# How to use BFD 1124 as subsonic filter



## bommers (Oct 30, 2010)

Hi - I have two ported subs running with correction with BFD and REW. I need a subsonic filter, the crown amp I am using does not have crossovers or EQ. Can I use the BFD to create a subsonic filter at 15 or 18 hz to prevent overexcursion of driver? What other options for a in line subsonic filter?


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## Wayne A. Pflughaupt (Apr 13, 2006)

Unfortunately, the BFD does not have a subsonic filter. Our DIY Subwoofers Forum has a section dedicated to hi pass filters – I’d suggest posting a question there.

Regards, 
Wayne


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## eyleron (Dec 31, 2009)

Can someone point me Illka's post that explains how to do this via creative use of the available filters? 
TIA


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## Wayne A. Pflughaupt (Apr 13, 2006)

Don’t know where the post is, but it’s actually more ridiculous than creative.

True, if you try hard enough the BFD it can be set up to “mimic” a high- or low-pass filter, but that requires a plethora of filters, and in the end _it is impossible for an equalizer to deliver the same kind of “constant slope” a crossover does,_ that continues to drop each octave out from the turn over frequency.

And let’s take a closer look at the required “plethora of filters.” As you may know, equalizer filters alter frequency response by shifting the phase of the signal passed through them (see this article for a more technically detailed description). Using an equalizer as it’s intended, that’s not a big deal because it’s essentially counteracting phase issues caused by the room that are altering a speaker’s perceived response (i.e., sound bouncing around the room and arriving at the listening position at different times).

But that’s not the case when you try to force an equalizer to behave like a crossover by using a bunch of filters. Each filter is introducing phase, but none of them are counteracting anything from the room. You’re just needlessly adding artifacts to the signal that don’t need to be there.

Bottom line: If a subsonic filter is what you need, then get a component that has one.

Regards, 
Wayne


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## eyleron (Dec 31, 2009)

Thanks, I'll look for that in a MiniDSP, iNuke DSP, etc. 

I'll refer Mark Seaton and Illka here next time they suggest one could/should be protecting their sub with a HPF using the BFD, giving one false hope.


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