REW + multisub - my journey begins

[paulspencer]

This is more of a "sharing the journey" than a "please help" post.

In the past I've used Ultracurve DEQ2496 to EQ flat at the listening position. Current system DIY omni speakers with two Rythmik servo subs. I used the subs as stands running stereo. Good results, but always looking to improve.

The multisub approach makes a lot of sense to me, and I've found REW is a brilliant tool to help with it.

So now I'm getting into this multisub, here's how I've started. First, I put one of the subs in the listening position with the driver at ear level, set crossover to 160 Hz 2nd order LP to see the effect up as high as possible. I'd never actually run them that high. Now I move the mic around to every thinkable sub position and measure. Then I use the averaging feature to see how different positions sum together. Ultimately I think I'll get even more serious and add a 3rd sub.

So far in this first test, I find that the best two positions are dead centre of the room, and the rear left corner. After measuring, I set up the system like this, then used DEQ to do the final adjustment. Well, Ultracurve was doing less work than ever, and subjectively I found the EQ works over a wider area than before. The corner seat in front of the sub now sounds right in the bass - before it was terrible, too boomy.

Please note: these are unsmoothed measurements! If I had 1/3 octave smoothed then the result at the end would probably be flat from 20 - 120 Hz +/- 3db! A more typical result would be +/- 15 db.

This is how the subs look when placed in the position of the mains:




Around 40 Hz I get a persistent mode, below this there are no modes and above quite a few. The top end roll off is NOT LP filtering.

This is showing variations of the sub height when placed in the left rear corner about 2m from my listening position side ways (I'm on the back wall).



Orange floor level, magenta ceiling level, blue midpoint (the worst). As soon as I move up from the floor, things go downhill.

Here are the 3 corners I can use:



Left and right are similar on the front wall behind the speakers, but the rear wall is clearly better. Note the flatness from 70 - 120 Hz. Sadly I had to low pass it and lose some of that benefit.

Now, coffee table sub - always thought this was a good idea to make the sub "nearfield" and reduce room interaction and put the listener in the "direct field." These measurements seem to indicate that this doesn't work. The nearfield range requires subwoofer headphones much closer than a coffee table!

These positions are all slight variations, allowing for a 1.5 x 0.5 x 0.5 m coffee table where the driver could be downfiring, or in a bandpass with vents on either end. I measured these positions, and also showed the dead centre of the room in black. Here:



The centre looks the best by far.

Here it gets interesting:


The two best positions are shown and averaged together. Rear left corner + dead centre in the room. Black is average, blue is the centre (the only position that seems to miss out on the 40 Hz peaks) and orange corner.

These are the locations that most would say "don't do it!" But I had to try. So I set the subs up this way, then ran ultracurve eq, and found I got the flattest response ever, with less eq required than usual. Also, sitting in front of the corner sub, it still sounds balanced. Before it was just too boomy. I still need eq for this to work. I needed to still cut around 40 Hz, and boost around 100 Hz. It would have been better had I been able to run the subs higher, but I actually have to low pass them lower than in the measurements. Otherwise, I may not have needed eq at all.

The mains are doing more work, and the result works over a wider area than before, and I won't call it a final result, more of a beginning.

[brucek]

I move the mic around to every thinkable sub position and measure. Then I use the averaging feature to see how different positions sum together.

I hadn't thought of that method for multiple subs. Really good idea..

brucek

[glaufman]

Actually, I would've thought dead center as being "rule of thumb" for achieving flatness... many/most times though it cuts the output so far that the tradeoff (especially in conjunction with interior design concerns) is not viewed as being worth it...

[paulspencer]

It turns out in my room it's a good spot for a coffee table, but rule of thumb? Definitely not.

[glaufman]

paulspencer wrote:

It turns out in my room it's a good spot for a coffee table, but rule of thumb? Definitely not.

Maybe it would be if more people were willing to give it a try...

Again, I'm only talking flatness here.

BTW, WRT to using a sub as a coffee table, watch out for rolloff

[paulspencer]

Good point! lol

The great thing is that with a mic and REW you don't have to - you can find out for yourself what works best.

[Ricci]

Good stuff Paul. Did you do an actual measurement with both subs one in each position to see if it resulted in the simulated average response? Should be close.

[paulspencer]

I now have them in those positions, but the result is different as I had to set the low pass filter lower in both subs, also one of them is set to 4th order. As as result, it's not as flat, but before I adjusted the crossover, I could see in the RTA that it was very flat already just looking at pink noise. So in practice I can't actually achieve that result without eq right now.

[Wayne A. Pflughaupt]

paulspencer wrote:

Here it gets interesting:

The two best positions are shown and averaged together. Rear left corner + dead centre in the room. Black is average, blue is the centre (the only position that seems to miss out on the 40 Hz peaks) and orange corner.

Is the black line REW’s averaging? If so you won’t get that response with an actual measurement. The measurement will more-or-less track whatever sub is dominating at whatever frequency range - “blue sub” below 30 Hz, “orange sub” between 30-70 Hz, etc.

Re the three-corner graph, while the blue trace was flatter across the 90-100 Hz range, overall it’s not as good as the orange trace, IMO. In the subwoofer range, below 90 Hz, the orange trace is flatter overall, and has better extension to boot. An broad EQ filter at about 40 Hz would get you good extension all the way out to 20 Hz. So you might want to reconsider that position, unless there’s just no other way to fill that 90-100 Hz hole (from your mains-position graph it appears you’re having problems in that region from the mains as well).

There seems to be at least a couple of thoughts about using multiple subs. One is to increase overall gain (read headroom). However, you only get this benefit with corner placement. Some people like corner placement, others think it is wretched. I expect the difference depends on the particular room. Works great in my room, across most of the seating area, but at a buddy’s house, corner placement got one of the worst readings I’ve ever seen.

Another thought on multiple subs is what you’re trying to accomplish, evening out response to a wider number of seating positions. Again, how successful it is will be largely room-dependant. But it sounds like you’re having some good success. I wouldn’t worry about having to use some equalization to achieve your objective. It will only make things better - subs are pretty forgiving.

Regards,
Wayne

[paulspencer]

Hi Wayne and thanks for the feedback. My goal is to get it as flat as I can in as many useful locations as I can, and then use EQ to improve further. I've been using eq the lazy way for years - Ultracurve eq for the seating position only. This works fine for the main couch (3 seats) but not for the corner. Now with the current setup it works in the corner as well.

When I had it set up with the centre and rear corner, with pink noise seen on the RTA it was dead flat up to the xo point around 80 - 100. But I had to set the lowpass lower to make it work! On paper (the RTA screen actually) it looked just like the black average (not that I checked with REW which is less forgiving).

This is just the beginning!