surround speakers place "in-wall" - AVS Forum
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post #1 of 17 Old 04-29-2013, 05:09 AM - Thread Starter
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I am building a theater, so I have have access to all the walls currently. I am placing some polk audio bookshelf speakers and the sides and rear of the room. Rather than place them on the wall, I was thinking about building some plywood boxes to mount into the wall and then allowing the speakers to just sit in the box. That way they will be recessed a bit into the wall. Is this a horrible idea? I a good idea? I have no idea. I just like the idea of pushing them in a bit.

If I make a box, should I make it just a bit smaller than the speaker, or maybe as wide as a stud (~16''). I have a bunch of wood to use, what kind of material would you cover it with?

Thats it.

Thanks for the input.
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post #2 of 17 Old 04-29-2013, 06:37 AM
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Hmm, well, it'll mess up something called the baffle step region of the speakers FR. But to be honest, whatever, just do it. They'll probably sound to warm/thick, but you can't use audessay or what ever you have to straighten it out. Try to get the speaker flush to the wall to avoid something called diffraction.

It's not the best, but not really worse than tight against the wall either. Sometimes practicality is more important and in this case I'd say do it.

Now if you want a true in wall speaker and aren't afraid to really DIY, let me know.
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post #3 of 17 Old 04-29-2013, 07:02 AM
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Hmm, well, it'll mess up something called the baffle step region of the speakers FR.
Maybe, maybe not. If the speakers are baffle step compensated it would lead to heightened bass/midbass response, but IME most commercial cabs in Polk price range do not have BSC. OTOH recessing them into the wall will raise the Allison Effect notch frequency, perhaps eliminating it entirely. The same applies to diffraction issues. Flush is best, but extending a few inches shouldn't be a problem.

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post #4 of 17 Old 04-29-2013, 08:34 AM
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Ya good point, there may not be any BSC, so in wall would help. But some of the Polk towers actually use a lot of BSC and even use 2.5 way configurations. This sounds like a simple TM though, so likely very little BSC.

How deep is the speaker flexinator? If it's shallow try to flush mount it. I doubt it though.
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post #5 of 17 Old 04-29-2013, 08:58 AM - Thread Starter
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no way i can flush mount it. I just built boxes that are 9'' deep. since its a 2x4 wall, and there is a piece of wood on the back of my box, it will protrude quite a bit. I figured putting it 5 inches into the wall was better than the whole thing poking out.

Making the walls thicker to accommodate the speaker is an option, but that part of the room is only 13.5' wide, I'd hate to lose any more space...

As an aside if you use sketchup, take a look at my plans. I am a novice with it, but here is what I have planned so far theater.skp.zip 81k .zip file

Its not a real zip file, just delete the .zip from the file name.
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File Type: zip theater.skp.zip (80.6 KB, 2 views)
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post #6 of 17 Old 04-29-2013, 09:01 AM - Thread Starter
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The speakers are polk monitor 30s, i have 4 of them for sides and surrounds.
http://www.amazon.com/Polk-Audio-AM3025-A-Bookshelf-Speakers/dp/B0002ZSFTG
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post #7 of 17 Old 04-29-2013, 09:05 AM - Thread Starter
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i can remove the rear feet and mounting stuff from the speaker to get it an inch closer to the back of the box, does the port on the back pose a problem with placing it inside a box as I am planning?
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post #8 of 17 Old 04-29-2013, 11:34 AM - Thread Starter
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sorry for the crappy picture. but I built them and put them up. take a peek.
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post #9 of 17 Old 04-29-2013, 12:55 PM
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Quote:
Originally Posted by flexinator View Post

no way i can flush mount it. I just built boxes that are 9'' deep. since its a 2x4 wall, and there is a piece of wood on the back of my box, it will protrude quite a bit. I figured putting it 5 inches into the wall was better than the whole thing poking out. .
Four inches of extension places the Allison Effect frequency at 850Hz. The baffle step on these is at about 1900Hz, so there will be some loss of response at 850Hz. But if they were placed so that the baffle was nine inches from a rear wall it would be worse, with the Allison effect response notch at 375Hz, and being more than an additional octave below the baffle step it would be a much deeper notch as well.
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does the port on the back pose a problem with placing it inside a box as I am planning
That's another story entirely. It could totally ball up the works if they are run full range. But if subs are used below 100Hz it should be no problem, as the ports wouldn't be doing anything anyway.

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post #10 of 17 Old 04-29-2013, 01:21 PM
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1900hz seems to high Bill? Baffle step F3 is around 400hz and the edge diffraction peak around 1200hz by my off the cuff guess. The OP should be relatively ok. Closer would be better, but pretty good.
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post #11 of 17 Old 04-29-2013, 02:23 PM
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1900hz seems to high Bill? Baffle step F3 is around 400hz and the edge diffraction peak around 1200hz by my off the cuff guess. The OP should be relatively ok. Closer would be better, but pretty good.
I quoted the baffle step where the baffle is one wavelength wide, above which there is no loss.

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post #12 of 17 Old 04-29-2013, 02:48 PM
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Ah, I see. Half that makes more sense for estimating how much reaches the back of the speaker. I just checked it out in EDGE to make sure I wasn't off, but it's fully forward radiating by 1000hz. Unless I inputed it wrong.
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post #13 of 17 Old 04-29-2013, 02:56 PM
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Ah, I see. Half that makes more sense for estimating how much reaches the back of the speaker. I just checked it out in EDGE to make sure I wasn't off, but it's fully forward radiating by 1000hz. Unless I inputed it wrong.
I don't know what EDGE is, I just calculate 1 wavelength dividing the speed of sound (1130FPS) by the smaller baffle dimension. Below that it takes a good two octaves to reach the full 6dB of baffle step loss.

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post #14 of 17 Old 04-29-2013, 03:17 PM
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Gotcha. I was looking at F3.

EDGE is a baffle step and diffraction simulation program.
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post #15 of 17 Old 05-04-2013, 02:21 PM
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Foam to the edge of the drivers and rounded corners will help too, right?
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post #16 of 17 Old 05-04-2013, 02:54 PM
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I think David Ralph has found felt to be the most effective, but ya, you're on the right path.
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post #17 of 17 Old 05-04-2013, 03:47 PM
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Foam to the edge of the drivers and rounded corners will help too, right?
Foam won't hurt, but may not help that much either. Rounded corners don't do much unless they're 1.5 inches or more radius. Diffraction effects are easily measured, but not that easily heard, our ears don't work anywhere near as well as microphones and measurement software do.
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I think David Ralph has found felt to be the most effective
Better than foam, yes. The best use for it is to cover the entire baffle, leaving the drivers either not recessed or recessed just enough so that when the felt is added the net result is a flush baffle and driver frames.

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