a 70's Electro-Harmonix brochure
This "noise gate" cuts out the hisses, hums, and other unwanted background noises produced by electric instruments and sound effects that can ruin a combo's stage presence. It operates on the principle that while most instruments and effects produce hisses and hums, this noise is at a much lower volume than the music that is being played, and therefore not audible. However when an instrument idles, the extraneous noise is no longer masked by the music, and it becomes an up front sound. The Silencer works like an extra hand on a master volume control. When the music startsit instantly turns up the volume; when the music stops, it instantly cuts the volume down to zero. The noise disappears.