User Guide

Sound System Design Reference Manual
4-5
As we saw in an earlier example, our system
only has 7.5 dB of maximum gain available with a
6 dB safety factor. By going to both a directional
microphone and a directional loudspeaker, we can
increase this by about 6 dB, yielding a maximum
gain of 13.5 dB — still some 16 dB short of what we
actually need.
The solution is obvious; a hand-held
microphone will be necessary in order to achieve the
required gain. For 16 dB of added gain, D
s
will have
to be reduced to the value calculated below:
16 = 20 log (1/x)
16/20 = log (1/x)
10
.8
= 1/x
Therefore: x = 1/10
.8
= 0.16 meter (6”)
Of course, the problem with a hand-held
microphone is that it is difficult for the user to
maintain a fixed distance between the microphone
and his mouth. As a result, the gain of the system will
vary considerably with only small changes in the
performer-microphone operating distance. It is
always better to use some kind of personal
microphone, one worn by the user. In this case, a
swivel type microphone attached to a headpiece
would be best, since it provides the minimum value
of D
S
. This type of microphone is now becoming very
popular on-stage, largely because a number of major
pop and country artists have adopted it. In other
cases a simple tietack microphone may be sufficient.
Conclusion
In this chapter, we have presented the
rudiments of gain calculation for sound systems, and
the methods of analysis form the basis for the study
of indoor systems, which we will cover in a later
chapter.