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post #1 of 18 Old 08-28-2014, 09:50 AM - Thread Starter
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Ported subwoofer placement behind a AT screen

I am still determining which subs to go with for my theater, and I have a question about the best way to place a ported sub when it is positioned behind an AT screen.

First, which direction should the ports face? Through the screen into the theater, towards the back wall, towards the outside corners, or towards each other (assuming 2)? Or some combination?

Second, how much, if any, space do I need to leave between the sub and the screen?
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post #2 of 18 Old 08-29-2014, 07:53 AM
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Depending on the size of your screen, the ports on most subwoofers end up being below the screen. Then it depends on what you have below the screen. If you're using a false wall, and it's also AT, it shouldn't matter at all, and the direction the ports face will be inconsequential. If you have a solid wall below the screen, you'll probably want to raise the sub so it's not trapped behind the solid part of the wall. That would then place the ports behind the AT fabric of the screen. At that point, you'll want to check for air movement out of the ports. If it's significant, you take a chance of the air movement causing the screen to move. In that case, just turn the ports left or right and you should eliminate the screen movement. Remember that frequencies in the range of subwoofer ports propagate omnidirectionally, so the direction the ports are aimed is not critical.

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post #3 of 18 Old 08-29-2014, 12:33 PM - Thread Starter
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Originally Posted by craig john View Post
Depending on the size of your screen, the ports on most subwoofers end up being below the screen. Then it depends on what you have below the screen. If you're using a false wall, and it's also AT, it shouldn't matter at all, and the direction the ports face will be inconsequential. If you have a solid wall below the screen, you'll probably want to raise the sub so it's not trapped behind the solid part of the wall. That would then place the ports behind the AT fabric of the screen. At that point, you'll want to check for air movement out of the ports. If it's significant, you take a chance of the air movement causing the screen to move. In that case, just turn the ports left or right and you should eliminate the screen movement. Remember that frequencies in the range of subwoofer ports propagate omnidirectionally, so the direction the ports are aimed is not critical.

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Thanks for the reply, Craig. With my current plan, the ports would be below the actual screen, and I would have black AT fabric in front of that area as well. The subs I was considering are known to move quite a bit of air, which is what brought my concern. Now if the ports were turned to either side, the driver would also face that direction. Would that matter at all?
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post #4 of 18 Old 08-29-2014, 01:10 PM
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Originally Posted by ChldsPlay View Post
if the ports were turned to either side, the driver would also face that direction. Would that matter at all?
Driver output in the subwoofer passband is omni-directional, just as the port output is. However, you may get quite a bit of above bandwidth harmonic output. Facing the driver towards the wall, away from the LP, will acoustically filter some of those harmonics out. But before deciding on any sub placement you need to do the sub crawl, preferably using an RTA, to find the best locations.

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post #5 of 18 Old 08-29-2014, 01:54 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post
Driver output in the subwoofer passband is omni-directional, just as the port output is. However, you may get quite a bit of above bandwidth harmonic output. Facing the driver towards the wall, away from the LP, will acoustically filter some of those harmonics out. But before deciding on any sub placement you need to do the sub crawl, preferably using an RTA, to find the best locations.
I won't be doing any crawling. Where they are going to be placed is determined already - behind the false wall behind the screen in between the LCRs. There isn't any flexibility on that matter. I'm just trying to determine if the ones I was thinking about would fit without adding more space behind the false wall. If I can turn them sideways, or if the ports can blow through the AT material, then there's no problem.

If the ports were facing the wall, would they need a certain amount of space to breathe?
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post #6 of 18 Old 08-29-2014, 01:59 PM
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Originally Posted by ChldsPlay View Post
I won't be doing any crawling. Where they are going to be placed is determined already - behind the false wall behind the screen in between the LCRs. There isn't any flexibility on that matter.


Fingers-crossed that it works out and your not sitting in a huge null....but don't be surprised if you are.

You got a diagram of your room (with sub placement), or at least some measurements? With those, we can come up with an estimate of what your response will be at the MLP.

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post #7 of 18 Old 08-29-2014, 02:05 PM
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I'm in somewhat the same situation as OP. Not sure how much room he has behind his AT screen, but if like me it's a few feet wide at the most. What's the sub crawl process for finding location? I understand if the sub would be in the main room...but I'm thinking he's restricted to just the area behind the screen. If that's that case (it is for me) is there a recommended starting point for achieving good results? For me, I can't use one big sub...so I'm thinking multiples...so 2.....is 3 (uneven) a bad thing? If so then 4 next?

