Optimization of seat to seat variance - AVS Forum
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post #1 of 34 Old 04-02-2013, 06:55 AM - Thread Starter
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I have just finished my room, and now the calibration process has begun. My room is approximately 23.5 x 11 feet and has two rows. I have four subwoofers which I have measured together at all seats. The result is below:



Each graph represents a seat and it is very clear that each row follows the same response but the variation between the two rows is very large!

My room and sub placement is done like this:


I dont have much flexibility in regards to sub placement, so the image above is the only possible way.

Would be very interested in some advice on how to improve this by using individual delay, filters etc on each sub (I have all the processing power for each sub you would want). I am very interested in the Harman SFM algorithm but my budget does not allow me to get the SDEC4500 eq and their software. But I have the exact same options available except their algorithm.

How would you approach this systematically and not just by guessing parameters? I have measurements of each sub in each seat as well if this can give any indication on were filters are best used.

I know it is a big question, but if someone in here has gone through the same exersize, then it would be great to hear about it.

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post #2 of 34 Old 04-02-2013, 08:44 AM
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The process JBL Synthesis uses is iterative. See my article home theater multiple seat optimization here: http://www.madronadigital.com/Library/Computer%20Optimization%20of%20Acoustics.html

You can read through that paper and follow some of their simplifications. Then attempt to optimize one parameter only. It will still require tons of manual labor as you have to optimize for multiple speakers.

The other thing you could do is ignore certain regions.

Another option is to optimize for sweet spot and then look for commonality of response. In your measurement for example, if blue and red are more important spaces, you can pull down their peaks at 40 to 50 Hz some since the other seats don't have opposite response there.

Sorry I can't be of more help smile.gif. I have the Synthesis system and having tried to just EQ all the channels and found it too much work, I would not even think about doing what you are aspiring smile.gif.

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post #3 of 34 Old 04-03-2013, 03:30 AM - Thread Starter
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It would be optimal to have the SDEC4500 but I am using the pro-version instead from BSS. (Exactly the same component, but I need the JBL software/calibrator for calibration).

I have thought about an alternative solution. I could buy a Procella P10Si subwoofer and place it strategically. I think rear wall would be good. Wouldn't this cancel out the length mode and maybe minimize the variation across the two rows? This is an expensive route though, as I would need an amp and more output on my DSP.

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post #4 of 34 Old 04-03-2013, 11:57 AM
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If you have the option of placing a 5th sub on the rear wall, why can't you move one of the existing subs there to try to improve response? confused.gif
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post #5 of 34 Old 04-03-2013, 12:06 PM - Thread Starter
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Quote:
Originally Posted by Alan P View Post

If you have the option of placing a 5th sub on the rear wall, why can't you move one of the existing subs there to try to improve response? confused.gif
Thats why smile.gif


I dont have enough room for the current subs on the rear wall. But I guess I could do a temporary test with one of the subs to try and see what happens to the response. I need a buddy to help me carry it though.

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post #6 of 34 Old 04-03-2013, 01:12 PM
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Oh, I thought your front subs were in-wall. If they are stand-alone and you have a rectangular room, put them in the four corners and see what you end up with. Hopefully you will get rid of the nulls and all you have is peaks which you can then dial out using your BSS EQ.

It is a heck of a lot less work to move those subs than to try to find EQ parameters out of a few million choices smile.gif.

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post #7 of 34 Old 04-03-2013, 01:15 PM - Thread Starter
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Originally Posted by amirm View Post

Oh, I thought your front subs were in-wall. If they are stand-alone and you have a rectangular room, put them in the four corners and see what you end up with. Hopefully you will get rid of the nulls and all you have is peaks which you can then dial out using your BSS EQ.

It is a heck of a lot less work to move those subs than to try to find EQ parameters out of a few million choices smile.gif.

My room is not very big and it is not possible with the corner placement. There is a stair in the left back side of the room. Thats why I wrote that the sub positioning was fixed in regards to the illustration. But I could position a sub like JBL S4S or Procella P10Si on the back wall.

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post #8 of 34 Old 04-03-2013, 11:04 PM
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Originally Posted by JonasHansen View Post

I think rear wall would be good. Wouldn't this cancel out the length mode and maybe minimize the variation across the two rows?
To cancel out the first two length modes, you would have to place the subs at the 1/4 and 3/4 divisions of room length: i.e., on the side walls, 70.5 inches from the front wall and 70.5 inches from the back wall.
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There is a stair in the left back side of the room.
Is the staircase inside the room or outside the room? If the latter, is there a door closing off the room or is the room open?

