SABINE FBX2420 Spécifications télécharger pdf (Page 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 amplied 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-

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 FBX1220 and FBX2420 systems solve feedback problems by precise attenu-

ation 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 delity of the signal. We call

this automatic lter an FBX Feedback Exterminator® lter, or FBX lter 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 lter offers three distinct advantages over graphic

lters.

1. FirstandmostobviousistheautomaticnatureofFBXlters.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-ltersarepreciselyplacedanywherefeedback

occurs(with1Hzresolution),whilegraphicEQltersarelimitedto31xedcenter

points.AnFBXlterrepresentsadirecthitonfeedback!Incontrast,agraphicEQ

ltercanonlyapproximatetheexactfrequencyofthefeedback,andthelter(orlters)

withtheclosestcenterfrequencymustbepulleddown.Suchltersaredeepestat

theircenters,andsuchimpreciseattenuationtakesabig(andunnecessary)chunk

outofyoursound(seegure7a).

3. Increasedclarityandgain-before-feedbackarefurtheraccomplishedbythethirdand

mostimportantadvantageofFBX:Sabine’smicro-ltersaretentimesnarrowerthan

31-bandEQlters.UsingFBXmicro-lterswillreturnupto90percentofthepower

removedbyEQlters.

Here’s a good place to make a very important distinction. Graphic EQ lters are typically

called “1/3-octave,” but it’s important to understand that this term refers to the spacing of

the lter centers (1/3-octave apart), and not the width of the lter (usually a full octave).

Graphic lters 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, gure 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 rst 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 gure 7b. Note how much more

of the program (the “good audio”) is eliminated using an EQ — whereas only feedback

is eliminated using FBX lters.

7.3. Parametric Filters and FBX

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.

1 2 3 4 5 6 7 8 9 10 11 12

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