Design
Usually, a standard telephone handset was placed into a cradle that had been engineered to fit closely (by the use of rubber seals) around the microphone and earpiece of the handset. A modem would modulate a loudspeaker in the cup attached to the handset's microphone, and sound from the loudspeaker in the telephone handset's earpiece would be picked up by a microphone in the cup attached to the earpiece. In this way signals could be passed in both directions.
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. The same is also shown in several films with deaf characters in it.
Read more about this topic: Acoustic Coupler
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