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Discussion Starter #1
Sorry for this repeat post - it is now in the correct place.

Hi- new to the forums:smile:

I have a pair of diy speakers that are 'open air' - i.e not even a baffle.

The two woofers each side have to be carefully equalised so as to not result in too much excursion at higher volumes.

I am using REW to generate different filters for the woofers and the mid/high unit.
Using eight uncompressed audio feeds via hdmi, enables me to allocate different filter corrections to the separate drivers

I need to include a brickwall filter - or something pretty steep - at 40hz, to make sure no lower gets to these woofers.
I would also quite like to make the crossover frequency quite steep as well.
Any idea how I can get more than 12db/octave using REW 'generic' filters?
I am using 'generic' filters because the filter data is exported to Equaliser APO for final output to a multichannel amp.

Any advice would be much appreciated

Don
 

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Each filter in REW is a single biquad, so 2nd order, 12 dB/octave. You would have to use multiple filters to get faster rolloff.
 

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Discussion Starter #3
Hi John, thanks for the reply.

So am I right in saying ( having searched this, as it is not my subject ), for a single 2 pole filter, a Q of .7071 is optimal?
If I cascade two ( i.e a 4 pole filter ) , then from what i read two different q values are needed? - that is .541 and 1.30?

Also will two cascading filters give 24db/octave, and so on?



Thanks

Keith
 

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Yes, that's pretty much it. Higher order filters can be decomposed into a series of biquads. If a set of LPQ filters with the same cutoff frequency and 0.7071 Q are cascaded then the attenuation at the cutoff frequency increases by 3 dB for each extra filter and the rolloff increases by 12 dB/octave. The different Q values compensate for the attenuation at the cutoff by adding some peaking to pull the response back up to -3 dB. You can see the effect in REW's EQ Window if you turn on the option to show individual filter responses.
 
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