
1) Telephony equipment
Found on
https://www.crosswordclues.com/clue/sidetone

Sidetone is audible feedback to someone who is speaking. The term is most used in the telecommunication field. ==Telephony== In telephony, sidetone is the effect of sound picked up by the telephone`s transmitter (mouthpiece) and instantly introduced at a low electronic signal level into the receiver (earpiece) of the same handset, a form of feedb....
Found on
http://en.wikipedia.org/wiki/Sidetone

(from the article `telephone`) Anti-sidetone circuit. The anti-sidetone circuit is an assemblage of transformers, resistors, and capacitors that perform a number of functions. The ...
Found on
http://www.britannica.com/eb/a-z/s/91

the reproduction in the telephone receiver of sounds picked up by its associated transmitter and transmitted without appreciable delay by the internal circuitry of the telephone set
Found on
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=722-15-21
No exact match found.