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Author Topic: Two LO Drivers - use DSP to roll off one of them?  (Read 4791 times)

Shane Ervin

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Two LO Drivers - use DSP to roll off one of them?
« on: October 10, 2016, 02:20:39 pm »

Hi.  Looking for comments from the roll-your-own crossover crowd (or anyone).

This product, the Meyer UP-4XP uses two drivers that
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that work in parallel at low frequencies delivering a combined acoustic output with one of the drivers rolling off at higher frequencies to prevent interference (due to comb filtering effects) in the crossover region.

Q: Has anyone tried something similar with larger drivers, specifically 2x 15" or 2x 18"?  (...in a bid to reduce comb filter effects between the mid driver and the more distant of the two LO drivers).

Background:  In an essentially a 3-way rig, and having been restricted to a mono 3-way (FDS-360), or a stereo 3-way (miniDrive) crossover, now I have 8 channels of DSP to play with, so I'm exploring this technique to create a different signal for the bottom 18".

Details:
  • top cab is a 3-way with 18" / 10" / 1"
  • bottom cab has just another identical 18" and is a nominally identical enclosure, except for slightly increased interior volume due to the absence of the mid chamber and horn
  • LO - MID X-Over f=600 Hz, using LR 36
  • distance from bottom driver's upper edge to mid driver's lower edge is 43"
  • Ground stack deployment, always
  • 4 amp channels per side, so no additional amps required in this scenario.  In fact, the new DSP is internal to a 4-channel amp, so...

Early results, pre-Smaart analysis:
  • As a 1st step, I chose a roll-off for the bottom 18" by listening to pop music with male vocals, and then seeking to attenuate the vocal signal level in the bottom LO driver by >= 10 dB, relative to that in the upper cabinet's LO driver.
  • I settled on a 250 Hz, Butterworth 24 corner.
  • In order to balance the overall SPL, i.e., divide the load evenly for typical program material, between the two differently processed LO drivers, I had to add 3dB of gain to the lower driver (with its narrower bandwidth).

I'm reaching out to the community for additional perspective.  In particular:
  • What are your thoughts on having different gains on otherwise identical LO drivers?  Is it un-wise to depart from a situation in which both 18's carry the same signal level in the 40 - 250 region?
  • Is the reduction in comb filtering, that would otherwise affect the portion of the spectrum from ~ 400 -to- ~ 800 Hz, worth the trouble to do this?
  • Given 8 channels of DSP, would you just leave well enough alone and simply keep going with identical signals to the two LO drivers?
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Shane

Newer: EAW 8" MicroWedges; QSC PLD 4.2 PLD 4.3; QSC TouchMix-16; UAD Apollo 16; Sennheiser G3 IEM wireless
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Riley Casey

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #1 on: October 10, 2016, 02:51:53 pm »

The original Meyer ULX speakers used a passive low pass in the box to achieve this, not more than 12db per octave from the looks of it.  600 hz seems ambitious for an 18 crossing into a ten.  I suggest treating the extra 18 as a sub driver and band passing it 30 - 80 hz then measure / listen again.  Actually I'd use the two 18s as sub drivers and install a 15 as the low driver crossing into the ten but that would probably be actual work which of course I oppose on principal.
« Last Edit: October 10, 2016, 02:54:07 pm by Riley Casey »
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Merlijn van Veen

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #2 on: October 10, 2016, 02:56:03 pm »

Meyer Sound's solution is one way of tackling the inherit interference issues that occur when a bipole arrangement becomes uncoupled. However if you resort to typical minimum phase filters, like the ones in your DSP, to introduce e.g. a LPF in either driver. You'll end up adding phase shift which is not present in the other driver. The 4th order BW you mentioned introduces 180 deg of phase shift at the corner frequency in one driver exclusively, making both drivers cancel at that same frequency. Unless you're capable of adding the same amount of phase shift to the other driver as well without limiting its bandwidth. Something that's beyond the capabilities of your DSP AFAIK. Correct me if I'm wrong.


