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In Reply to: RE: Subwoofer For Acoustat Model 3s posted by davidbeinct on November 04, 2015 at 11:46:21
L-R stands for Linkwitz-Riley. It's a crossover alignment that's become popular in recent years because it offers better performance as you move off axis. It's so popular that it's become pretty universal and your gear is likely to have it or to use it by default.
The "3" refers to the order, or number of poles. But forgive me for screwing up, since unlike other alignemnts LR filters come in only multiples of two! So we'd be talking second or fourth order. Each pole is 6 dB/octave, so the filters in a fourth-order crossover roll off at 24 dB per octave.
The rule of thumb here is that you can hear a driver until the crossover is about 20 dB down, so with a 4-pole crossover at 100 Hz, you'd be hearing a significant contribution from the woofer up to somewhat less than an octave up. Conversely, the stats will be 24 dB down at 50 Hz. You can see how both slope and crossover point affect output and the audibility of the crossover.
Follow Ups:
Thanks again. Is there any benefit in blending the filters, e.g. 12 dB per octave high pass to the mains but 24 dB per octave low pass to the sub? Obviously if I do that it won't tame panel slap quite as aggressively, but maybe it would help blending?
You have to be careful that the slopes complement one another, otherwise you'll get peaks and dips and other consequences. Asymmetrical slopes are used for various reasons but you likely have to have more control over the crossover characteristics and some measurement equipment to use them. Drivers can also be offset.
Another concern is that drivers have their own frequency response characteristics and these act as filters as well. So the net effect of the crossover is a combination of both filters. You'll sometimes see asymmetrical crossover filters used for that reason.
Basically, I'd try it and listen or better still measure -- measurements are great for aligning subs because you can tell what's going on, levels and frequency response, much more quickly with measurements than by ear.
You also have to consider that the lobing (off-axis behavior) can be a bit funky depending on the relative position of the two speakers. So some experimentation may be required. Safest is when they're the same distance. Also, the lobes will be tilted some between a line source and a point source. This will all have a greater effect at higher frequencies than lower ones -- a 100 Hz wavelength is about 10' and the closer the speeakers the less lobing error you'll get. But in practice, a sub can be put just about anywhere with minor adjustments as long as you can adjust delay so that they stay in phase.
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