A squelch system (used in wireless equipment) whereby the transmitter emits a tone at an inaudible frequency. The receiver, upon detecting any signal checks to see if that tone is present. If so, it allows the main signal to be heard, otherwise it stays muted.

Source: [http://www.allchurchsound.com/ACS/glossary/index.html Church Audio & Acoustics Glossary] [[Category: Acoustics]]