Dual-subwoofer tradeoffs for DSP integration

[stigger]

I'm trying to pick a subwoofer model to buy two of them, and run co-located with two mains (which are yet to be chosen later, when the subs are dealt with). I have an intermediate understanding of FIR-based DSP correction using REW and rePhase, as well as many hours of practical experience tweaking my current system. Now that I feel capable of steering the sound in the right direction, I want to do everything "right". The problem is that the more I read about subwoofers, the less capable I feel of making a decision. So, here are my thoughts so far.

- I have a not particularly large, but very asymmetrical L-shaped room. Space is limited: I have two fixed positions for the subs, can't go wider than 38cm. Fortunately the nulls seem to be different in these two spots, so I expect that two subs can compliment each other.
- From what I understand, sealed subs are "less room-dependent", so in a bad room they should be easier to integrate properly (I'm planning to get sealed mains as well)
- Also, group delay is important, read that here times and times again
- If I were in the US, I would get Rythmik F12, but getting them in Germany nowadays seems like an adventure of its own
- The only practical alternative are SVS subs, like SB-2000. But, they use DSPs, which ruin group delay (do I understand correctly, that this is the case even for the classic models, with no app?)

The options I see:
- Get SVS SB 2000 Classic, live with the group delay (can try to mitigate a little bit with DSP, but generally not fixable)
- Relax the "has to be sealed" requirement, and then look at the ported subs with good group delay. This should still allow me to integrate with the mains using linear crossovers, right?
- Either import pre-built Rythmiks, or get DIY kits from them and order enclosures locally. Both options are painful enough to even think about.

So, I'm stuck: the third option is just not practical, and I don't know how to pick between the first two. Less group delay, or less "room-dependency"? Or am I missing something?

 

[Chrispy]

Why run them so close together rather than getting a single better sub? Multiple subs have advantages outside of that scenario....

 

[stigger]

The distance between the subs would be at least 1.5m, and the room is very asymmetrical, so acoustically the spots are very different. Besides, putting them next to corresponding mains allows to use linear-phase crossovers. (or so I read)

Oh, and I'm all for getting better subs, as long as they aren't wider than 38cm

 

[alex-z]

Sealed and ported subs are equally room dependent, the room modes and room gain created are equal. Sealed subs simply have an earlier but shallower rolloff, which in small rooms can make them sound less boomy if you are comparing two subs with no signal processing. I do advocate for sealed subs when possible, as there won't be secondary port resonances or compression.

Group delay is important but is no reason to avoid signal processing. Virtually all modern subs use signal processing, including Rythmik. On the Rythmik F12 for example they advertise -2dB at 14Hz, that number would be closer to 40Hz for a sealed 12" sub with no signal processing.

In most cases the group delay of the subwoofer itself is masked by the modal issues of the room. Signal processing is actually a huge benefit overall because you first use proper seat/subwoofer positioning to minimize nulls then EQ away modal peaks.

 

[Devnull]

DSP adds processing delay, not group delay. If you look at some of the info at SVS you can see that their sealed and ported subwoofers have about the same group delay down to around 30Hz. Below that the group delay increases for the ported box. Not a lot of music below 30Hz so it's really not a problem.

 

[Ellebob]

If you're trying to fix room modes, you don't want both of your subs in the front sound stage. If you're placing your subs up front you will not be able to fix room modes in the length dimension, only in the width dimension. Look at some of the Harman papers about placement.

A quick tutorial is if you place the sub in the null, it will help get reduce that mode. Using the width dimension for example, a commonplace would be to place the sub in the middle of the width dimension. For convenience it's usually the middle of the front wall or back wall, but technically could be more on the middle of the room.
When using two subs you can place the subs in the front corners and this is the equivalent of placing one sub in the middle. This is called the virtual technique, when you place two subs it's the equivalent of placing one in the middle of them for frequency response. PS you can also play some at the 1/4 and 3/4 points and that will actually fix not just the middle room mode but ones at the quarter points.

The problem is, none of this did anything for the length room modes. Common placements for that are one sub in the middle of the front wall and one sub in the middle of the sidewall. Another placement would be one sub in the middle of the left wall and one sub in the middle of the right wall. You can also use quarter points, like one placed at the quarter point on the on the front wall on the left side and one at the quarter point on the back wall on the right side. You can do this on the sidewalls as well. You can also place them in opposite corners. You won't get as many of the room modes with that configuration, but you will get more output and if that's needed then it's worth the trade off.

These configurations will help with width and length modes. With an odd shape room you'll have to experiment a little more and take measurements. But either way the subs need to be at opposite ends.

www.harman.com/documents/multsubs_0.pdf

 

[levimax]

I have 2 SVS SB3000 which I am disappointed in. While sealed subs can have less group delay many commercial sub manufacturers like SVS push their designs for marketing reasons for maximum LF extension by boosting LF and then they having to use a strong high pass "protection filter" which is what causes high group delay. Well designed subs whether sealed or ported can have group delay of less than 20 ms which will is not a problem (many subs like SVS have over 100 ms GD which is a problem especially for integration). In addition these heavily DSP subs have constant delay. All of this DSP, which you can not adjust or defeat, makes integration more difficult rather than easier as advertised.