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post #9 of 34 Old 04-04-2013, 12:47 AM - Thread Starter
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Originally Posted by sdurani View Post

To cancel out the first two length modes, you would have to place the subs at the 1/4 and 3/4 divisions of room length: i.e., on the side walls, 70.5 inches from the front wall and 70.5 inches from the back wall.
Is the staircase inside the room or outside the room? If the latter, is there a door closing off the room or is the room open?

Can't I "virtually" move the subs by adjusting delay of the sub on the rear wall? 70 inches from the front wall won't be possible.

The stair is inside the room and it is open to the area above the basement. Easier to describe with pictures (sorry about the quality - smartphone camera)











As a temporary solution I have a very thin plate to put over the hole to keep light out. But the bass will not be kept in. You can see where the sub on the side wall is currently placed on the pictures too. The depth of the screen wall matches the depth of the subs, and the width of the screen wall is exactly the width of the three subs behind the screen, so thats why I cant just move them.

This is what behind the screen wall looks - not much room to move the subs (Entire area is filled with insulation after the picture was taken):

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post #10 of 34 Old 04-04-2013, 08:24 AM
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You acn adjust relative phase between different drivers using the distance settings. But a sub is going to interact with the room in the same way regardless of its delay. It will excite the same modes, and on its own generate the same frequency dips whether it issues its sound RIGHT NOW or in 2 milliseconds.
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post #11 of 34 Old 04-04-2013, 08:30 AM - Thread Starter
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Originally Posted by JHAz View Post

You acn adjust relative phase between different drivers using the distance settings. But a sub is going to interact with the room in the same way regardless of its delay. It will excite the same modes, and on its own generate the same frequency dips whether it issues its sound RIGHT NOW or in 2 milliseconds.
But lets say my subs in the front exite the length mode. Cant I move the location of the dip by using a sub on the rear wall? If I delay it the exact amount so where there before was a dip caused by the front subs, the rear sub will have its maximum amplitude which will cancel out the dip.

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post #12 of 34 Old 04-04-2013, 08:44 AM - Thread Starter
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Maybe I'm thinking too much in terms of smaller wavelengths like this.


I forget that the wavelengths are so long in the lower frequencies that the image above is not a valid way to think...

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post #13 of 34 Old 04-04-2013, 09:59 AM
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Well afaik a delay is a delay snd does't chane room interaction. As I said you csn change how 2the sources interact throug delay that aligns their phases in a desiranle way. But thevdelay itself does not change how the driver in a specific location interacts wiyh the room. Sometimes easier tobser w hen pushed to the absurd. Will the sub measure differently if you test it next week? THAT'S a looong delay. . .
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post #14 of 34 Old 04-04-2013, 11:09 AM
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Quote:
Originally Posted by JonasHansen View Post

Can't I "virtually" move the subs by adjusting delay of the sub on the rear wall?
I wish, since it would allow me to place a sub where convenient and "virtually" move it to the ideal spot using delay. If you can move one of your subs to the back wall, you'd at least be able to see whether that yields at least some improvement in consistency (maybe you can create a virtual subwoofer at the middle of the seating area).

If you can't move your subs, then attempting to do SFM manually is going to be difficult to impossible. You'll have use your BSS units to find the right combination of delay, level and EQ for each of the 4 subs that, when combined, improve consistency across all four seats. How many possible combinations would that be for 3 parameters and 4 subs: thousands, tens of thousands? Having to measure each attempt from four seats just makes the trial & error process even more difficult.

The only other option I can think of for a room like that is lots of bass trapping on the back wall. Maybe smoothening/flattening out the response across the entire room is the best way to achieve consistency in your situation.

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post #15 of 34 Old 04-04-2013, 11:51 AM
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Quote:
Originally Posted by JonasHansen View Post

But lets say my subs in the front exite the length mode. Cant I move the location of the dip by using a sub on the rear wall? If I delay it the exact amount so where there before was a dip caused by the front subs, the rear sub will have its maximum amplitude which will cancel out the dip.

you cannot move whatever dip your sub on the front wall causes by using another sub. You CAN use additional subs to smooth out overall frequency response, a topic much discussed around here. In that context, getting the smoothest response in any given location may well be helped by changing the relative phases of the two subs, and the distance setting is a fine way to accomplish that, as I said. but FWIW, each sub's individual room interaction is unchanged. You change the way they play together by offsetting their timing because they are now doing what they were doing before but at different times. .
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post #16 of 34 Old 04-04-2013, 02:39 PM
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As you have obviously found out having multiple subs does not necessarily guarantee smooth bass and low seat to seat variability. Properly placed you should have virtually zero seat to seat variability.

