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Figure 1. Speaker pointed at the center of a room, in this case showing uneven coverage.
Figure 2. Speaker pointed at the back of the room, showing more consistent coverage.
carefully route the microphone to the speakers and quietly
verify the signal is flowing where you expect it to be.
There are a couple of different types of equalizers one might
encounter in a sound system. The typical filter in an equalizer
has three parameters: Gain (+ or -), a center frequency, and a
width parameter, which is referred to as bandwidth or sometimes
Q. A graphic equalizer is a series of filters which have
fixed frequency and bandwidth parameters. Most often seen
in a 1/3 octave spacing configuration, a third-octave equalizer
provides about 30 filters, which gives the engineer reasonable,
although not absolute, control over the signal. Graphic equalizers
are relatively easy to use as two of the three filter parameters
are fixed, leaving gain as the only moderated parameter.
Parametric equalizers provide precise control over all three filter
parameters. Parametric equalizers are much more difficult to
use for quickly ringing out a room, and are more often used for
system tuning and calibration. Mixing console channel equalizers
are often parametric equalizers.
The ringing out procedure consists of the iterative process of
slowly raising the level of the microphone until you can just hear the
first frequency begin to feedback, then controlling the feedback by
varying the position of the input fader while identifying the frequency
of the feedback and adjusting the output equalizer to eliminate
the ringing. Every cut one makes with an equalizer is effectively a
reduction in gain. This exercise is designed to maximize gain, so only
adjust the equalizer the minimum amount necessary to stop the
ringing. Identifying the ringing frequency can be a challenge. For a
little ear training have a look at the “Simple Feedback Trainer” (http://
sft.sourceforge.net/).
Carefully raise the level of the microphone. If the same frequency
begins to feedback again, reduce that frequency further with the
equalizer. If a different frequency begins to ring, identify and reduce
that frequency using the equalizer. Repeat this process until you
have either achieved enough gain to comfortably operate the system
or until you have identified and eliminated about five frequencies.
At that point the system is probably reaching the breakeven
point between problem frequencies and overall gain reduction. The
idea is to maximize gain before feedback. If too much cut equalization
is applied, it becomes the same thing as simply turning it down.
the speaker is all in the audience and not bouncing off of the walls.
For the vertical axis, generally speaking, point speakers toward
the back of the room, not to the middle of the room. This will produce
a more even response throughout the space. Speaker coverage
patterns have shapes, try and make those shapes complement
the shape of the room. Figure 1 shows a speaker on a stand pointed
at the center of the room. Notice the increase in level at in the third
row. In Figure 2 the speaker is pointed at the back of the room, notice
that the sound pressure levels are more even for each row of the
audience.
This method isn’t a substitute for a more extensive sound system
tuning and calibration, but sometimes ringing out the room is all
that is required to get through the gig. The process can also be used
as a tool to help the engineer understand the limits of the sound
system.
Extra Notes
There are a few other items of which to be aware before attempting
to ring out the room. It should go without saying, but the
physical relationship between the microphones and speakers is very
important. Putting microphones in front of the speakers is almost
always impossible, or at very least frustrating. A microphone should
maintain several feet of distance behind or to the side of any speaker.
When pointing the speakers, be aware of the walls. Sound reflects
off of hard surfaces. Try and point the speakers so that the pattern of
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