also: おんきょうカプラー
noun
acoustic coupler
A device that converts digital signals into sound for transmission over telephone lines, used historically with modems.
昔のパソコン通信では音響カプラを使って電話回線に接続した。
In the early days of PC communication, we connected to the telephone line using an acoustic coupler.
モデム is the modern general term for a modulator-demodulator, while 音響カプラ specifically refers to the older acoustic type that uses a telephone handset.
Compound of 音響 (sound) and カプラ (coupler), a direct borrowing from English 'acoustic coupler'.