This is the response in our demo theater across a single row of three seats:



The main question is how important is low seat to seat variability to you?

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post #17 of 34 Old 04-07-2013, 09:01 AM - Thread Starter
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I am not really sure what the "conclusion" is. The length mode dip which the front row is placed in, is it impossible to get rid of even by placing aux-subs elsewhere in the room?

My result right now is this:


I think it is an unacceptable response and I am really frustrated about having used so much money/time on this project without having it performing well.

If the only solution is to move the front subs, then it could be possible to move them 23 inches into the room. But this solution requires a redesign of the screen wall so if it can be fixed with aux-subs, then I would definately prefer that.

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post #18 of 34 Old 04-07-2013, 09:42 AM
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Interesting exercise.

The key in this case, given the understandable constraints, is massive rear wall LF damping, and then find the best case combo of time/phase alignment (and also low pass filtration of all subs) of the primary sub group, with that of the fourth sub.

WRT the rear wall damping, it can offer MF/HF diffusion/scattering if needed. Either wood slat techniques, 6 mil plastic, a variety of approaches for that discussion. Either way, adequate rear wall damping, either focused and freq specific (increased difficulty), or broadband (easier) perhaps faced with scattering, etc.

Good luck

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post #19 of 34 Old 04-07-2013, 01:06 PM - Thread Starter
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Originally Posted by FOH View Post

Interesting exercise.

The key in this case, given the understandable constraints, is massive rear wall LF damping, and then find the best case combo of time/phase alignment (and also low pass filtration of all subs) of the primary sub group, with that of the fourth sub.

WRT the rear wall damping, it can offer MF/HF diffusion/scattering if needed. Either wood slat techniques, 6 mil plastic, a variety of approaches for that discussion. Either way, adequate rear wall damping, either focused and freq specific (increased difficulty), or broadband (easier) perhaps faced with scattering, etc.

Good luck

The back wall already has 6 inches of mineral wool with 2 inch air gap. But I guess 25hz is way too low and must be handled by membrane absorbers. I had a chat with the GIK people, and they say that 25hz is not easy to tame.

Right now I am hoping that two aux-subs in the rear of the room can solve the problem. I cant move the chairs forward, as they are already placed 125 inches away from a 124 inch wide screen. I like the screen to be big, but closer will be too big smile.gif I cant move the chairs backwards either, as I have built a riser and is using the space for a sofa section.

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post #20 of 34 Old 04-07-2013, 01:32 PM
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No, 25hz is a tall order, however you can affect change that low IMO.

What's the rear wall construction? Could you bring that 6" of treatment out even further? As you likely know, it's as much a function of the gap depth, as it is panel thickness. If you really want to treat this, a tuned/freq specific pressure trap, combined with a thick velocity panel, across the entire rear surface could work, but how much? I don't know.

Yes, two sub rearward of the center line would enable more response taming.

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post #21 of 34 Old 04-07-2013, 01:40 PM - Thread Starter
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Quote:
Originally Posted by FOH View Post

No, 25hz is a tall order, however you can affect change that low IMO.

What's the rear wall construction? Could you bring that 6" of treatment out even further? As you likely know, it's as much a function of the gap depth, as it is panel thickness. If you really want to treat this, a tuned/freq specific pressure trap, combined with a thick velocity panel, across the entire rear surface could work, but how much? I don't know.

Yes, two sub rearward of the center line would enable more response taming.

Unfortunately I cannot move the treatment out even further. When going down the stairs I have to go around the sofa along the back wall where there only is 22 inches. I know the room is probably too small for two rows - but I really love to have both recliners and sofa and room for more than 3 people.

Rear wall construction is concrete.

When you say that two rearsubs would give more response taming, would this include improvement of seat to seat? I have gotten a good offer on two of these: http://earthquakesounds.co.uk/p/91/earthquake-sound-in-wall-subwoofer

People say they are good, and it would be a good way to test it out.

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post #22 of 34 Old 04-07-2013, 01:47 PM
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Quote:
Originally Posted by JonasHansen View Post

Rear wall construction is concrete.

