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DFR11EQ Theory
Feedback and DFR11EQ Operation
When
acoustical feedback occurs in a sound system, it is because the gain of the
system
is
too high. Since no sound system (microphones, loudspeakers, room acoustics, etc.) has
an absolutely flat frequency response, feedback will occur at specific frequencies before
others; these are the frequencies with the most gain. If the gain at only these specific
frequencies
is lowered, then the system can operate with more overall gain before it
feeds
back, without
a
perceptible dif
ference in tonal quality
. This is the operating principle of the
DFR11EQ.
At
the heart of the DFR1
1EQ is a very
powerful algorithm that can accurately and quickly
discriminate between feedback and non-feedback sounds (speech and music). When this
algorithm
detects feedback, it smoothly inserts a –3 dB,
1
/
10
-octave notch filter into the audio
path
to reduce the gain at the frequency which is feeding back. If
the feedback does not stop,
the
filter depth is increased in 3 dB increments (up to –18 dB) until the feedback stops.
After the DFR11EQ stops the feedback at one frequency, the sound system may start
feeding back at another frequency. In this case, the DFR11EQ inserts another notch filter
into
the audio path at the new frequency
. The DFR1
1EQ can insert a total of 10 notch filters to
reduce
feedback.
DFR11EQ Limitations
The
DFR1
1EQ (or any other notch filter system) cannot entirely eliminate feedback in a
sound
system, it can only help to reduce it.
In a typical system, a point of diminishing returns
is reached after 4 to 8 notch filters are set. This is because generally there are only a few
dominant
frequency
response peaks above the response of the entire system (see diagram
below).
The DFR1
1EQ works very well controlling these peaks. The user can expect a 6 to 9
dB improvement of gain-before-feedback in a typical system. However
, if the system has
too
much
overall gain, then all of the frequencies have too much gain; instead of trying to notch
out
all of the frequencies, better results will be obtained by lowering the gain of the system. If
the
system still has insuf
ficient gain, then other changes must be made to the sound system
such as different microphone or loudspeaker placement. There are several excellent
publications which delve deeper into the issue of feedback and sound systems, including:
Acoustic Feedback – How to Avoid It, Vivian Capel, Bernard Babani Publishing, ISBN
0–85934–255–7; Sound Reinforcement Handbook
, Gary Davis and Ralph Jones, Hal
Leonard
Publishing, ISBN 0–88188–900–8; Sound System Engineering
, Don and
Carolyn
Davis, Howard W. Sams and Co., ISBN 0–672–21857–7; “Understanding Sound System
Design”,
Rick Frank, Shure
Brothers, # AL1
174; “Basic Handbook of Feedback Reduction”,
Matt
Anderson and Jon T
atooles, Shure Brothers, # AL 1280.
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FREQUENCY (Hz)
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