Bruce,
This is indeed a complex and interesting topic.
Quote:
|
I find it very hard to blend a speaker with a 18-36dB/octave low frequency rolloff/slope --that many ported speakers have-- plus the 12dB/octave electrical slope of the HT processor for the high-pass side of this crossover equation (equals 30-48dB/octave slope) with the low-pass side of this equation which the HT processor models on a 24dB/octave slope.
|
I have struggled with this myself. However, since most receivers and pre-pros have symmetrical slopes, one must assume that they beleive the actual speaker rolloff is far enough away (in frequency) from the crossover to be insignificant. The filter design experts would say you should be a decade apart to fully eliminate interaction. Nobody makes a main speaker with 8Hz rolloff, so this clearly is not the case. In my experience, however, if you're an octave away, it's pretty safe. A lot of main speakers do have 40Hz capability.
The idea of matching the crossover frequency/slope with the speaker rolloff characteristics is not new, but is rarely considered or implemented. It's just not practical for the mass market. As you pointed out, receivers and most pre-pros have limited crossover options... and few main speakers have precise 12dB, 80Hz rolloff. It's hard enough to educate the masses on picking the right crossover frequency...let alone matching asymmetrical slopes to speaker rolloff characteristics.
One of my favorite designs, on paper, is to match a passive RC filter (between pre and main amps) to sealed mains for a true third-order rolloff. This avoids active crossover circuitry in the signal path to the mains and minimizes noise and distortion. Then just use a third-order low-pass on the sub for seamless integration. I've done a few of these designs and they work well. A variation on this is to use 2nd order highpass with sealed mains and 4th order sub lowpass for matched Linkwitz-Reilly responses -but this requires active filters in the main signal path.
However, I've had equal success with symmetrical 24dB LR filters (using my own analog active filter designs or the 24dB LR option in my Lexicon MC-12). In fact, my current theater system has symmetrical 24dB 70Hz slopes with 12db/52Hz speakers
. This measures quite flat through the crossover region in my room. The key here is that rooms often have ragged response this region that easily swamps out a few dB of peak or dip in sub integration. If you have some test gear and are willing to experiment a bit, you can often
flatten overall room response with a deliberate crossover mismatch!
Happy experimenting!
My home page with theater and speaker details