"What, again, does the BFD do? In any event it sounds very impressive... "
Management summary: "it sounds better when you are done".
Some details if you want to read on:
As Dennis said, the BFD (no, it doesn't stand for what you think - rather it is "Behringer Feedback Destroyer") is, among other things, a parametric equalizer. For all practical purposes, it allows you to fine tune the frequency response (FR) of your listening environment. The goal is to get rid of the peaks and valleys of sound pressure (loud and soft areas) in the bass region of your overall room FR. Due to room effects, some bass frequencies can overpower (peaks) what you are hearing in the bass range, or conversely, you can lose bass in the range where you have dips.
Almost any HT can benefit and get better bass with some form of equalization. Even great subs like the Axiom EPs (and of course my SVSs :->) that are designed and tested to have an essentially "flat" FR in their range do this in an anechoic environment. Put them into a room and suddenly you can find yourself getting some frequencies boosted and others eaten by the way the bass frequencies positively or negatively reinforce each other (remember your wave theory in physics?) as they bounce off of walls, ceilings, couches, marble floors (like Dennis has), etc. Add large openings into other parts of the house, drapes, pictures on the walls - and you can see how crazy it can get.
In order to counteract the effects of the room on bass, what we do is measure the FR of the room using test tones and a SPL meter. We plot those on a graph of frequency versus SPL. In an anechoic environment, what we would see is a fairly flat line at a given SPL across the sampled frequencies that usually match the manufacturer's specs pretty well. In the real world, we'll see places where the SPL will be +/- several dB (sometimes 10 - 15 or more in extreme cases) which of course does not allow you to faithfully reproduce the bass from the source material.
Using an equalizer like the BFD, we can cut the peaks and boost the valleys (unless it is a null - another story) to get to as close to a flat FR as possible. We may then apply a "house curve" to the bass that essentially boosts (or doesn't cut as much if needed) the lower frequencies since the human ear is less sensitive to bass as the frequencies get lower. What the result will be is a uniform reproduction of bass, in your room, that has the effect of cutting out "boominess" and tightens up the perceived bass response significantly.
A parametric equalizer differs from a graphic equalizer (you've all seen graphic equalizers - they are the gizmo's with all the sliders at different frequencies to cut and boost the sound at those frequencies) in that it allows you to arbitrarily set the center frequency of the band you are equalizing, and to apply a variable width of frequencies around that mid-point to which you will then apply boost or cuts. Graphic equalizers are less flexible due to the fixed nature of frequencies that they can affect, and the width of the frequencies they affect.
EQing a room for bass is part science and part art form. I know I've EQ'ed my room many times and gotten different results each time - some better and some worse. In any case it is simple to see what your room looks like if you have a SPL meter (we all do, don't we?) and some test tones. We play the test tones (usually start at 16Hz and go up at 1/6 octave intervals) and graph the result. If there are wild swings, or significant peaks and valleys in the resulting graph, you will probably realize benefits by applying some EQ.
Hope that helps.
Cheers,
Doug
