Axiom Home Page Forums General Discussion Advice from Axiom Owners Dual subwoofers vs and oddly shaped room round #1

Ok I just got done spending about 24 hours over the last two days playing with subwoofers. Sean dropped off his Dominator D-10s and tripod on Saturday so I figured I’d just get it done.

I’m including links to an article if found very useful both for it’s content and the links it provides, even a couple to Axiom. Much thanks to Big Daddy at the Blu-ray.com forum for putting all this information together in one place and summarizing it.

http://forum.blu-ray.com/subwoofers/48286-guide-subwoofers-part-ii-standing-waves-room-modes.html

http://forum.blu-ray.com/subwoofers/4366...djustments.html

I’m not sure how much external validity my results will have for other except in the most general sense which is actually one of the most important findings I made. The picture below shows the basic layout of my room(s). The brown block is my seating, purple line is the screen, green dot is the best single subwoofer location, yellow dots are other subwoofer locations I tried, red areas measured the worst for subwoofer placement. Besides the odd shape the room also has a vaulted ceiling.



Prior running the subs place my EP500 on the sweat spot and set up a grid on the floor running frequency sweeps using Sean’s HSU CD to find the best single subwoofer locations. After all that work I found that I had already identified 3 of the 4 best locations by ear. However, all was not wasted since the single spot I’d missed actually turned out to be the best one.

For each test I ran frequency sweeps in each seat after the subs had been calibrated for their new positions. I did one sweep with the AS-EQ1 off to see what two subs alone would do and then ran another sweep in each seat with the AS-EQ1 on again with both subs running. For the AS-EQ1 I had to leave the phase setting to 0 since it handles that. For the non-EQ runs I ran a test tone for phase setting on each sub and adjusted the dials accordingly. I also ran several tests with my SPL meter and figured that my margin for error was about 3dB or +/- 1.5dB.

For comparing the overall results of EQ on multiple seats I’m summing the variation’s recorded in each of the 3 seats along with noting any significant other results.

I ran a total of 7 dual subwoofer tests with the D-10s and comparing them only to themselves (i.e. the single sub in the best location the dual D-10s are compared to is also a D-10). I did opposite corners (using two of the best locations), both back corners, opposite side walls in 3 configurations and front and back walls in two configurations.

In every case when EQ was not applied I got worse results with dual subs than with one single sub in the best spot. About half these results were within the margin of error but in all cases the dual subs did worse. EQ improved the results of using dual subs but when compared to the single sub in the best spot using EQ the duals still did worse.

Using the combination I got the best dual results in I put the EP500 in one spot and a D-10 in the other to see how two very dissimilar subs did together. W/o EQ the results were the same as before with the single sub in the best location beating the duals. OTOH when EQ was applied to the two dissimilar subs the results got worse by twice the margin of error.

So what can I conclude from this. Well first read the fine print on using dual subwoofers that says “usually get better results.” Although I’ve only tested one room I’m going to make a leap of faith and suggest that my poor dual subwoofer results stem from a non-rectangular room. Virtually all the math and theory on dual subs is about rectangular rooms. Actually the only thing I ever found to read about non rectangular rooms basically says good luck cause it’s up to trial an error.

Not all of this was a waste of time. It’s saved me the effort of buying another EP500 or other brand of duals (I’m not fond of the changes to the EP500 amp controls). But it does still leave the door open for using EQ which is for another thread:

http://www.axiomaudio.com/boards/ubbthreads.php?ubb=showflat&Number=308860#Post308860

Remember: We only call it that with oddly-shaped rooms!

I have been debating buying another sub for a while but I just don't think the gains will be worth the money and I am "glad" to hear your results to be honest. I think buying an EQ would be a better way to go about it, as I don't need more output at this point.

I thought that dual subs was a given considering the really bad bass response across my seating but never figured a second sub wouldn’t help at all. Given the results I’m getting with one sub in the best location using EQ I wouldn’t even consider trying 3-4 subs.

