Theatre Design & Technology - Winter 1985 - 18

SOUND
by Rollins Brook

FEEDBACK m ~hsa~
dreaded of all sound system problems!
Surely, if we can put a man on the
moon, we must have the technology to
design a sound system that will not feed
back. Alas, we do not. At present, we
are unable to guarantee no feedback;
but there are a few anti-feedback aids.
The most effective prevention involves
careful selection of equipment and an
understanding of how to use the sound
system in a particular theatre.
The measure of a system's sensitivity
to feedback is called available gainbefore-feedback. For speech reinforcement work the practical questions are:
In the back row of the theatre, how
much does the system increase the level
of a sound from the stage? Does it increase the intelligibility of speech sufficiently while remaining free of feedback? For music reinforcement, the
question is: Is the sound loud enough
to produce the desired physical / emotional impact?
Feedback occurs when too much
sound from the loudspeaker finds its
way back to the microphone. The diagram illustrates the four basic elements
in the loop that can affect both the onset and the control of feedback: the
microphones, the electronic chain, the
loudspeaker, and the acoustical path
between loudspeaker and microphone.
Feedback occurs first at the frequency where the loop gain is greatest.
As system gain is increased, feedback
will occur at more frequencies. Usually
the first four to twelve of these feedback points (or ring modes) are determined by narrow response peaks in the
loudspeaker; they are often reinforced
by acoustical features of the room.
While microphones are usually much
smoother, they can contribute to the
problem.
In both microphones and loudspeakers the factors of restricted directional pattern as well as smoothness of
frequency response are important in
feedback control. In feedback-prone
situations, a cardioid pattern micro16

by Rollins Brook

phone will usually work better than an
omnidirectional one, often improving
gain-before-feedback by 3 to 5 dB. (3
dB can be very important to speech intelligibility even though it is not a very
big increase in loudness.) Since ring
modes will usually coincide with peaks
in the frequency response of the microphone or loudspeaker, feedback is even
more likely to occur where each have
peaks at the same frequency (or frequencies). Using microphones and
loudspeakers with a smooth, or nonpeaky, frequency response is an important step in securing adequate gainbefore- feedback. (See detailed articles
on microphones and loudspeakers in
TD&TWinter 1981 and Summer 1983.)
The electronic chain can be used to
make feedback problems better or
worse. The widely-used Y3 octave
equalizer-sometimes called a "graphic" equalizer-can be an important tool
for removing broad peaks from the
loudspeaker response. A more narrowband equalizer ('I. to 'I" octave) is
sometimes used to remove a few of the
most troublesome peaks in the loudspeaker. Narrowband filters (usually
called notch filters) are also used to
reduce troublesome acoustical ring
modes. A parametric equalizer can also
be used as a notch filter. As with all
feedback tools, only a few dB of
improvement can be expected from
equalizers.
Most modern mixing boards contain adjustable equalizers that can be
used to alter sound arriving from the
microphones. In feedback-prone situations, the operator must use these
equalizers carefully in order not to aggravate the tendency to feedback.
Two specialized electronic devices
have been developed especially for the
control of feedback: the frequency
shifter and the feedback (or ring) suppressor. The frequency shifter acts to
break the loop by slightly changing the
frequency of all sounds passing through
it. This small shift in frequency (usually less than 10 hertz) is enough to break
the loop and prevent feedback from occurring. But there are several limitations. Feedback can bridge this fre-

Theatre Design & Teehno(ogy/USITT/Winter 1985

quency gap if the gain is high enough.
As the system approaches the onset of
feedback, the shifter creates an unworldly sound-somewhat akin to the
voices of the robot creatures on Battlestar Galactica. The sound coloration is
generally more obvious and disconcerting than the more pure tone of normal
edge-of- feedback.
Another objection to the shifter
comes from musicians. They play or
sing a note, yet they hear it coming out
of the system slightly off key. This can
be very annoying to performers as well
as to audience close enough to hear
both the live and reinforced sounds.
Obviously, the frequency shifter has
very limited applications in music reinforcement; it is best used as a speech
device.
The shifter can, under ideal situations, yield as much as 10 dB of additional gain - before- feedback. Its usefulness is largely dependent upon the
acoustical qualities of the room-the
shorter the reverberation time, the more
the shifter helps. It does not work well
in a large, reverberant, multipurpose
auditorium. Unfortunately, it is these
large reverberant rooms that usually
need the most help in feedback control.
The ring suppressor is a gain-shifting
device controlled by a "smart" electronic circuit that is able to recognize
the onset of feedback. At the first sign
of feedback, the control circuit lowers
the gain of the system until the feedback early-warning signs disappear.
The original gain setting can be restored
by manually pressing a reset button.
The control circuit can, on rare occasions, be fooled by sustained musical
notes. One could also imagine a tugof-war between an operator sneaking
up the level control of the mixer while
the suppressor is reducing its own gain.
But the real problem with this device is
that it reduces the volume of the whole
spectrum even though the feedback is
only a narrowband problem. With
modern digital circuitry it is possiblealthough still too expensive-to design
a control circuit that can identify the
frequency at which the feedback starts
and then synthesize a narrowband filter

Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1985