Inside Automatic Microphone Mixer Systems

An automatic microphone system is a good option whenever multiple microphones (four or more) are being used, particularly if the sound system is intended to run without a live operator
shure mixer

Every time the number of open or active microphones in your church system increases, the system gain (or volume) also increases.

The effect of this is greater potential for feedback as more microphones are added, just as if the master volume control were being turned up.

In addition, unwanted background noise increases with the number of open microphones. Here, the effect is a loss of intelligibility as the background noise level rises closer to the level of the desired sound.

A good solution is to activate microphones only when they are addressed and to keep them attenuated (turned down) when not being addressed.

In addition, when more than one microphone is addressed at a time, the system volume must be reduced appropriately to prevent feedback and insure minimum noise pickup.

An automatic microphone mixing system can be a lot of help in this situation. Essentially, it’s comprised of a special mixer and an associated group of microphones, and it’s function is twofold: to automatically activate microphones as needed and to automatically adjust the system volume in a corresponding manner.

In some automatic microphone systems, ordinary microphones are used and all of the control is provided by the mixer. In others, special microphones are integrated with the mixer to provide enhanced control.

There are several techniques used to accomplish channel activation or (gating) in an automatic microphone system.

A look at a basic automatic microphone mixer setup. (click to enlarge)
In most systems, a microphone is gated on when the sound that it picks up is louder than some “threshold” or reference level.


When the sound level falls below the threshold, the microphone is gated off. This threshold may be fixed, adjustable, or even automatically adjustable.

In any case, the threshold should be set so that the microphone is not activated by background noise but will be activated by normal sound levels.

Traditional threshold systems distinguish between background noise and the desired sound only by level.

However, if background noise becomes sufficiently loud, it may activate microphones unless the threshold is adjusted to a higher level.

Subsequently, if the background noise decreases, normal sounds may fail to gate the microphones on unless the threshold is lowered as well. Threshold adjustment is critical to automatic mic systems of this type.

Some recent automatic mixers incorporate noise adaptive threshold circuitry. These have the ability to distinguish steady signals such as background noise from rapidly changing signals like speech.

They can automatically and continuously adjust individual channel thresholds as ambient noise conditions change.

In addition, some designs can recognize that the same signal is being picked up by more than one microphone.

In that case, only the channel with the strongest signal is activated. This prevents both microphones from being activated when a talker is in between two microphones for example.

Certain other automatic systems, with integrated microphones, can actually sense the location of the sound source relative to the ambient noise and activate microphones only when the sound comes from the desired direction. These “directional gating” systems do not require any threshold adjustments.

There is another circuit within every automatic mixer that continuously senses the number of open microphones (NOM) and adjusts the gain of the mixer accordingly.

With a properly functioning automatic system, if each individual microphone is adjusted to a level below the feedback point, then any combination of microphones will also be below the feedback point.

Many automatic microphone mixers have additional control circuitry, often in the form of logic connections.

These are electrical terminals that can be used for a variety of functions, including: microphone status indicators, mute switches, loudspeaker attenuation, and the selection of “priority” channels.

Some automatic mixers have an adjustable “off attenuation” control: instead of gating the microphone completely off, it can be “attenuated” or turned down by some finite amount, to make the gating effect less noticeable in certain applications.

Another control included on some units is an adjustable “hold time”: when the desired sound stops, the channel is held on for a short time to avoid gating the microphone off between words or short pauses.

In addition, a function which locks on the last microphone activated insures that at least one microphone is on, even if no one is speaking.

Finally, most automatic mixing systems are able to be expanded by adding individual channels and/or by linking multiple mixers together to control large numbers of microphones simultaneously.

An automatic microphone system should be considered whenever multiple microphones (four or more) are being used, particularly if the sound system is intended to run hands-free, that is, without a live operator.

This is often the case not only in the worship facility itself but in fellowship halls, conference rooms, and auditorium systems.

Microphones should be selected and placed according to the normal guidelines (integrated systems require a microphone choice from the selection available for those systems).

It is recommended that the manufacturer or a qualified installed sound professional be consulted on the details of a particular automatic microphone system.

(Copyright Shure Incorporated, used by permission.)

David McLain | The Mixer Guy | CCI SOLUTIONS
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Olympia, WA 98507-0481
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