My advice is avoid built in DSP for both your subs and mains and use DSP that you can control to accurately integrate your subs. As far as subs go look for ones engineered with sound quality rather than marketing hype. You are going to have to compromise on either size or LF extension but giving up some LF extension to get reasonable group delay is a good one for sound quality / ease of integration. Not sure what is available where you live but there are some really good "flat pak" subs that are pre-cut and fairly easy to assemble available in the US. GSG is an example.

Proper sub integration is much harder than most would lead you to believe and the best way to ensure success is to start with mains and subs that are as easy to integrate as possible. Good luck and have fun.

 

[Sancus]

Not sure how pricing compares but Arendal makes great sealed subs with low group delay.

https://www.audioholics.com/subwoofer-reviews/arendal-1961-1s-1v/1961groupdelay.jpg/image

 

[stigger]

Thanks everyone for additional details and refinements, I've seen them in different discussions here and there, but it really helps to have everything important summed up in one place. I do understand that the better positioning can give the most benefits, but physical space is very limited in my case, so I have to squeeze everything I can from the room I have.

Not sure how pricing compares but Arendal makes great sealed subs with low group delay

Thank you! I somehow forgot about them, probably dismissed subconsciously due to the lack of understanding of the implications that the side-firing driver brings. Is there some clearance required on the side of the driver? What if I want to put it next to the right floorstanding speaker, for example, so the subwoofer "plays into" the speaker? What if the driver plays into a piece of furniture, like a TV cabinet? Questions like that make it hard to reason about them.

On paper, 1723 1S seems like a very good option for me though, so if the side-firing driver does not make the sub less practical, then I'll seriously consider it. I would probably buy one first, experiment a bit with the Arendal and my current mediocre ported sub, and then decide whether I want to buy the second.

 

[Devnull]

If you want to get a feeling for how two subs will perform, measure your existing system and your current sub in the two locations you're thinking about. Then use that data in MSO, multi sub optimizer, to see how two subs might perform.

 

[stigger]

I actually am planning to do that this weekend, although was hoping that REW and rePhase will suffice to get a feeling of the combined result. My current sub is not particularly special though (Canton SUB 650), so replacing it might make sense regardless of the simulation results.

 

[Oddball]

REW should give you a solid preview. MSO could do more, but eventually will depend on what you will use for bass management and EQ overall.

I would primarily focus on frequency response and time decay as that is, at least for me, most important.

 

[Keith_W]

DSP adds processing delay, not group delay. If you look at some of the info at SVS you can see that their sealed and ported subwoofers have about the same group delay down to around 30Hz. Below that the group delay increases for the ported box. Not a lot of music below 30Hz so it's really not a problem.

Whilst that is true, it is still delay that he has to compensate for when tuning the system. My objection to DSP in subs is not so much that it adds delay, but it is a feature you pay for that you won't be using. It's redundant and not needed, I would personally prefer a "dumb" subwoofer.

As for the rest of the OP's questions:

1. Sealed vs. ported subs. Ported subs do add extra group delay. The trade-off for LF extension is group delay.
2. For linear-phase FIR, the subs do not necessarily have to be co-located with the mains. Whilst it is true that elimination of pre and post-ringing relies on proper summation of LPF/HPF, in practice it is quite manageable and not much of a problem. Bass waves are really long, and as long as the subs are properly time aligned it won't be an issue.

As for other posters recommending MSO - don't bother. You are using linear-phase FIR. You need MSO if you are using minphase IIR. For linphase there is no need.

And FWIW if you are using REW to make linphase filters, I hope you know what you are doing. It is an extremely manual process and everything needs to be done in the correct order. For example, did you know that when you export your .WAV file you have to save it with the correct tap count and manually place the impulse peak?

 

[stigger]

I'm not an expert in REW, but I think I do know what I'm doing. At least I know enough to start evaluating Acourate this week The primary motivation is saving time, because I keep coming up with ideas I want to test, and doing everything manually quickly becomes tedious. If I can interate faster, I might buy it (provided, it's going to work well enough in a Windows VM).

 

[beagleman]

I use dual Velodyne CT-120 subs. Honestly it was the best improvement in actual sound quality, not for volume or deepness of bass, but simply evened out standing waves and made bass FAR more neutral as I move to different seats and even in my one main seat.

Once in a while I shut one off and marvel at how uneven it sounds as I do bass sweeps.

 

[Keith_W]

I see. Good luck with it then. IMO it's not REW or Acourate, I think you need both. Acourate is better for making filters, REW is better for taking measurements.