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View Full Version : VOLUME SHAPING QUESTION



R Beardsley
12-17-2007, 01:36 PM
I'm trying to design my theater system to have two towers, which would also house my regular 4412's for use with music, as well as house a pair of 2245's. 18" in enclosure depth (inside volume depth) would be perfect for where these are going. In order to achieve the 14.13 cubic ft woof volume, I'm ending up with 6 3/8 depth at the upper section, behind the 4412's and 4408's. Is this a bad thing to do, shape wise, or is the volume amount all that is important?

Joe Alesi
12-17-2007, 02:46 PM
Hello Bear,

Off hand, I would have said that Volume is volume and that is it, but given the shape in particular of the slender upper volume of the cabinet I'm not so sure.

It is entirely possible and it is a S T R E T C H that an enclosure like this may be seen acoustically as one ported enclosure (helmholtz resonator) connected in series to another helmholtz resonator. Even if this were the case would the resonant frequency of the secondary helmholtz resonator be signiifcant in its impact in the sound of the subwoofer in it's normal operating band (say sub 80 Hz)? Hmmmmm???? My guess would be that it would not impinge much in the 80 Hz range, given the volume of the upper chamber.

Sketch to follow.

boputnam
12-17-2007, 03:39 PM
...which would also house my regular 4412's for use with music...Confused...

So, you would not use the 2245's except with DVD's...? And the 2245's cabinet is merely a clever "bookshelf" for the 4412 and 4408's...?

The design, as trick as it is, and if it will work as planned, will need hella stiff bracing in those upper cavities to reduce unwanted resonances. This seems a big risk to this layman.

OTOH, why go through this multiple cabinet system menagerie? If you start running the 4412 and 4408 at-the-same-time you are asking for phasing and comb-filtering issues which will/could compromise the overall goal. Why not just make proper DIY cabinet housing the driver compliment you like and use it for all purposes. Maybe I'm missing something... :blink:

R Beardsley
12-17-2007, 04:12 PM
Ah, not a bookshelf. What you are seeing is the Simplified Volume. Bracing will be quite strong. The phase issue is appreciated, and not a factor here. This will be used for theater and music, with one system at a time. 4408's and 2245's for movies, and 4412's for music. This place is a mess with all these speakers sitting around, and I'm trying to keep the boss happy (well, on this one issue anyway - being the foremost of 19562 other issues. LOL) The whole purpose of this is to make it all "go away". If this looks unrealistic, I'll just find another way. Maybe another way would be to add another volume to the side down by the main volume instead, which could be hidden by fake cabinet fronts under the screen. I could add an area +/- 16w x 23h x whatever I need to get the additional volume that way. Maybe that's the way to go?

A9X
12-19-2007, 07:30 AM
It's a complicated build, but if you can do that, go ahead.

For all sealed/ported cabs now I use B&W type Matrix bracing and aim to contain the rear wave in the longest, most damped labyrinth I can. Sort of like a sealed end TL or well damped aperiodic (Drew's design with the 2123) or B&W Nautilus. Use for design inspiration, not specific details. I don't want to hear any of the rear wave back through the cone. Cootn waste / shredded cotton on my preference for stuffing these sort of enclosures.

Still, if it's possible to add the 2245 box with the driver and port down near the floor/rear wall a la Allison that would be my preference. Put the midbass and up where they'll work best in the room.

boputnam
12-19-2007, 09:40 AM
Ah, not a bookshelf. But it looks like the design is intended to support both the smaller cabinets - isn't that the plan?

R Beardsley
12-20-2007, 02:14 PM
But it looks like the design is intended to support both the smaller cabinets - isn't that the plan?

Well, kind of. The idea was for one large enclosure, which incorporated all the drivers. The sketch was the volume for the 2245 out of that enclosure.....all of the partitions, bracing, and outer enclosure removed.
After thinking about it, I guess it was rather a lame idea. I'm working on another idea which reminds me of the Everest Enclosure - only in basic appearance. I have a tendency to complicate things, and strive to complicate them further. :banghead: This is a genetic flaw, which I have had to deal with since birth. I think you guys would like my new idea a little better, at least I hope so. I'll post it, hopefully yet this afternoon.
It's really nice to be able come in here and get an honest opinion about these things.

boputnam
12-20-2007, 04:21 PM
Well, kind of. The idea was for one large enclosure, which incorporated all the drivers. The sketch was the volume for the 2245 out of that enclosure.....all of the partitions, bracing, and outer enclosure removed.Ah, I see. The dimensions suggested you could fit one cabinet in vertical orientation, then the other horizontally above it, topping the stack. I was drawn to that because I once attempted to "hide" various cabinets in a wall bookshelf design. Then, Ken Pachkowsky offered his 4345's for sale, and all my plans at keeping things within the existing decor went away. Now, the 4345's are the decor and everything else works into them... :rotfl:

R Beardsley
12-21-2007, 01:12 PM
This may be a better solution. My thinking was that the 4408 may be better for THX than 4412's, since low freq. is pretty much bypassed. I would appreciate some opinions about this.
The new drawing shows what I'm thinking with regard to enclosures on each side of a 104" screen. These will butt to walls on the side and back. The lower enclosure will have a volume of 14.13cf, tuned to 20hz.
I liked the idea of the B&W compartmentalized bracing, and would like some input about other ways to brace this enclosure.
Remembering an ancient post of Giskard's, regarding 2245's not doing well in corners, has me a little concerned about using them in the corner like this. Fabric will run floor to ceiling from just left of the vents.

Joe Alesi
12-21-2007, 04:39 PM
I liked the idea of the B&W compartmentalized bracing, and would like some input about other ways to brace this enclosure.
Remembering an ancient post of Giskard's, regarding 2245's not doing well in corners, has me a little concerned about using them in the corner like this. Fabric will run floor to ceiling from just left of the vents.

Hello Bear,

I would use a series of horizontal panels (cutoff rectangles/trapezoids?) like the top and bottom panels (perhaps 5 over the height of the enclosure- i.e: support every 1 foot of height) with their centres cut out and joined by something like 2x2 vertically.

Of course the corner position will augment the bass (not unlike a corner horn), but it may reinforce the bass at frequencies where you dont want it to causing significant variation it the response. Some eq and a low enough crosover may help in such situations. It would be hard to predict with certainty.

JBL 4645
12-23-2007, 05:55 AM
Hello Bear,

I would use a series of horizontal panels (cutoff rectangles/trapezoids?) like the top and bottom panels (perhaps 5 over the height of the enclosure- i.e: support every 1 foot of height) with their centres cut out and joined by something like 2x2 vertically.

Of course the corner position will augment the bass (not unlike a corner horn), but it may reinforce the bass at frequencies where you dont want it to causing significant variation it the response. Some eq and a low enough crosover may help in such situations. It would be hard to predict with certainty.


Thatís right most of what I have read about stacking them in the corner is a bad idea where it will add a few db of increase on certain low frequencies ((boom)).

Peace.:)

:tree:

A9X
12-23-2007, 04:20 PM
On large panels with high moving mass, ie large or multiple drivers, I never leave more an 6"x6" unsupported (usually 4x4) and where possible try to go push-push to reduce vibration.

Corner loading can enhance certain room modes, but it's impossible to say how badly until it's in situ. Then notching will reduce the worst of it (not eliminate).