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post #8 of 18 Old 08-29-2014, 02:52 PM - Thread Starter
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Quote:
Originally Posted by Alan P View Post


Fingers-crossed that it works out and your not sitting in a huge null....but don't be surprised if you are.

You got a diagram of your room (with sub placement), or at least some measurements? With those, we can come up with an estimate of what your response will be at the MLP.


As shown, it's a 23' x 13.5 room. I have it with a 2' area behind the false wall, and 21' from the screen to the back wall, and approximately 11.5' from screen to the first row.

The white boxes represent the speakers and subs behind the screen.
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post #9 of 18 Old 08-29-2014, 03:05 PM
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You are going to have serious bass nulls from the wave reflection off the wall behind the seats. Since the rows are different distances from that wall the nulls will occur at different frequencies in each row. One fix with two subs is to have one in the front of the room and one in the back. The other option is to make the rear wall totally absorbent so that there is no reflected wave, but that would require a very thick absorption layer over the entire wall.
This might upset your plans, but Murphy's Law of Sub Placement is that wherever you decide subs look the best or are most conveniently placed is where they will give the worst results.

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post #10 of 18 Old 08-29-2014, 03:21 PM
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I will never understand people who design these big-buck home theaters without flexible sub placement in the plans.

They just assume that wherever they decide to place them (for convenience and/or looks) is "good enough"....well folks, it usually ain't "good enough". You can use room simulators and mode calculators to get close, but you'll never really know where your sub(s) will perform best until you get them in the room.

That being said, I agree with Bill - you're gonna need at least one sub in the back of the room. That back right corner looks like a good candidate.

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post #11 of 18 Old 08-29-2014, 03:26 PM - Thread Starter
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Quote:
Originally Posted by Alan P View Post
I will never understand people who design these big-buck home theaters without flexible sub placement in the plans.

They just assume that wherever they decide to place them (for convenience and/or looks) is "good enough"....well folks, it usually ain't "good enough". You can use room simulators and mode calculators to get close, but you'll never really know where your sub(s) will perform best until you get them in the room.

That being said, I agree with Bill - you're gonna need at least one sub in the back of the room. That back right corner looks like a good candidate.
The back right corner would completely eliminate that walking path.
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post #12 of 18 Old 08-29-2014, 03:34 PM
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Well, if you're not willing to make sacrifices in aesthetics, you're going to have to make some sonic sacrifices. You can't have your cake and eat it to.

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post #13 of 18 Old 08-29-2014, 05:04 PM - Thread Starter
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It is more of a functional sacrifice, but there will always be sacrifices. The only way I can put one back there is to cut out a spot in that wall and slide it underneath the stairs. I imagine tjere is a new slew of problems with that.
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post #14 of 18 Old 08-29-2014, 05:14 PM
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couldn't you put one in the back right corner? You'd need to walk to the left to get to the back row seats, but it wouldn't block them as far as I can tell.

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post #15 of 18 Old 08-29-2014, 09:04 PM
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I have a wall of subs behind my screen about only about 4-5" between the subs and the back of the AT.

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post #16 of 18 Old 08-30-2014, 12:43 PM - Thread Starter
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Originally Posted by barhoram View Post
couldn't you put one in the back right corner? You'd need to walk to the left to get to the back row seats, but it wouldn't block them as far as I can tell.
The entrance to the room is the back left corner. I'd like for people to have the ability to go to either side. If I had a little extra space there is a lot more I could do. Another option, which I have no idea if it is realistic, is to slide a small sub into the riser.

I should have enough space that I don't have to have the subs between the LCR's. I may be able to put them in the corners behind the screen wall too.
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post #17 of 18 Old 08-30-2014, 04:35 PM
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You could build a sub integral with the riser. Could you fit a sub in the left corner behind the door?

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post #18 of 18 Old 08-30-2014, 04:52 PM - Thread Starter
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You could build a sub integral with the riser. Could you fit a sub in the left corner behind the door?
No, that is the entrance to theater. It is also only a small little area at the bottom of the stairs. Also, wouldn't that basically be like putting the sub in another room?
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