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Old 3rd January 2003, 08:04 PM   #21
Pjotr is offline Pjotr
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BTW: Q=0.5 is only true for 2.order (12dB) LR-filter whereas 4.order LR-filter(24dB) is nothing less than two cascaded 2.order Butterworth filters (Q=0.7)
Hi Cocolino,

If you put two Butterworth filters with a Q of 0.71 in cascade you will end up with an overall Q of 0.5 isn’t it? But in general I agree, x-overs involve much more than only the x-over filter characteristics. It is the total response that counts.

Hi Bjed,

Think I understand what you mean. But my “Dutch English” is of help maybe

I did a lot of experimenting with taming the Qb of closed boxes with electronic filtering. I have found that for small living rooms of say 4m x 6m a gentle bass roll-off (Qb =< 0.5) gives me the most natural bass reproduction. Then room modes/resonance’s are less aggressive exited: If you don’t put the energy in it can’t come out.

Even if the bass starts to drop at say 60 Hz but it drops gentle and 20 Hz is more than 15 dB down, then that 20 Hz can be heard, or better “perceived”, quite natural. Although it will not shake your stomach. To me this sounds more natural than a ruler flat response to 30 Hz and then dropping steeply as with some BR alignments. There is also something like natural “bass amplification” of the volume of the room itself that needs to be taken into account. I have put a small and quick engineering note about the background of filtering closed boxes online HERE for download as a .zip file. But please don’t blame for typo and grammar errors. It also contains an Excel sheet to calculate the filter response and values for a wanted target response. With the filter you can even make Qb as low as 0.25 but lower than 0.5 gives a rather thin bass.

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Old 3rd January 2003, 10:19 PM   #22
cocolino is offline cocolino  Germany
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If you put two Butterworth filters with a Q of 0.71 in cascade you will end up with an overall Q of 0.5 isn’t it?
I assumed the reason behind Your (unfounded) claiming that Butterworth filter were no good for high quality audio and that LR-filters would be a better choice was related to the more "smooth-shaped/ better pulse response thing" rather for just the level of attenuation (what IMO makes even less a point regarding this ) hence I wanted to explain that 4.order LR-filter actually = two 2.order Butterworth in series. Of course You`re right in that the 4.order LR-filter has 6dB (Q=0.5) attenuation at fx-over.

I fully second what You said about taming with electronic filtering!
Due to room gain at low frequencies speakers with flat low-frequency free-field response tend to sound boomy (particular in smaller rooms).
Electronic manipulation with a LR-transform circuit together with high-pass filtering is a very handy tool in compensating this effect and to achieve more natural though deeper bass.
Most people tend to believe the electronic manipulation will yield to huge amplification factors and need monster amps. Not so when applied accordingly and reasonable (eventually in conjunction with a subsonic-filter for protecting against weird ultra-low frequency effects). Taken into account the filters compensating slope for the room gain, in fact the power demand for such electronic filtered designs do not neccesarily have to exceed the unfiltered/unequalized by such a big amount (and besides this, most of the recorded music rarely contains really low frequency/ high level information).
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