Bummer, boundary impedance significantly impacts modal Q.
i.e., the more inert a boundary, the better defined the mode


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Originally Posted by JonasHansen View Post

When you say that two rearsubs would give more response taming, would this include improvement of seat to seat?

Hard to say, but I'd suspect it would. That just conjecture redface.gif

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post #23 of 34 Old 04-11-2013, 03:43 AM - Thread Starter
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I saved the money for extra subs/amps and went with a simpler solution. It is very rare that the two rows will be occupied at the same time. Either it will be the sofa or it will be the chairs.

I bought this, which is a remote for my BSS DSP:


Then I can make presets for front row, back row and an "average" preset. I think its the best possible solution right now.

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post #24 of 34 Old 09-10-2013, 01:26 PM - Thread Starter
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I want to bring up this subject again as I am still not sure exactly which position is the best for an additional subwoofer in my room with my seat to seat variance.

If I have my subs behind the screen and have a problem with the length mode, wouldn't the optimal position for another subwoofer be in the null of the length mode (i.e. the middle of the room)?

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post #25 of 34 Old 09-10-2013, 01:43 PM
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Quote:
Originally Posted by JonasHansen View Post

I want to bring up this subject again as I am still not sure exactly which position is the best for an additional subwoofer in my room with my seat to seat variance.

If I have my subs behind the screen and have a problem with the length mode, wouldn't the optimal position for another subwoofer be in the null of the length mode (i.e. the middle of the room)?

You need to cancel the offending room mode by placing subwoofers outputting equal SPL in both polarities of the offending room mode.

So first things first, what is the offending room mode?

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post #26 of 34 Old 09-10-2013, 01:50 PM - Thread Starter
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This is my room: https://dl.dropboxusercontent.com/u/10759510/ForumPics/JBLCinemaV2/2013_05_06/sub_placement.jpg

Look at the pink boxes - this is where my seatings are placed. Subs and screen is currently placed at 0 feet (subs are about 2 feet deep).

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post #27 of 34 Old 09-10-2013, 02:03 PM
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Quote:
Originally Posted by JonasHansen View Post

This is my room: https://dl.dropboxusercontent.com/u/10759510/ForumPics/JBLCinemaV2/2013_05_06/sub_placement.jpg

Look at the pink boxes - this is where my seatings are placed. Subs and screen is currently placed at 0 feet (subs are about 2 feet deep).

Is it the 26Hz area you are having a front/back issue with now?

And you only have subs at the front of the room?

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post #28 of 34 Old 09-10-2013, 02:06 PM - Thread Starter
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Quote:
Originally Posted by Nyal Mellor View Post

Is it the 26Hz area you are having a front/back issue with now?

And you only have subs at the front of the room?

Yes, it is 26hz. The rest of the seat to seat variation at other frequencies is acceptable. You can see my subwoofer placement in the first post in the thread. I have a sub at the sidewall around the midpoint of the room. When measuring this subwoofer alone, the 26hz issue is still present in the first row - i find this very weird as I though a single sub at the null would not excite the mode?

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post #29 of 34 Old 09-10-2013, 04:41 PM
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Quote:
Originally Posted by JonasHansen View Post

Yes, it is 26hz. The rest of the seat to seat variation at other frequencies is acceptable. You can see my subwoofer placement in the first post in the thread. I have a sub at the sidewall around the midpoint of the room. When measuring this subwoofer alone, the 26hz issue is still present in the first row - i find this very weird as I though a single sub at the null would not excite the mode?

The way to solve your issue is to leave the front subs where they are, put the side sub centered on the back wall of the room. Level match the front subs so they equal the output of the single rear sub when fed the same test signal.

For this to work all three subs have to be identical, or you have to be very lucky.

You may also need the ability to independently delay the front and rear subs to get everything optimally integrated.

The issue with this approach is that the back sub is going to run out of displacement before the front three, but you may have enough headroom. If not then you should put two subs front, two rear.

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post #30 of 34 Old 09-11-2013, 01:18 AM - Thread Starter
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I will try the solution you are recommending. It will require that I move all seatings forward though, as I dont have enough space in the back of the room.

What if I place one subwoofer in the back corner of the room? Would this have the same effect as midpoint of back wall? I have individual DSP for each sub.

JBL ScreenArray Cinema | 9.4 Surround | 136" Scope
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