Maxiumum Octave Range for BFD or FBQ

1759 Views 6 Replies 3 Participants Last post by mpotoka, Feb 20, 2009

mpotoka

Discussion Starter · Feb 19, 2009

I am modeling a small sealed sub that would fit inside my Ikea Expedit bookcase. The hole is 13.25"x13.25" I already have 1 Dayton DVC 10" driver so I figured I'd pick another one up. Not the idea drivers but a neat little experiment non the less.

I was researching building an LT for this box but was able to basically reach the same values by adding a few PEQ cuts of the 12 dB range. However the Q I was using in WinISD was that of .08. Is this a range capable by either the BFD or the FBQ? I know I have seen 120/60 listed before--is that the max width? The conversions I have looked at here: http://www.sengpielaudio.com/calculator-bandwidth.htm suggest that a 120/60 is a Q of .667, and that my .08 Q would be 7 octaves (or 420/60). Are my numbers correct? Does that mean I would just need to use a series of 2 octave cuts? Although if my sub is being cut by the receiver at 80 hz then it "seems" I wouldn't need more than a couple octaves anyways...

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brucek

· #2 · Feb 19, 2009

Can you explain what it is you're attempting to do?

I don't understand?

brucek

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Wayne A. Pflughaupt

· #3 · Feb 19, 2009

Yeah, its a bit peculiar what you're asking. 0.08 Q = 7 octaves; the FBQ has bandwidth settings up to 10 octaves, but from what I can see modeling its filters in REW, it's not accurate above about 2-octaves. The 10-octave setting is more accurately a 5-octave filter.

That said, there has to be a fundamental miscalculation or misunderstanding somewhere on your part. Keep in mind that the entire audio spectrum from 20Hz-20 kHz is 10 octaves. Also, a so-designated 7-octave filter will affect that much both above and below the center frequency. As such, a 7-octave filter will effectively wipe out the entire audio spectrum. Somehow I doubt this is what you really want/need to accomplish...

Regards,
Wayne

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mpotoka

Discussion Starter · #4 · Feb 19, 2009

Well--I guess doing this during the day works out a little bit better. I want to take my two subs (modeled as 2 drivers in 1 box but actually will be 2 boxes 1.2cf each (I can't go any bigger)

Instead of making a Linkwitz Transform for the subs I want to use a BFD. I want to turn the yellow line into the green line. I cannot use an LT because I don't have a high excursion or high power handling driver. But this is a living room in an apartment with neighbors--and I feel this is not a bad result from 2 $40 drivers and a small sized amp.

So I am limited to 96.5 db (plus room gain). I would have an f3 of 22.8hz and an f10 of 14.5hz. All theoretical of course--but I have one of the drivers and an amp, so for the cost of one more driver, some MDF, and a BFD it sure seems like a fun experiemnt.

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mpotoka

Discussion Starter · #5 · Feb 19, 2009

Man those pictures turned out small. Anyways--I guess really the only bigger than 2 octave curve I am using anymore is for the house curve--but I didn't put the lowpass of my receiver in the model either so who knows what I'd actually need. Of course some REW will show me once they are built...

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