iansr
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Post by iansr on Jan 2, 2011 7:05:56 GMT -7
I plan on installing 2 ceiling manifolds each using a pair of 12" woofers. The gap between the joists means that the opening will be limited to 12" in "width". The "length" of the opening is flexible within reason, but for aesthetic reasons I was thinking somewhere between 15" and 18". Do the resident experts think that opening will be viable?
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Post by ThomasW on Jan 2, 2011 7:48:59 GMT -7
The dimension of the 'sides' manifold opening only need to be as large as the dimensions of the panel needed to mount the drivers.
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iansr
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Post by iansr on Jan 2, 2011 8:03:37 GMT -7
OK thanks Thomas
The panels to mount the 12" drivers I guess will need to be about 14" or 15", but I can only have an opening 12" wide. However if I make the other side of the opening, say, 18" then hopefully that will compensate for the 12" side being in theory a couple of inches too short.
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Post by chrisbee on Jan 2, 2011 11:24:17 GMT -7
To rephrase Thomas' response: A box to house 12" drivers need be no larger than 12" on a side. I hope your intended 12" drivers are extremely long throw or you will break them on the very first film you watch. Eight long throw, 12" drivers are much safer. You can fit a 12" driver on all four sides of your two ceiling manifolds. Have look at Mike's driver chart for examples of potentially useful IB drivers. pages.sbcglobal.net/ginmtb/woofer_comparison_chart.htmNow look at the driver's individual displacements. You need a minimum of around 10 litres of total displacement to be safe. More, if you like to play loud.
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Post by ThomasW on Jan 2, 2011 13:28:14 GMT -7
I fail to see why this is so difficult to understand.
If the driver is mounted on a 15" OD board and that board is the side of a box with a 13" ID, make a 13" opening into the room
One doesn't 'compensate' for one dimension of the box/opening being too small by making the adjacent sides too big
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Post by ThomasW on Jan 2, 2011 13:29:55 GMT -7
To rephrase Thomas' response: A box to house 12" drivers need be no larger than 12" on a side. A 12" board with a cutout for a 12" driver will have no strength
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iansr
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Post by iansr on Jan 2, 2011 13:52:45 GMT -7
I fail to see why this is so difficult to understand. If the driver is mounted on a 15" OD board and that board is the side of a box with a 13" ID, make a 13" opening into the room One doesn't 'compensate' for one dimension of the box/opening being too small by making the adjacent sides too big As I have said at least twice, 12" is the maximum distance I have between the joists - I cannot make the opening bigger than 12" on the side that is 90 degrees to the joists
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Post by chrisbee on Jan 2, 2011 13:57:14 GMT -7
A 12" board with a cutout for a 12" driver will have no strength I was thinking about internal dimensions but didn't make this clear. A 12" board? Perhaps not. A cube? Only just. (assuming an ~11" cut-out diameter) Better add some extra space to aid fitting the screws then. Shall we settle on a 13" minimum internal dimensions for a 1.5" thickness cube? Just to be on the safe side? A four x 12" driver cube had better increase to 14" minimum internal dimensions to avoid overlapping basket rims if fitted internally to the manifold.
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Post by chrisbee on Jan 2, 2011 14:00:20 GMT -7
As I have said at least twice, 12" is the maximum distance I have between the joists - I cannot make the opening bigger than 12" on the side that is 90 degrees to the joists Let the manifold rest on the joists. If there is a small step between manifold and joist opening it is neither here nor there.
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iansr
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Post by iansr on Jan 2, 2011 14:17:40 GMT -7
Chrisbee thanks for the suggestion. I have thought about that, however my problem is that my only access into the roof space is by passing things up between the joists. Actually physically constructing the manifold above the joists would be just far too difficult, so I am left with constructing a manifold that is small enough to pass up between the joists.
