Acoustic coupler by K & N Electronics, c. 1973.

The acoustic data coupler was an early type of computer modem, used to connect a computer to a telephone line to link it to another computer or a network.

The acoustic coupler is plugged into a computer and converts computer digital information into sound that can be sent down the telephone line to another computer, where they are converted back into digital computer information. The coupler plugs directly into the computer, but to connect to a telephone line a telephone handset has to be laid into the cradle on top.

This particular data coupler was used by Ferranti in Manchester in the 1970s to connect to a computer in the USA for processing bills and materials costs, an unusual arrangement at the time.