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Using front speakers to get multiple (virtual) subwoofers

K man

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Maybe this is a known technique but I haven't seen anything written about it, so I decided to share the idea since it's been very successful for me.

I have a 6.2 system but for the sake of this article I'll focus on the 2.2 part of it.

Equipment
Denon AVR-X4400H
miniDSP C-DSP 8x12 DL
Buckeye Amps 8xNC252MP
Monitor Audio Platinum PL200 II (pair)
Rythmik E15
Rythmik F12

Problem
I'm using Multi-Sub Optimizer (MSO) to get an even bass landscape in my very asymmetric room. However, in two of the listening positions (which are indeed in a corner) I had a persistent problem with a +14db mode at 35Hz that I couldn't get rid of using just two subwoofers.

I instead tried to integrate the fronts with the subwoofers in MSO and after a lot of playing around with asymmetric crossovers, a few PEQs on the fronts, some all pass filters and inverting one of the subwoofers I could eventually get some ok results.

I loaded up the results in the miniDSP (manually on the fronts) and then went into running Dirac Live. (In reality I did this process many times.) There I ran into the real problems.

In Dirac I had essentially two options:
1) Use same curtains (same group) for the fronts and the subwoofer channel.
2) Use separate curtains (separate groups) for the fronts and the subwoofer channel.

In option 1) Dirac was trying to correct all the channels separately to fit with the target curve. Since I have the fronts corrected up to 333Hz, it meant Direct also lifted up the upper end of the subwoofers as much as it could. This caused the subwoofers to become very localizable (especially the E15). (Remember I had to use fixed crossovers to solve the mode problem, so couldn't do much there.)

In options 2) Dirac effectively messed up the result of MSO, since Dirac was adjusting the channels separately - and back was my 35Hz mode...

Conclusion: I need a third subwoofer to be able to solve this.

The problem is that my room and furniture doesn't allow for it. I spent a fair amount of time thinking on how to fit in a third subwoofer. I went on to discuss with Brian and Enrico at Rythmik about the possibility to cram in an upward firing FM8 in a tight space, but it was still not an optimal solution. (I was also looking into a SVS 3000 Micro, but still had no good placement for it. The KEF KC62 wasn't really an option since it changes it's frequency response with volume, rendering it useless for MSO.)

Then I got an idea...

Solution
Instead of integrating in the fronts with both the subwoofers in MSO, I added in all four as subwoofers. I gave some limited freedom (3 PEQs) to the new "front subwoofers" and was eventually able to find a configuration that was better than anything I've been able to generate up to that point. In particular, the 35Hz mode was now fully gone.

In the miniDSP I then configured the Diract sub channel to output to both of the fronts and both of the subs:
1671388269450.png


This effectively gave me a 6.4 system instead and I could then use Diract to smoothen out the bass channel without messing up the delicate relationships between all the four subwoofers.

I could then setup the crossover normally. No need for asymmetric of complex crossovers. I also found that I could set the crossovers very high without any problems with localization, since the sound it coming from the same front speakers anyway (although in mono)! This is indeed favorable, since the bass is more smooth from the four subwoofers than from the fronts alone. (This ability to crossover high without localization may not be general, but in my case the PEQs on the subwoofers were set so that most of the higher frequencies went to the fronts. I kept track of this when choosing the configuration in MSO.)
1671386576133.png

1671386610332.png

(I could go into 160Hz or even 200Hz without localization of the subwoofers, but decided to keep it somewhat low while still benefitting from the smoother bass around 100-120Hz.)

Result
I now have not three but even four subwoofers in my room, without having to refurniture the room!

Thanks to those four subwoofers I now have a very even bass landscape in the room, and that pesky 35Hz mode in the corner is fully gone! :)
 
Last edited:

waynel

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If I understand correctly you are routing sub bass to your main speakers. I believe that this is a flawed approach as you are losing the sub bass headroom advantage of using subs in order to chase a flatter response.

My advice is to try running your subs in mono with a ~80hz 4th order LR. Adjust your subs so they are in phase with each other at your main listening position ( this is easier with identical subs). If you still have a bill at 35hz in some positions try moving the subs . Otherwise better to live with the null than lose your sub bass headroom.
 
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K man

K man

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If I understand correctly you are routing sub bass to your main speakers. I believe that this is a flawed approach as you are losing the sub bass headroom advantage of using subs in order to chase a flatter response.
I have some fairly capable front speakers and I never push them at any high volumes, so the headroom really isn't a problem for me.

My intentions with adding the subwoofers was to even out the bass and to fill out the lowest frequencies (<30Hz), where the fronts gets weak.

With that said I did take care to use only negative PEQs on the "front subs".

My advice is to try running your subs in mono with a ~80hz 4th order LR. Adjust your subs so they are in phase with each other at your main listening position ( this is easier with identical subs). If you still have a bill at 35hz in some positions try moving the subs .
Then I don't think you understand the complexity of the problem. Even with MSO it was tricky to find a solution to the 35Hz mode. Solving it required several subwoofers, several PEQs on each subwoofer, inverting one of them and adding in two all pass filters. Doing it with only two subwoofers was impossible.

Otherwise better to live with the null than lose your sub bass headroom.
Not in my case. That mode (not null) was very auditable and disturbing, while I never had any problems with bass headroom on the front speakers.
 

Kervel

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Interesting solution. Did you consider using MSO for the subwoofer channel with 2 subs and 2 fronts, but with a fixed 40 hertz high pass on the fronts? Then they are still used, but just a bit less. I recommend checking the filtered response in MSO, to make sure the subs do the heavy lifting and the fronts are not used too much in very deep bass
 
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K man

K man

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Did you consider using MSO for the subwoofer channel with 2 subs and 2 fronts, but with a fixed 40 hertz high pass on the fronts? Then they are still used, but just a bit less.
Yes. I wrote about the problems with that in the "Problem" section.

I recommend checking the filtered response in MSO, to make sure the subs do the heavy lifting and the fronts are not used too much in very deep bass
Yes, as I wrote to @waynel above I did take this into consideration. Making sure no boost was added to the lower end of the front speakers. I also picked a solution in MSO where the lower end was mostly loaded into the real subs and most of the upper frequencies was loaded into the "front subs".
 

Kervel

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Very nice; I doubt waynels point is an issue. Happy to hear your solution works well!
 
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