and depended on the widespread standardisation of the dimensions of telephone handsets. Direct electrical connections to telephone networks, once they were made legal, rapidly became the preferred method of attaching modems, and the use of acoustic couplers dwindled. Acoustic couplers are still used by people travelling in areas of the world where electrical connection to the telephone network is illegal or impractical. Many models of TDDs (Telecommunications Device for the Deaf) still have a built-in acoustic coupler, which allow more universal use with pay phones and for 911 calls by deaf people.

An acoustic coupler is prominently shown early in the 1983 film "WarGames", when character David Lightman (depicted by actor Matthew Broderick) places a telephone handset into the cradle of a film prop acoustic modem to accentuate the act of using telephone lines for interconnection to the developing computer networks of the period, in this case, a military command computer.

This acoustic coupler is a Moore Reed Acoustic Coupler type T.C. 301/m with a serial number of 06213. Unfortunately the pictures do not show the inside of the coupler box as the foam packing is perished at the moment but it does have a lovely varnished purpose made wooden case.