Handbook for Sound Engineers

Preamplifiers and Mixers 743

increases. All audio systems face the same problems

whenever multiple open microphones are needed. These

problems are:

1. Build-up of background noise and reverberation.


2. Reduced gain before feedback.


3. Comb filtering.

These problems can plague boardrooms, city council

chambers, conference centers, houses of worship, tele-

conferencing rooms, radio talk shows—anywhere

multiple microphones are used. Since audio quality

rapidly deteriorates as the number of open microphones

increases, the solution is to keep the minimum number

of microphones open that will handle the audio. An

automatic microphone mixer keeps all unused micro-

phone input channels attenuated, and activates any

microphone spoken into within milliseconds.

21.3.1.1 Buildup of Background Noise and

Reverberation

The first problem of multiple open microphones is the
buildup of background noise and reverberation. This
buildup can adversely affect the quality of recordings or
broadcasts originating from the audio system. Consider
the case of a city council with eight members and eight
microphones. For this example, only one member is
talking. If all eight microphones are open when only
one microphone is needed, the audio output will contain
the background noise and reverberation of all eight
microphones. This means the audio signal will contain
substantially more background noise and reverberation
than if only the talker’s microphone were open. This
buildup of background noise and reverberation greatly
deteriorates the audio quality. Speech clarity and intelli-
gibility always suffer as background noise and reverber-
ation increase.
As the number of open microphones increases, the
background noise and reverberation in the audio output
also increase. In our city council example, the audio
output from eight open microphones would contain
9 dB more background noise and reverberation than a
single open microphone. To the human ear, the noise
would sound almost twice as loud when all eight micro-
phones were open.

To minimize background noise and reverberation
buildup, an automatic microphone mixer activates only
the microphone(s) being addressed and employs a
NOMA circuit. NOMA is an acronym for number of
open microphones attenuator. NOMA systematically

decreases the master gainwhenever the number of open

microphones increases. Without NOMA, the audio

system would produce objectionable noise modulation

(pumping and breathing) as background noise and

reverberation increase and decrease with the number of

open microphones. With a properly designed automatic

microphone mixer, background noise and reverberation

remain constant no matter how many or few micro-

phones are activated.

21.3.1.2 Reduced Gain Before Feedback

The second problem of multiple open microphones is

reduced gain before feedback. Acoustic feedback

(“howling”) can be a problem anytime a sound rein-

forcement (PA) system is used. To avoid feedback, PA

systems are operated below the point where the system

becomes unstable and starts to howl. However, this

feedback safety margin is reduced each time another

microphone is opened. Have one too many open micro-

phones and the result is feedback.

The automatic microphone mixer solution is to keep

unused microphones turned off and utilize NOMA. As

more microphones are activated, the overall gain will

remain constant thanks to the NOMA circuit. An auto-

matic microphone mixer assures that if the audio system

does not feedback when any one microphone is open,

the system will remain feedback free even if all the

microphones are open.

21.3.1.3 Comb Filtering

The third problem of multiple open microphones is comb

filtering. Comb filtering occurs when open microphones

at different distances from a talker are mixed together,

Fig. 21-14. Since sound travels at a finite speed, the

talker’s voice arrives at the microphones at different

times. When combined in a mixer, these out-of-step

microphone signals produce a combined frequency

response very different from the frequency response of a

single microphone. (A frequency response chart of the

out-of-step signals looks like the teeth of a hair comb,

thus the name.) The aural result of comb filtering is an

audio signal that sounds hollow, diffuse, and thin.

The solution to comb filtering also is keeping the

number of open microphones to an absolute minimum.

By automatically turning off unused microphones, an

automatic microphone mixer reduces comb filtering and

the resultant poor audio.