When I first bought the house my plan was to build a wall across the opening from the living room to the dining area (behind the screen) since I figured I’d need something to mount the screen to and have a symmetrical wall behind my mains. However, I quickly realized that the partial wall wasn’t effecting my front soundstage so decided to hang the screen from the ceiling instead. At this point I doing even see a reason to try that now since I thing the single sub w/EQ is doing well enough that I can justify the cost of adding a wall and second sub just to try things out when I could just pick up an EQ system instead.

I’m going to spend more time listening to various material from all my seats to see if I can live with the results I’m getting right now. My preliminary finding are that I can live with it but that might just be because of the improvement over the previous setup. I’ve notice from doing lots of experimenting that when first changing something things often seem much better only to have the novelty wear off and have something about the sound bugging me.

Dean,

This is exactly what happened to me prior to getting dual subs. I was just missing something...now with dual EP500's, I see the what that "something" was...

Great write up btw!

Dana

I'm not sure how much this will help, but when I compared my EP500 to the PB13 Ultra, the difference was minimal until I played with the phase settings. Both subs were next to each other in the best spot in the room, set to 0 phase. I don't know why(well, the room I suppose), but the sound improvement was dramatic on the PB13 when the phase was changed to 180 degrees, a difference I couldn't replicate on the EP500. I don't know if phase experimentation would be worth a try in your case with 2 subs, but worth consideration perhaps?

Dean, thanks for the exhaustive results from what I'd guess was literally the "sweat spot". What are the other three of the best spots?

Yes, the discussion by Dr. Toole and others is almost always prefaced with "in a rectangular room". Looking at your room, there's one of the common two sub arrangements that might be more closely satisfied despite the room irregularity. If you still have the D-10s, trying the diagonally opposite corner arrangement using the red "worst" corner and putting the other one in the lower left corner(which apparently wasn't tried)might be interesting.

Yeah Dana it was quite obvious once I got to listening that the LFE was way off in two of my three seats. What surprised me most about trying out two subs is that so far they haven’t done a bit of good in any of the configurations I’ve tried.

I’m guess the reason you needed different phase settings for the EP500 and PB13 Ultra in the same place is the DSP circuitry in the EP500. Another thing I finally got a hard number on was that the DSP circuitry in my version of the EP500 v2 adds 8 feet to the physical distance. Prior to that I was only guessing based on a friend, who’s a part time sound engineer, calibrating it for me by ear. He said by the sound of it 8-9 feet extra was being added. Also the Onkyo bass management is adding 5 feet and Doug from SVS said that the EQ1 adds about 8 feet.

I did play with the phase settings when not using EQ. luckily the D-10 have a dial so I could get the phase as close as possible by ear. However, the EQ1 takes care of the phase setting when it’s hooked up and from the sound of it especially when blending with the mains it’s doing a great job of it.

I even try running them out of phase intentionally but that only made things worse.

Thanks for the suggestion John. When the best locations weren’t panning out I thought I would try pairing off a couple other locations with the bad (I mean really bad) corner since they were off at different frequencies and figured they might balance each other out. I hadn’t really considered the lower left corner since there’s no real “corner” (I’m assuming you mean just south of the screen not clear at the far end of the kitchen). However, by the time I got done on Sunday I was sick of whole process and figured I’d take a break. I do still have the D-10s and will get around to another round of testing shortly, but I just want to do some longer term listening with the present configuration to see if I can live with it since it’s a nice setup that conveniently keeps the subwoofer out of the way.

I wish both the Onkyo and the EQ1 could save measurements from at least 2 positions so I could try out a new setup and keep the best one w/o having to re-run all the EQ measurements. The SN R-972 was suppose to be able to save two sets of measurements but I never kept up with it much after the release since it seemed to be having so many problems.

Being able to save more than one set of EQ measurements will be high on my list of wants when I next upgrade processor(s), and not just for tweaking but also to be able to have two or more speaker configurations depending on what I’m doing.