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An acronym of Number of Open Mics, believed first created in 1967, or 1968, by Bill Snow after he retired from Bell Labs and went to work at Altec Lansing Research. Dan Dugan, the father of the automatic microphone mixer and Altec Lansing, the manufacturer of his first design, popularized its use. In Dan’s original design, the automatic mic mixer, like human operators, turned the gain down on unused mic channels and turned the gain up on active channels, all the while ensuring that the overall level remained roughly constant. As a rough approximation, each doubling of the number of open mics (NOM) cuts the gain by 3dB, i.e., as more mics are opened up the mic mixer reduces overall gain. If not, as mics open and close, the reverberation and ambient noise fluctuates unacceptably. NOM attenuation techniques work to provide the gain, stability, and low noise qualities of a single open mic with the benefits of multiple mics. The NOM acronym is still used in some sound reinforcement circles.

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