Handbook for Sound Engineers

744 Chapter 21

21.3.1.4 Summary

1. Keeping the number of open microphones to a
   minimum always improves overall audio quality.


2. The primary function of an automatic microphone

mixer is to keep unused microphone input channels

attenuated (turned down or off) and to instanta-

neously activate microphones when needed.

3. Buildup of background and reverberant noise,

reduced gain before feedback, and comb filtering

can all be controlled by using an automatic micro-

phone mixer.

21.3.2 Design Objectives for Automatic
Microphone Mixers

As shown in Fig. 21-15, a conventional microphone

mixer in a sound system amplifies the signal from each

microphone and combines these amplified signals
together to produce a single output. This output feeds a

power amplifier and then one or more loudspeakers.

Each doubling of the number of open microphones
feeding into a sound system reduces the available gain

before feedback by 3 dB. This fact surprises the layman

who often believes that more microphones equate to the
sound system being louder, not softer. A sound system

with numerous microphones easily becomes ineffective

if a sound engineer is not present to control levels and
switch off unused microphones. Since gain before feed-

back can often be marginal because of the acoustical

characteristics of a room, an automatic microphone
mixer may be the only way to provide adequately loud

program levels to the audience with an unattended

sound system.

21.3.2.1 Examples of Design Objectives for an

Automatic Microphone Mixer

1. Keeps the sound system gain below the threshold
   of feedback instability.


2. Requires no operator or sound technician at the
   controls.


3. Does not introduce spurious, undesirable noise or
   distortion of the program signals.


4. Can be installed as easily as a conventional mixer.


5. Responds only to the desired speech input signals
   and is relatively unaffected by extraneous back-
   ground noise signals.


6. Activates input channels fast enough that no
   audible loss of speech signals occurs.


7. Allows more than one talker on the system when
   required by the discussion content while still main-
   taining control of the overall sound system gain.


8. Adjusts the system gain to compensate for a range
   of talker input levels.


9. Provides system status outputs for peripheral
   equipment control and can interface with external
   control systems for advanced system design if
   required.

The automatic microphone mixer operation should

provide relatively easy and very rapid input activation.

Desired speech from a talker should cause immediate

activation of the appropriate input channel, which may

not always happen if the design of an automatic micro-

phone mixer is poor. Also, random false activation of

microphones remote from the talker can occur with

some automatic microphone mixer designs. However,

this false activation is typically not troublesome as the

false signals are normally much lower in level than the

desired talker signal. The automatic microphone mixer

is doing its job if all talkers are clearly heard by the

audience when they speak, and the sound system

remains below the point of feedback.

Figure 21-14. Comb filtering occurs whenever open micro-
phones at different distances from a talker are mixed
together. Figure 21-15. Simplified diagram of a microphone mixer.

To

amplifiers

as above

Input 1

Input 2

Input 3

Auxiliary

input Auxiliary

output