User Guide

Sound System Design Reference Manual
However, all of the preceding calculations have
assumed that the microphone is an omnidirectional
unit. What happens if we substitute a directional
microphone? Figure 6-7 shows the additional
geometrical relationships needed to calculate the
increase in gain produced by a directional
microphone.
Note that the distance from talker to
microphone is still .6 meters and that the talker is
assumed to be located along the major axis of the
microphone. The loudspeaker is located 5.4 meters
from the microphone along an angle of 75° from the
major axis.
Figure 6-7 also shows a typical cardioid pattern
for a directional microphone. The directivity index of
such a microphone along its major axis is about 5 dB.
Since the talker is located on the major axis of
the microphone, it “hears” his signal 5 dB louder than
the random incidence reverberant field. In theory this
should increase potential system gain by a factor of
5 dB.
But we must also consider the microphone’s
directional characteristics with relation to the
loudspeaker. If the directivity index of the microphone
at 0° is 5 dB, the polar pattern indicates that its
directivity index at 75° must be about 3 dB. This tells
us that even though the loudspeaker is 75° off the
major axis of the microphone, it still provides 3 dB of
discrimination in favor of the direct sound from the
loudspeaker.
We know that the loudspeaker’s directivity
index is -3 dB along the axis between the
loudspeaker and the microphone. We also know that
the microphone’s directivity index along this axis is
+3 dB. The combined directivity indices along this
axis must therefore, be 0 dB and we can find the
equivalent critical distance from Figure 6-6.
The combined critical distance of loudspeaker
and microphone along their common axis is about
1.3 meters. Since the distance between the two is
more than 3 times this figure, the microphone still lies
within the reverberant field of the loudspeaker. Using
the directional microphone should therefore allow an
increase in potential system gain before feedback of
about 5 dB. (In practice, little more than 3 or 4 dB of
additional gain can be achieved.)
Figure 6-7. Characteristics of a cardioid microphone
6-7