An open microphone in a room with a loudspeaker system creates a potential source of feedback as the sound from the speaker system reenters the microphone. As someone speaks, the microphone picks up the sound and
This recirculation cycle can be repeated many times in reflective rooms or where very little margin exists between the necessary loudness level and the maximum possible loudness level. If the sound level in the room is high enough, the recirculation will build up to the point where a loud howling or squealing will occur. This oscillation is referred to as acoustic feedback. As more microphones are added to the system, the problem grows worse. An automatic mixer solves this basic problem by applying a process commonly referred to as NOM attenuation.
NOM stands for: Number of Open Microphones. An automatic mixer differentiates between active and inactive microphones, turns down the inactive ones and then adjusts the the overall sound system level according to how many active microphones remain. The result is that all of the microphones in the room act the same as a single microphone with respect to feedback.
There are many different types of automatic mixers available from different manufacturers. Simple, low cost types offer nothing more than a switch that opens and closes the microphone with voice activity. Better ones use more involved algorithms to differentiate between active and inactive microphones. The best automatic mixers utilize sophisticated algorithms with either analog or digital controls to smoothly turn mics up and down or distribute gain among the channels to apply NOM attenuation in a seamless, inaudible manner.
Improved Sound Quality
An automatic mixer also minimizes recirculated sound and background noise to dramatically improve sound quality in public address, teleconferencing and recording systems.
In a public address system, the sound will recirculate through the microphones and speakers and hit the listener's ears numerous times, spaced a few milliseconds apart, making it very difficult to understand what is being said. Background noise gathered by unused microphones in any type of sound or recording system will also add an unnatural "hollow" character to the sound, reducing intelligibility.
Automatic mixers can also be an important addition to a studio production facility where multiple microphone systems are used to produce talk shows, by reducing background noise and applying NOM attenuation. The lower background noise improves the clarity of the sound and the NOM attenuation automatically limits the overall sound level to avoid overloading the audio signal chain.
Automatic Hands Free Operations
In modern sound systems such as boardrooms, courtrooms, churches and conference centers, multiple microphone systems are often used for sound reinforcement, teleconferencing and recording all at the same time. It is virtually impossible for someone to operate a sound system like this manually and keep up with rapid voice activity. The sound system operator would also have to be in the room with the talkers and microphones to have any hope of keeping up with the activity. Thus, automatic mixers are virtually mandatory in multiple microphone sound systems to provide dependable response to the voice activity and the convenience of unattended hands free operation.