[Wayne A. Pflughaupt]

Otto wrote:

Hi Wayne,

Yeah, it's for sub only. The thing I'm looking for is a clean way to make a LPF, so that I can disengage bass management from my preamp. I think I could do this more effectively with the DEQ as opposed to the BFD because of the GEQ and shelf filters in the DEQ. Whaddya think?

A shelving filter doesn’t act like a low- or high-pass crossover filter, where response is 12 dB down an octave out, 24 dB down two octaves out, and so on. It’s merely another type of EQ filter, and as such its action is fully dependent on the amount of boost or cut you apply. It works like this: From flat response with a 0 dB reference line, if you apply a 6 dB cut, response drops 6 dB, and then is flat again below that point. In other words, it “plateaus” response, not endless drops it.

As such, describing a shelving filter having a 6- or 12-dB slope, as Behringer does, is just wrong. For instance, how can you have a true 6-dB slope if the filter is adustable +/- 15 dB?? I dunno, maybe the DEQ locks down the amount of boost or cut when you engage the shelving, or maybe the 6- or 12-dB reference has to do with how steep the “hinging” action at the center frequency is. The manual doesn’t say much about it either way.

Regarding mimicking a shelving or low-pass filter with the BFD or DEQ, you may find that difficult to accomplish.

Recently I decided that EQing my sub flat wasn’t getting it for me, so I thought I’d try my buddy brucek’s famous house curve filter to get me about a 6 dB or so reduction at 100 Hz. Well, it didn’t go too well.

I established a reference 32 Hz SPL reading, to make sure my filter wasn’t making any changes at that point – it was only supposed to affect above that. But nothing I tried worked the way I needed it to. I first started with brucek’s recommended two-octave 366 Hz filter cut 15 dB, but that didn’t get it. I forget exactly what it was, but I think that it wasn’t getting me enough cut at 100 Hz. Deepening the cut, when I got what I wanted at 100 Hz, I found that it had affected all the way down to 32 Hz – my reference SPL level had dropped. I trued moving the frequency up and down, different (mostly tighter) bandwidth settings and cut values. I eventually gave up and totally re-equalized for the house curve. I guess the problem was that according to brucek’s predicted filter response picture (in my house curve sticky thread), the house curve filter only gets about a 4 dB reduction at 100 Hz. If you need something different, you’re on your on. You’ll pretty much need a real-time display to figure it out (or whatever that nifty program is that brucek created that picture with!).

Based on that, I’d hate to try to figure out a shelving filter without something like TrueRTA to let you see exactly what’s going on. Keep in mind too, that by the time you achieve a 12 dB slope, you’re probably going to end up with a fairly tight filter, and it will only be a maximum of 12 dB down at the center frequency. Going deeper, or trying to achieve a steeper slope is going to require an even tighter filter. This is especially problematic since you want to hinge at 30 Hz or so: You’re probably going to find that by the time you get to 70 Hz or so, everything will be back up!

Anyway you cut it, Otto, you just can’t create a decent low- or high pass filter with an equalizer, especially at the low frequency you want to. If that’s ultimately what you want, I suggest just getting a real crossover. Behringer has some cheap ones. I might suggest moving up to something like the Rane AC-22

Regards,
Wayne