# 2nd-Order BW filters

Discussion in 'Archived Threads 2001-2004' started by Travis G, Mar 10, 2002.

1. ### Travis G Stunt Coordinator

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Perhaps someone here can explain this better to me. I've been stuck on this paragraph for a couple days now. This paragraph was from Dickasons LDC:

2. ### Ryan Schnacke Supporting Actor

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This confuses me too. I have a feeling that it is related to the fact that adding a 2nd subwoofer (identical and placed in the same corner) will gain you 6dB while doubling the power to your existing subwoofer will only gain you 3dB.

3. ### Patrick Sun Studio Mogul

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When you sum the response of the HP driver (tweeter), and the LP driver (woofer) with BW filters at the same XO frequency, the nature of the BW filters will produce a summed hump of 3dB at the XO frequency. This has to do with the logarithmic summation of 2 sources both at -3dB in SPL. (Either do the math, or accept it as fact).

To smooth out the hump, Dickason is telling you calcalate the XO frequency at different frequencies. For the HP XO BW frequency, use 1.3x(XO Frequency), and for the LP XO BW frequency, use (XO Frequency)/1.3.

For example. If I want to use BW filter for XO frequency at 2000Hz, and want to smooth out the 3dB hump, I would find the values for the BW HP filter at frequency equal to 1.3x2000 = 2600Hz. For the LP filter, I'd find the values of the BW LP filter at frequency equal to 2000/1.3 = 1538Hz. This should produce a flat response at 2000Hz when the driver's SPL output is summed together.

To have filter responses to sum at a given XO frequency, you need to use L-R (Linkwitz-Riley) filters which provide the response down -6dB at the XO frequency, and when you sum to 2 responses, you get a flat response at the XO frequency. Again, that's just how the logarithmic math works out to get a flat response.

4. ### Travis G Stunt Coordinator

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I don't know Ryan. Pay no attention to me; I don't know what the hell I am talking about. I thought it Dickason was talking about how two drivers sum together (+3dB is the opposite phase of -3dB.)

Thanks Patrick Sun. I am beginning to see the light. I should have known that you would come to the rescue. I kind of was hoping for a hundered or so illustrations. JK

Travis

5. ### Patrick Sun Studio Mogul

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2nd order BW SPL curve:
/----------^-----------
2nd order L-R SPL curve:
/----------------------
There you go!

6. ### Brian Bunge Producer

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Pat,
I like those illustrations! Is that typical GA Tech geek humor? Remember, I went to DeVry so I may not be as quick to pick up on such things!
Brian

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