An acoustic coupler is an older hardware interface used to transmit digital data over analog telephone lines. Instead of a direct electrical connection, the user pushed a standard telephone handset into two rubber cups containing a microphone and a speaker. The device converted digital bits into audible audio tones that were played into the phone's mouthpiece and listened for incoming tones from the earpiece.
Heard on the job
Before we had RJ11 jacks on everything, you had to jam the handset into an acoustic coupler and pray no one slammed a door nearby.
Insider tip
Because they relied on ambient sound isolation, acoustic couplers were notoriously sensitive to room noise and physical vibrations, which would introduce errors into the data stream.