# Passive 6dB/oct filter results



## Anthony (Oct 5, 2006)

The problem I have been having is a steep rolloff with my dipole subwoofer. It was to be expected, but I was hoping my BFD and boost in the amp could EQ it flat. I tried this and the extra resonance and interference from all the filters made the sound weird, boomy, and just "off".

A quick primer on dipoles. Based on baffle size, the rear wave interferes with the front wave and cancels out. This starts at a frequency known as Feq, which is where the infinite baffle response equals the dipole response. Below that, the dipoles roll off at 6dB/octave to the driver resonance, then the rolloff is 12 db/oct. 

A solution many have tried is to put in a passive low-pass filter with a 6dB/octave slope. Properly placed, it can balance out the dipole roll off perfectly -- at the expense of efficiency. This is why drivers with large X-max and power handling are desirable (along with an amp powerful enough to drive them).

With help from other forum members and a few websites, I made such a filter. It is a simple R-C circuit with the resistor in series with the signal path and the capacitor shorting across the signal to ground. I'll leave you to my other thread to read the theory and all to selecting the proper values, but here's an overview of the math. 2*pi*f = 1/(R*C) where f is the -3dB frequency. 

Below you can see the immediate effects of varying f.
Purple is no filter
Blue is 28 Hz
Red is 16 Hz
Green is 12 Hz








I decided to stick with the 16 Hz filter (Red). There's still a peak at 60 Hz or so, but I decided to use the amp's parametric filter to snub that one out. Using the RTA feature of REW, I dialed in the smallest bandwidth filter I could, found the center, then set the level. It's nice to do that real-time and see the results.

Below, Red is still with the filter, Blue is with the notch filter dialed in, and Tan is run through the receiver (80 Hz, 2nd order crossover).








So I still have the nulls at 100 Hz and 30 Hz to deal with, but those are room-related. But this seems to be the correct path. The bass still sounds natural, but MUCH deeper.

Right now it's just held together on a terminal strip. I need to get a project box, some RCA ends and solder together something more permanent.


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## brucek (Apr 11, 2006)

Yeah, looks pretty good Anthony.......


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

Pretty interesting, Anthony! Hehe, I guess it's hard to find an active 6 db/octave LP filter at 12-16 Hz, huh? 

Still, I don't know why you couldn't get the same result using a BFD and the famous "brucek shelving filter." Did you try that? 










Regards,
Wayne


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## Anthony (Oct 5, 2006)

I did not try that, but I have given up on my current BFD. I tried moving the transformer around and it still hums/buzzes even with the sub amp off. I may replace it later, but for now this was made with stuff I had laying around 

Now my priority is fixing those 30 Hz and 100 Hz room modes!


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