8
SeCtIon SeVen — FBX tHeorY & PrACtICe
7.1. Introduction to FBX®
WHY FBX? Feedback is certainly the most pervasive challenge to the audio industry. The
potential appearance of sudden, loud, out-of-control feedback is every sound engineer’s
and musician’s nightmare. Unlike more subtle audio quality problems or shortcomings,
feedback is embarrassingly obvious — it disturbs the performer, the audience, and the
technician, and can damage equipment and just generally ruin your day.
Feedback is a potential problem in any amplified sound system that places a microphone
or pickup in proximity to a loudspeaker. Poor acoustical conditions or misguided use by
unsophisticated sound system operators only aggravate the situation. To make matters still
worse, a non-Sabine variety of wireless microphone adds yet another level of feedback
danger to the picture. Since feedback erupts whenever the distance, location, and gain
relationships between a speaker and a microphone reach a critical combination, a mic
that can move anywhere results in an ever changing potential for feedback. A step in the
wrong direction may change a clear sound to a piercing shriek in less than a second.
This enhanced potential for feedback with a wireless system gets worse if lavalier mi
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crophones are used. Such microphones are usually placed farther from the mouth than
handheld or head set microphones, thus requiring more gain. Also, the polar pattern
of a lavalier microphone is frequently omnidirectional. Thus, the likelihood of feedback
increases, due to the microphone’s increased off-axis sensitivity to the sound emanating
from the loudspeakers.
The Sabine FBX2410 systems solve feedback problems by precise attenuation of very
narrow bands of feedback-prone frequencies. The process is automatic, simple to use,
adaptable to changing acoustical conditions and relationships, powerful in its application,
and has minimal consequences to the audio fidelity of the signal. We call this automatic
filter an FBX Feedback Exterminator® filter, or FBX filter for short.
7.2. the Advantages of FBX Filters
Before the invention of FBX, the most common device for controlling feedback was the
31-band graphic EQ. However, an FBX filter offers three distinct advantages over graphic
filters.
1. First and most obvious is the automatic nature of FBX filters. When feedback occurs,
FBX responds more quickly than even the most experienced engineer. Automatic
FBX placement works even in the presence of audio program material, intelligently
distinguishing feedback from music or speech.
2. A second advantage is that FBX micro-filters are precisely placed anywhere feedback
occurs (with 1 Hz resolution), while graphic EQ filters are limited to 31 fixed center
points. An FBX filter represents a direct hit on feedback! In contrast, a graphic EQ
filter can only approximate the exact frequency of the feedback, and the filter (or filters)
with the closest center frequency must be pulled down. Such filters are deepest at
their centers, and such imprecise attenuation takes a big (and unnecessary) chunk
out of your sound (see figure 7a).
3. Increased clarity and gain-before-feedback are further accomplished by the third and
most important advantage of FBX: Sabine’s micro-filters are ten times narrower than
31-band EQ filters. Using FBX micro-filters will return up to 90 percent of the power
removed by EQ filters.
Here’s a good place to make a very important distinction. Graphic EQ filters are typically
called “1/3-octave,” but it’s important to understand that this term refers to the spacing of
the filter centers (1/3-octave apart), and not the width of the filter (usually a full octave).
Graphic filters thus overlap one another, and affect frequencies well above and below
the center point frequency, including frequencies of adjacent bands. This makes graphic
equalizers very practical tools for shaping sound “with broad strokes,” such as dialing in
overall system EQ, but results in destructive audio quality overkill when they are used
to eliminate feedback. A graphic equalizer would need more than 10,000 narrow-band
sliders to be as precise and powerful as your FBX.
As an example of the power of FBX, figure 7b shows test results measured with a PA set
up consisting of a microphone, mixer, FBX Feedback Exterminator®, power amp and two
speakers. The system’s gain was first raised until the FBX removed nine feedback points.
Next, the FBX was replaced with a graphic EQ. The EQ was adjusted while the system
gain was raised to the same level achieved with the FBX. The frequency response curves
of each device were then plotted and are compared in figure 7b. Note how much more
of the program (the “good audio”) is eliminated using an EQ — whereas only feedback
is eliminated using FBX filters.
If the graphic EQ really had 1/3 octave
filter widths, the frequency response curve
would vary 6 dB between sliders. This
would ruin the sound.
Graphic EQs usually use one-octave-
wide overlapping filters that provide much
smoother frequency response curves.
Notice that the overlapping filters add
together to cut -16 dB when the sliders
are only pulled down -10 dB.
-10 dB cut at 500, 630, 1K, 1.25K, 1.6K & 2K Hz
Fig. 7a.