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Post by chrisbee on Jan 2, 2011 14:35:13 GMT -7
Chrisbee thanks for the suggestion. I have thought about that, however my problem is that my only access into the roof space is by passing things up between the joists. Actually physically constructing the manifold above the joists would be just far too difficult, so I am left with constructing a manifold that is small enough to pass up between the joists. Do you have your 12" drivers yet? If not I'd aim for two 15" per manifold. Make the manifold to slip between the joists. Then screw outward into the joists through pre-drilled holes in the manifold. Fix the drivers from the inside of the boxes using recessed T-Nuts hot glued to the outside of the box. The glue will ensure none of the T-nuts drop out during driver fixing. Use a T-spade wood drill bit to cut a recess for each T-nut on the outside of the manifold before drilling out the fixing screw holes. Ignore the mouth opening dimensions you end up with.
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iansr
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Post by iansr on Jan 2, 2011 15:02:51 GMT -7
Thanks for that. I would like to use 15" drivers but I have limited height above the joists (it is a decent size void in total because its long, but not too high, and made worse by the fact its a sloping roof so the height decreases). I'm going to have another careful look to see if there is any way I can get 15" ones to fit in there
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Post by ThomasW on Jan 2, 2011 15:38:29 GMT -7
Chrisbee thanks for the suggestion. I have thought about that, however my problem is that my only access into the roof space is by passing things up between the joists. Actually physically constructing the manifold above the joists would be just far too difficult, so I am left with constructing a manifold that is small enough to pass up between the joists. People have built the manifold in sections, passed those up through the cutout, then done the assembly work through the opening. One such build was done by Chasw98. I'll see if I can find a picture of his install.... Edit to add this link to the photo album of Chasw98's IB build s21.photobucket.com/albums/b297/chasw98/IB/
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iansr
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Post by iansr on Jan 2, 2011 15:44:08 GMT -7
that would be interesting to see.
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iansr
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Post by iansr on Jan 3, 2011 10:40:38 GMT -7
OK I have carefully measured how much head room I have above the joists and I could get a manifold in there with outer dimension height of 18". So I reckon I can just about get away with 15" drivers i.e. there will be just about enough distance between the cut-out (which is 14" diameter for the drivers I'm considering) and the edge of the panel. Do you concur?
If that is right then I just need to decide whether to semi construct the manifolds above the joists as Thomasw suggested (to maximise the opening width) or be satisfied with a narrower opening and construct a manifold that will fit between the joists.
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Post by chrisbee on Jan 3, 2011 11:45:31 GMT -7
The IB315 has an SD of 810 or 1620cm^2 for two = 250"^2 for two.
250/12 =21". Assuming a 3/4" panel thickness the difference in manifold opening is 2 x 0.75" = 1.5" x 21" = 31.5"^2. 250/31.5 ~= 8:1.
Given the difficulty of building a manifold above the ceiling I wouldn't worry about the reduced manifold opening area.
If you decide to slide the finished manifold between the joists you could use a stepped panel arrangement internally. This would keep the "skirt" between the joists thinner but beefier higher up where the drivers sit.
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Post by chrisbee on Jan 3, 2011 12:13:24 GMT -7
No loss of throat area nor strength.
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iansr
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Post by iansr on Jan 3, 2011 14:05:00 GMT -7
Thanks Chrisbee. What i was thinking of doing involves a small amount of above-joist construction; construct the inner box which will just slide up snugly between the joists but made longer so that bottom edge is level with the bottom of the joists. I would then screw throught the joists to secure the box in place. I would then glue the outer panels to the part of the box that protrudes above the joists so as to give me my 2 layers. can you see any problems with that?
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Post by chrisbee on Jan 3, 2011 14:46:51 GMT -7
Whatever you feel most comfortable with. You're doing the work and know your own capabilities better than anyone else. If you pre-drill the sides you can slip a couple of dowels through each side to rest on top of the joists. These will take the load while you continue with your permanent fixing. Much easier than trying to work and support a heavy box at the same time. Pre-drilling for small dowels to locate the outer layers will ensure concentricity of your driver cut-outs and squareness of your panels prior to assembly. You are going to be working through the driver cut-outs. So have everything within reach before fixing the box to the joists. While supported by dowels you can move the box along the joists if it proves necessary to reach the loose panels. Keyhole surgery takes on new meaning!
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iansr
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Post by iansr on Jan 3, 2011 15:15:59 GMT -7
some good tips there, thanks
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