cross-over question

[nightflier]

I've had my HT setup pretty much the way I like it and I usually don't tweak with it. But I was wondering about cross-over thresholds. If the crossover in a typical receiver is set to hand the output below 80 Hz. to the sub, then how is this cut-off really made?

Is it a sharp cut-off; anything below 80 goes to the sub and everything above to the speakers? or is it blended slowly, where it begins to drop off all the speakers at 85 Hz.-ish, the sub begins to pick it up there, and doesn't get everything until the 75 Hz.-ish point?

[GMichael]

It's usually rolled off slowly. I think that you can even pick how slowly you want it to drop off with some receivers or processors.

[Woochifer]

Every crossover does the rolloff using a specified slope, so there will always be some sort of transitional range. The crossover slope determines how wide this range is. The crossover slopes that you will typically see on a midrange home theater receiver are 2nd- (-12 db/octave), 3rd- (-18 db/octave), or 4th-order (-24 db/octave). A 4th-order crossover slope and a 80 Hz crossover point means that the signal at 40 Hz is -24 db. Some higher end processors allow you to choose the high and low pass crossover slope.

Generally, there are inconsistencies in how the bass management is implemented on different models, except with THX setups, which are built around a 4th-order crossover slope with the standard crossover point of 80 Hz (this slope is implemented with 2nd-order crossovers on both the THX surrounds and the receiver [summed together, this creates a 4th-order slope], and a 4th-order crossover on the subwoofer output). Some articles I've read though note that a steep crossover slope can cause ringing, so you have a trade off in that you have a shorter transitional range between the speaker and the subwoofer, and the potential for ringing.

This article explains a little bit about some of the anomalies you'll see in how bass management has been implemented. The Yamaha receiver that I use has many issues as well (the transition around the 90 Hz crossover point has significant overlap, which I have to dial down using a parametric EQ). All of this keeps the integration between the sub and speakers from performing as well as they could.

http://www.hometheaterhifi.com/volume_12_2/feature-article-slope-troubles-6-2005.html

[nightflier]

Very interesting. My processor (Outlaw 970) does not have a configurable cross-over slope. According to the documentation high-pass is hard wired for 12dB / 2nd order and low-pass is 24dB / 4th order. I presume these are different because of the difference in how a typical person would hear these.

[Woochifer]

Very interesting. My processor (Outlaw 970) does not have a configurable cross-over slope. According to the documentation high-pass is hard wired for 12dB / 2nd order and low-pass is 24dB / 4th order. I presume these are different because of the difference in how a typical person would hear these.

That sounds like the THX spec. As the article points out, the problem with using a 2nd order crossover slope is that you need to pair that with speakers that also use a 2nd order crossover that starts at 80 Hz. Generally speaking, I'm not a fan of THX surrounds because until the Ultra 2 spec came out, surround speakers had to use a dipolar design in order to receive the THX certification. But, the THX surrounds are also the only ones that can match the subwoofer 4th order crossover slope when the high-pass filter uses a 2nd order crossover. I think it's a mistake for so much of the receiver/processor market to blindly follow the THX guidelines, knowing full well that the vast majority of the surround speakers in use do not use the THX-mandated crossover design. Either way, you got compromises all the way around -- you either have to adjust to mismatched crossover slopes, or you have to limit your choices to THX surround speakers, the majority of which are dipoles that IMO sound like crap with multichannel music or movie soundtracks that have a lot of directional sounds steered into the surround channels from the front soundstage.

My receiver uses a 3rd order low pass filter and a 2nd order high pass filter, and this combination creates an overlap in the range around the crossover point that I use a parametric equalizer to correct. Part of this is room-induced, but I don't doubt that the crossover slopes also have a role.

I think eventually this situation will be corrected, because it's long overdue. For now, selectable crossover profiles are increasingly used in high end processors and some subwoofer auto calibrations now use them as well.

[nightflier]

Well I'm using Axiom M22Ti's for fronts, VP150 center, and QS4's for surrounds. There is no documentation about slopes online, but the graph for my fronts does lead me to believe that they are second order cross-overs. I'll have to send them an email.

[Woochifer]

Well I'm using Axiom M22Ti's for fronts, VP150 center, and QS4's for surrounds. There is no documentation about slopes online, but the graph for my fronts does lead me to believe that they are second order cross-overs. I'll have to send them an email.

On your front speakers, I doubt that the designers would deliberately use a crossover to limit the low end response, which is how the THX surrounds are designed. Any rolloff that you see with your front speakers would follow the driver characteristics and the port alignment, which probably uses a tuned frequency lower than 80 Hz. Typically, the rolloff below the tuned port frequency is fairly steep (similar to a 4th-order alignment) unless the driver starts to reach its physical limits well before that point.