Handbook for Sound Engineers

742 Chapter 21

be considered when the number of microphones
required for the sound system is four or greater.

When used in a sound reinforcement system, an
automatic microphone mixer provides a significant
increase in gain before feedback when multiple micro-
phones must be used without a sound engineer. It also
improves the quality of the sound system output by
reducing the amount of extraneous room sound being
picked up, and by reducing comb filtering. In addition,
it automatically adjusts system gain to compensate for
the number of microphones in use at any instant. Thus,
an automatic microphone mixer attempts to provide the
same system control that might be produced by a human
sound engineer.

As automatic microphone mixers are optimized for
speech applications, their use in musical applications is not

recommended. Mixing microphones for music is as much

art as science, and therefore the artistic judgment of a

human sound engineer is much preferred to the electronic

decision process of an automatic microphone mixer.

In summary, when used in a speech sound system

with multiple microphones, the ideal automatic micro-

phone mixer assures that the number of active micro-

phones at any moment equals the number of active

talkers at the same moment. All unused microphones at

that moment are attenuated.

21.3.1 The Audio Problems Caused by Multiple

Open Microphones

High-quality audio becomes progressively more diffi-

cult to achieve as the number of open microphones

Figure 21-12. Block diagram of the Cooper Sound CS 104.

Figure 21-13. Signal path schematic of Jensen Twin-Servo 990 microphone preamplifier. Courtesy Jensen Transformers, Inc.

Mic gainline

mic 1mic 2

mic 3

Mic gainline

mic 1mic 2

mic 3

Mic gainline

mic 1mic 2

mic 3

Mic gainline

mic 1mic 2

mic 3

1

2

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4

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Mic pwr48P/12T

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