Sabine POSITIVE FEEDBACK User Manual

Version 3

™

Positive Feedback

The Advantages of FBX Feedback Exterminators

INSIDE:

Story of Feedback 1

Equalization 2

The FBX Solution 3

Glossary of Tech Terms 5-8

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®

The Story of Feedback

By Doran Oster, President

Ever since Lee DeForest
invented the first vacuum tube,
engineers have walked the
tightrope between feedback
and system gain. The pur­pose of this guide is to give
you the tools to get all the
gain you need without the
agony of feedback. We’ll start
with a common-sense discus­sion of the techniques sound
engineers now use to control
feedback to get the most gain
and clarity out of their sound
systems.

Our imaginary
work bench

Imagine a mic and speak­ers set up in a tiny shower
room. Clap your hands. The
sound reverberates back and
forth between the tile walls
and floor. Just a touch of the
volume fader fills the room
with screeching feedback.
Now move our sound sys­tem out to an open
grassy field. Clap your
hands. There is no
echo. The speakers are
well away from the
microphone and there
are no reflections, so
now we can really crank
up the system without a
bit of feedback.
Most sound systems
have characteristics that
fall between these two
examples, but examin­ing the extreme cases makes
it easier to understand the
more common in-between situ­ations.

Fig. 1: Feedback Loop

What is acoustic
feedback?

Feedback is the loud ringing
sound that occurs when the
sound leaving a speaker is
picked up by a microphone and
reamplified again and again.
(See Fig. 1.) The cycle repeats
until the feedback reaches the
system’s maximum loudness or
until someone turns down the
volume. Virtually every sound
system that has a microphone
and a speaker in the same room
is susceptible to feedback.
Which frequencies feed
back? All acoustic systems
have distinct resonant frequen­cies. Regardless of where you
thump a guitar’s top, it always
responds with the same tone.
This is the natural resonant fre­quency of the guitar. It is the
frequency where all of the
instrument’s components vibrate
naturally as a unit. In sound
systems, these resonant points

are the frequencies where feed­back occurs.
Each of the system’s com­ponents, including and especial-

ly the room itself, has its own
set of resonant frequencies.
Each component adds togeth­er to produce the total sys­tem’s resonant frequencies. It
is almost impossible to predict
which frequencies will feed
back without first “thumping”
the system, but you only have
to turn up the amp for them to
rudely reveal themselves.
The frequency that feeds
back first is the one that
requires the least amount of
energy to excite the resonance.
If you remove the first feedback
frequency, the next feedback
frequency will be the one that
requires the second least
amount of energy, and so on.

Controlling feedback

In order for feedback to
occur, the amplifier has to be
turned up enough so that
sound from the speaker re­enters the microphone louder
than the original sound. In our
imaginary experiment, feed-

back easily
occurred in the
shower room
because the
sound leaving the
speakers did not
dissipate very
much before re­entering the
microphone. But
when we move
the speakers
away in the open

field, the sound
energy dissipates as it radi­ates away from the speakers.
If there are no surfaces to
reflect the sound back to the

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EQUALIZATION

mic, the sound quickly loses
energy, dropping to one quar­ter the energy every time the
distance from the speakers is
doubled. By the time the
sound finally reaches the
microphone, the sound energy
is weaker than the original
sound, so there is no feed­back. From this example we
deduce the Prime Directive of
Feedback Control:
Keep the sound emanat-

ing from the speakers away
from the microphones as
much as possible.

Here are the most com­mon tricks of the trade for
controlling feedback:

• Stand close to the micro­phone. Speak loudly and

clearly so that you do not have
to amplify the sound too much.

• Each open microphone
has a chance to feed back.

Mute or turn down the gain of
any microphone that is not in
use. Noise gates can be help­ful for this.

• Mount the microphones in
fixed positions. Moving the

microphone around on the
stage increases the chances
that the microphone and the
speaker will form new resonant
paths.

• Use cardioid or hyper-car­dioid microphones, and point
the mics away from the
speakers. They pick up much

less sound from the back side
of the mic, which protects
against monitor feedback. Be
careful not to put your hand on
or too close to the micro­phone’s screen, since this can
cover the ports that enable the
heart-shaped (hence cardioid)
rejection pattern.

• Place the speakers in front
of the microphones so there

is not a direct path back to the
microphone.

• Aim the speakers so the
sound does not reflect

directly off a wall back into
the mic. You can estimate the

speaker’s dispersion pattern
(the area that is directly
“sprayed” with sound) for the
mids and high frequencies by
imagining rays of light radiating
out of the speaker’s horns. If
you can see the center part of
the horn, you are probably in
the dispersion pattern. Lower
frequency sounds tend to radi­ate out in all directions from all
sides of the speakers.

• Make the surfaces of the
room as sound absorbent as
possible to reduce sound

reflections. Use acoustical
absorbing tiles in the ceiling, put
down carpeting, and hang
curtains.
In the real world of most
performance spaces, you can­not always follow these anti­feedback techniques. Lead
singers insist on pointing the
monitors directly at the mic.
Worship leaders insist on the
mobility of a wireless micro­phone, and night club owners
will not likely carpet the dance
floor and hang velvet curtains.
Even after you’ve tried all these
tricks, you may still not have
enough gain and clarity to satis­fy the audience. Do the best
you can, and then go on to the
next level of feedback control:
equalization.

Equalization

Equalizers (EQs) are sets
of filters, or volume controls, for
different parts of the audio
spectrum.
Since the earliest days,
sound engineers have used
equalizers for two distinctly dif­ferent purposes: 1) To improve
the tone quality and balance of
the sound, and 2) To control
feedback for extra gain and
microphone mobility. Some
types of EQs are best at shap­ing the tone and other types are
better at controlling feedback.

It may seem paradoxical to
add filters to a sound system in
order to increase the gain. But
if you can use extremely narrow
filters to turn down the frequen­cies that are feeding back, you
will be able to increase the gain
of all the other frequencies for a
total net gain. There are essen­tially three categories of equal­izers: graphic, parametric and
adaptive parametric.

Graphic EQ

Graphic EQs are basically
a set of volume controls for
individual sections of the audio
spectrum. The earliest music
equalizers were the bass and
treble tone knobs. As technolo­gy advanced, these filters were
narrowed to give more precise
control. Today, the industry
standard is called a 1/3-octave
graphic equalizer, which has 31
individual volume controls
spaced 3 per octave.
There is a common miscon­ception in the industry about
1/3-octave EQs that is impor­tant to this discussion. Many
industry veterans incorrectly
presume that 1/3-octave EQs
use 1/3-octave wide filters. If
this were the case, the EQ fil­ters would not be wide enough
to create smooth curves.
Instead, they would produce a
notched frequency response
that would make the EQ use­less for shaping the sound and
useless for controlling feedback
frequencies between the slid­ers. Actually, most manufactur­ers use 3/4 to 1-octave wide
overlapping filters placed on
1/3-octave center points. These
wider filters provide the neces­sary smooth frequency
response. (See Fig. 2.) It’s

important to understand that
the term “1/3-octave” refers
to the spacing of the sliders,
not the filter width.

Graphic EQs are excellent
for shaping the sound, and they

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