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Shane Ervin

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #3 on: October 10, 2016, 03:42:33 pm »

<snip>600 hz seems ambitious for an 18 crossing into a ten. <snip>
JBL's original choice for the 18" -to- 10" crossover was actually 600 Hz for these SR-4738's.  For a few years after having removed the passives, I used a lower value, and made cards for both f=250 and also f=400.  As a sidefill carrying vocals, it was nice to have more of the vocal emit mainly from the 10".  Kik and Bass ruled the roost in the 18's while vocals came from the 10".  (Dave Ratt has written about 3-way monitors being almost as good as using separate wedges to keep the singer's mic in its own wedge).  Later though, my thinking reverted back to an f=600 crossover, in a bid to increase the protection for the mid.

p.s., I can't really add a 15", plus I prefer sidefills to be 3-way, anyway.  However, when on FoH duty, I always add a cross-rented sub, then hi-pass my part of the rig to 80 Hz.  With four 18"s hi-passed at 80, and rented subs taking the beating, it works very well (for a ground stack.)  ;)
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Shane

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Shane Ervin

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #4 on: October 10, 2016, 03:51:29 pm »

<snip>You'll end up adding phase shift which is not present in the other driver. The 4th order BW you mentioned introduces 180 deg of phase shift at the corner frequency in one driver exclusively, making both drivers cancel at that same frequency.<snip>
I'll start experimenting with BW 12's and other corner freq's, and report back.
BTW, I'm using the QSC PLD 4.3 for the 18" drivers, and a PLD 4.2 for the MIDs and HIs, so I only have access to the DSP features they offer.

I must say, relative to the previous setup, it's a boon to reduce the rack space requirements from 9 RU... to 4RU !!!  That's the main attraction here, and I get lower current draw as a bonus, and these extra channels of processing as gravy.  Hence the question: Is it better to stand pat with these advantages, or press harder and try to make use of the new, higher, DSP channel count to tailor that bottom cab's drive signal?

Fun topic and has me thinking!  Thanks for replying.
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Shane

Newer: EAW 8" MicroWedges; QSC PLD 4.2 PLD 4.3; QSC TouchMix-16; UAD Apollo 16; Sennheiser G3 IEM wireless
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Chris Grimshaw

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #5 on: October 11, 2016, 08:32:53 am »

By boosting the LF-only 18", you're leaving a bit of headroom on the table in the bass department. By pushing that one harder, its running at larger excursions compared to the midbass one, so you're not getting the most out of the upper 18" when the lower one runs out of steam.
If you want more <250Hz, use the system EQ to boost them both equally.

The above assumes, of course, that the upper range of the upper 18" doesn't suffer much when you apply some power at low-frequencies. If it does, working the lower driver harder is probably the way to go.

For a starting point, I'd probably aim at 150Hz BW12 and see how that does. Don't worry so much about the phase shift - Butterworth filters are -3dB (Linkwitz-Riley are -6dB) at the selected frequency, so any cancellation it'll cause is limited.

Have you tried running both of the 18"s with the same signal, though? - it'll get you some vertical directivity by having such a large (vertically) radiating area. That might be a good thing, keeping the low-mids off the ceiling.

Chris
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Shane Ervin

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #6 on: October 11, 2016, 10:18:50 am »

By boosting the LF-only 18", you're leaving a bit of headroom on the table in the bass department.
That's been my main concern since I began pondering whether to employ the two extra (until now: superfluous) processing channels that the new amp rig affords me.

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Have you tried running both of the 18"s with the same signal, though? - it'll get you some vertical directivity by having such a large (vertically) radiating area. That might be a good thing, keeping the low-mids off the ceiling.
Yes, and that's been the way I've run them all along, since I only had a mono or stereo 3-way crossover.  And I'd been been happy with it: simplicity; equal headroom; etc.

