How do you pick a crossover point??

I'm setting up a new processor and am finding it difficult to choose an initial crossover point for each of our speakers.

Many processors use Lucasfilm's suggestion of a 'small' speaker setting with the crossover set at 80HZ. Nowheres on the Lucas website could I find out why they chose 80HZ, but it is a scheme that seems to work well.

Our new processor (a CB3) has extensive bass management capabilitles and the ability to use one of several crossover types set at any crossover point. My fundamental question is how do I choose the correct crossover point?

Our system is as follows:

ReQuests for both front mains and L/R surrounds 30~24000 Hz
Logos center 75~20000Hz
Two daisy-chained Descents 18~150 Hz

In theory, the ReQuests should qualify as 'full range' speakers (AKA THX Large) and probably dont need to be crossed over. Thats if you believe the published specs. Most ReQuest owners would probably admit that they fall a bit short in the bass department, to which I agree.

Obviously the Logos needs to be crossed over.

I think the Requests should be also, mainly to prevent over-driving the woofer on hard passages, particulary on tracks with extensive LFE content. But at what point should it be done?

I was hoping someone on the forum could look at the specs mentioned above and offer an educated suggestion at a crossover point for the two types of speakers we are using (Requests + Logos). Sure, the CB is quite flexible and it is easy enough to change a setting, but the configuration as a whole is quite extensive and I've never really given crossovers much thought.....at least not enough to avoid taking anything but a wild guess.

Thanks and sorry for the long post...

Tj

What you run into in bass is a trade off between ideal positioning to take care of room modes (corners are generally ideal) and the directionality of the sound. You don't want to be able to localize the bass as coming from your subs...as that's a different spot in your room than the rest of the music is coming out of. I think generally 80Hz is considered the point at which the waves are pretty much omni-directional, and thus unable to be localized...so make for the ideal crossover point. If you have your subs right next to your speakers, you could likely cross over higher (100Hz or so), at the cost of some low end room mode reinforcement.

Hi,
The published spec for a speakers low end is typically the -3db point, i.e. the frequency where the ouput drops to one half of what it is above there.

I normally double that frequency, and set the crossover point there. The advantage of that is the speaker response is normally quite flat at that frequency, and I find it gives a better overall frequency response.

If you set the crossover at the published low frequency, then the speakers output is already down to one half, before the crossover starts cutting if further. The sub will start cutting off at that frequency on the high end also, leaving a 3db dip in the response due to the main speakers rolloff down low.

A second benefit is that since the main speaker doesn't have to handle so much low bass with its greater cone movement, it will be better able to handle the higher frequencies. YMMV...

Peter

So if you were using ReQuests you'd cross over at 60Hz and 150 for the Logos? Or would you cross the Logos over a bit lower, since the sub starts at 150?

Tj

Tj Bassi said:

So if you were using ReQuests you'd cross over at 60Hz and 150 for the Logos? Or would you cross the Logos over a bit lower, since the sub starts at 150?

Tj

Click to expand...

TJ, having owned the Logos (and pushed it hard), I can vouch for the recommendation to cross it over at 150 or 160hz. Definetly no less than 120Hz. Even then I'd bottom mine out at 120Hz. Last x-over I used was 160Hz.

Even the reQuests I'd do at least 60Hz, but more like 80Hz is what I'd run them at. Also, depends on the slopes used (I beleive a CB3 will do L-R 24Db/Octave) so 60HZ might be OK if it's a 24/db/octave x-over.

The CB3 will do 6,12,18,24 Db/Octave per channel. Using your suggestion of the Logos X-ing at 160 and the ReQuests at 80, would you stick to 24Db/Octave for both or go a bit gentler, say 18 on the Requests but stay at 24 for the center?
All this is for a Linkwitz-Riley crossover, correct?

I wish I had a better understanding of acoustics---but I appreciate your help.

Tj

TJ, I'd use L-R 24's all around. Using other slopes has an impact on the phase response and unless you have really good measuring gear (Calibrated mic, a mic pre-amp and software like ETF), getting the phase response between center and L/R is going to be really hard. And that assumes the CB3 let's you adjust phase in one degree increments like my DriveRack 260 does.

So I'd recommend the following:

Logos crossed over at 160 using a 24db/Octave L-R

reQuests crossed over at 60Hz using a24db/Octave L-R

But also try an 80Hz setting on the reQuests, you might be pleasantly surprised. A lot depends on room interactions. and mains to sub integration. Again, phase and positioning have a lot to do with that as well.

The other thing to pay really close attention to is the time domain. Make sure the delays for each channel (and especially the sub) are set to the correct values based on careful measurements (simpler tools, just a tape measure required here )

I can tell you that spending >16 hrs tuning my front channels yielded an incredibly cohesive sound field. And that time delay, phase and location all interact, so set and measure, set and measure, etc. etc. until it’s just right. Your CB3 is a wonderful tool as are your ML’s, so it’s worth the time to get the most out of them.

JonFo said:

TJ, I'd use L-R 24's all around. Using other slopes has an impact on the phase response and unless you have really good measuring gear (Calibrated mic, a mic pre-amp and software like ETF), getting the phase response between center and L/R is going to be really hard. And that assumes the CB3 let's you adjust phase in one degree increments like my DriveRack 260 does.

So I'd recommend the following:

Logos crossed over at 160 using a 24db/Octave L-R

reQuests crossed over at 60Hz using a24db/Octave L-R

But also try an 80Hz setting on the reQuests, you might be pleasantly surprised. A lot depends on room interactions. and mains to sub integration. Again, phase and positioning have a lot to do with that as well.

The other thing to pay really close attention to is the time domain. Make sure the delays for each channel (and especially the sub) are set to the correct values based on careful measurements (simpler tools, just a tape measure required here )

I can tell you that spending >16 hrs tuning my front channels yielded an incredibly cohesive sound field. And that time delay, phase and location all interact, so set and measure, set and measure, etc. etc. until it’s just right. Your CB3 is a wonderful tool as are your ML’s, so it’s worth the time to get the most out of them.

Click to expand...

tj - let us know the diff if you try this because it basically goes against martin logan recommendations for the mains at least. They recommend 40Hz on the descent for their floorstanders - as you know for stereo listening. Do you think it should be different for the mains in multi-channel?

I am with the recommendations from manufacturers and THX. My Denon receiver manual also states using the -3 dB to find the crossover point.

Using Denon's Auto-EQ setup mode (test tones into mic at sitting area), my Denon crossed my Vantage at 40 Hz and subwoofer at 80 Hz. It did cross my Vignettes at 100 Hz, but i lowered it to 80 Hz because of Denon's manual recommendation. My Stage CC is also crossed at 80 Hz. I assume that the Vantage needs to be low also for stereo listening.

This is with my system...so, your system may need to be crossed over higher. It is strange that the Logos would bottom out at 120 Hz...most decent car speakers (thus, worst quality than Logos) can be crossed over at 80 Hz without a problem. In car audio (my previous experience), 24Db/Octave crossover is used if you have a subwoofer, and 12Db/Octave crossover if you don't.

If you have measurement equipment to properly time and phase align your speakers with your subs, you should be able to cross over wherever you want...and there are gains to be had from having this additional flexibility. But that requires a lot of additional equipment, and using a blanket recommendation like denon or thx is much easier to do if you're one of the most who don't have that measurement equipment.

timm said:

tj - let us know the diff if you try this because it basically goes against martin logan recommendations for the mains at least. They recommend 40Hz on the descent for their floorstanders - as you know for stereo listening. Do you think it should be different for the mains in multi-channel?

Click to expand...

I believe you are referring to the sub's low pass filter setting (70 vs 40), but the filter is only available in two-channel mode. If you are using the LFE either by the single ended input or XLR connector, the filter is inactive.

Guys, one reason for recommending higher crossover points is that the power curve of the 12” driver in that request is probably generating a good bit of second and third order harmonic distortion products when pushed at 40Hz.
For one, a reQuest is anywhere from 10 to 7 years old. Its 12” driver is likely no longer at factory spec.
But even brand new 12”s (unless designed for Sub duty) will put out >3% THD at 40Hz at high SPL’s.

By pushing 40 Hz (and lower, remember that even at 24 db per octave the driver will still be trying to reproduce 30hz at a fair clip) signals through that driver, you will probably have >6% THD in that driver at 90db SPL. While not excessively loud, that kind of distortion is definitely audible. Getting a driver more suited for it (the sub), you get clean upper bass and mid-bass.

Here is an example a good 12” driver, a Peerless 830500, doing 30Hz:

Note that second harmonics are >5% and third is approaching 1%. And this is not pushing it.