I like your comment about narrowed vertical directivity of the very same vocal part of the spectrum that I've sought to remove from the bottom driver.  That's compelling, and it may just prove the tipping point in the decision to just keep running with 3-channels of processing - la regular 3-way.
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Shane

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Don Boomer

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #7 on: October 11, 2016, 11:25:17 am »

Band limiting one of a pair of woofers has been done many times in commercial products. But are you having a particular problem with audible comb filtering with your rig?
Depending on the type of program material you are intested in, I usually also look at minimizing the low end of vocals from my subs although I typically come up with a number an octave lower than the 250Hz you've settled on.

Again, not knowing what you are after performance wise, have you looked at using your second 18 with an aux fed sub arrangement?
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Len Zenith Jr

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #8 on: October 11, 2016, 12:02:56 pm »

Using a second woofer rolled off in designs is an attempt to make up for the baffle step losses. Essentially low frequencies radiate into full space where as higher frequencies radiate into half space, the point of that transition depends on the size of the cabinets baffle. Full space radiation is -6 dB's lower than half space so adding a second woofer in parallel to cover that area brings the frequency response back up 6 dB's. If they didn't roll off the second woofer at that transition point, the frequency response would be no flatter than a single woofer, just louder. I don't think eliminating mid range comb filtering has anything to do with the roll off and is just marketing hype, flat frequency response would be the motivating factor.
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Shane Ervin

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #9 on: October 11, 2016, 12:06:24 pm »

... are you having a particular problem with audible comb filtering with your rig?
No particular problem, and there's no comb filtering that stands out when all bands are un-muted.  I always get good comments on the rig when run vanilla 3-way.

As I began exploring the application of the lower corner freq to the "distant" driver (43" distant from the MID with which it's crossed over), it was intriguing to hear a more localized image of the vocal signal from the tightly clustered LO/MID driver pair of the top cabinet, without those vocals emitting from the bottom LO driver.

So, I just want to explore the best use (if any) for those 2 new spare channels (1 per side) of DSP processing.  It's sort of a continuous improvement / TQM motivated thinking process, I suppose, and not wanting to leave any performance tweaks on the table, especially if easy to implement and devoid of serious drawbacks.
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... have you looked at using your second 18 with an aux fed sub arrangement?

I'm of the Jeff Berryman school when it comes to Aux-fed subs, generally speaking, which is to say I consider the advantages too small to outweigh the potential pitfalls (when in the wrong hands).

Here with this basic 3-way rig (no subs), it's hi-passed at 60 Hz for sidefill jobs, and 40 Hz for small FoH jobs.  So both 18"s have always been fed the same 60 to 600 Hz bandpass.

On those occasions when I do add cross-rented subs, its as much motivated by protecting the front loaded LO drivers, as it is for increased bottom end power capacity.  In any case, I cross over from the subs to the 4738 / 4718 rig at 80 Hz.

With subs or without, however, the initial appeal of reduced comb filter effects from 400 to 800 Hz was an academic curiosity and I wanted to see if it panned out in any worthwhile tangible way in actual use.

I also wanted to discover what hoops I'd need to jump through to pull it off correctly.

Merlijn posted regarding the need to avoid using a filter with 180 deg phase shift at the f=cutoff freq for that bottom LO driver.

And Len just posted a good point about maintaining flat freq response - something I've been worried about also.
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Shane

Newer: EAW 8" MicroWedges; QSC PLD 4.2 PLD 4.3; QSC TouchMix-16; UAD Apollo 16; Sennheiser G3 IEM wireless
Older: JBL SR 4738 / SR4718; BSS MiniDrive; Klark Teknik DN-301; Yorkville AP4040 AP2020; Motion Labs Rac Pac;

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Re: Two LO Drivers - use DSP to roll off one of them?
« Reply #9 on: October 11, 2016, 12:06:24 pm »


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