My "Coffee table Sub": Questions on planning... (1 Viewer)

[Clay Autery]

Working name is "Coffee Table Sub" due to my obligation to produce a new coffee table for my wife, need for a sub cabinet for my new Tempest driver, and the need to conserve floor space in my HT room.
The link below is to a view of the plan from the top down. I am maintaining the Vented Adire Alignment but massaging the dimensions to achieve the desired shape.
Top Plan View
I am going to build it as follows:
1) double thickness 0.75" MDF all around.
2) internal bracing
3) plate amp box in the bottom center (so I can add a plate amp later, or take it out and seal the wire pass-throughs if need be.
4) down-firing driver
5) 2 FP3 ports x 11" as spec'd in the Adire Vented plans.
6) Not shown: The inner top and outer top will be outside caps. The outer top will overhang all sides by 1/2" and be under-routed with a 1/8" round over and probably be veneered with maple (or cherry.... spouse choice)
7) Not Shown: Inner and outer bottoms will be inset to the ends and sides.
8) Not shown: All internal braces will be perforated (probably with 4" diameter holes).
The current outside dimensions of the sub are 54" length (including 1/2" overhang) x 24" (including 1/2" overhang) x 22.62" (including 4" legs with inset ball-bearing transfers at the four corners to make it possible to move).
The internal dimensions are 50" x 20" x 15.62", and after subtracting all volume reductions, I am right at 214L.
Here's the questions: I need to reduce the internal volume reduction by 620 in^2 so that I can reduce the internal height to 15" and the external height to 22" max.
Looking at the plan, and keping in mind that the box is 1.5" thick on all sides, would I be able to delete the longitudinal brace and one of the cross braces to gain some internal volume back to trade off for reduced cabinet height.
Currently, the bracing is set for panel spans of just about 9.75" as in the single width Adire plan, but if I used only three cross braces splitting the internal volume into thirds, would I have any problems with flexing... it is my gut feeling that I won't, but I'd like the opinion of some more experienced builders.
BTW, a close look at the plan would indicate that the amp box is only single thickness 0.75" MDF, but with it being reinforced with two braces in the current plan, or one brace across the center in the three cross-brace only plan, I doubt seriously if the 10.75 x 10.75 x 2.75" internal box will offer any flex problems.
Any comments or suggestions would be appreciated....
...and Yes, I know it is way over-built and will be very, very heavy... It should come in at right around 260lbs or a little more...
Thanks,

 

[Chris Tsutsui]

I doubt the box will have flex problems with 0.75" MDF used for the entire enclosure plus bracing. I have held a 3/4" A-grade sheet of MDF and it doesn't flex easily. Add bracing and the panels will have little or no resonating. You'll also be adding 4 lbs of polyfill.

Not double siding the walls will save you about 100lbs and would be easier. If you weren't bracing so close together as in that pic, I'd encourage the use of double siding more but as I see it, I would lose the 1.5" walls.

Today I talked with a custom home theater builder and he commented on my Adire alignment tempest:

He said to watch the polyfill as it can actually burn if touching a hot driver, and don't clog the vent. He also said that small holes creates air noise within the box and he recommends one or two large squares to be routed out of the brace.

He said plastic ports have different resonating frequencies and a thicker port is recommended. (I'm still going to use the FP-3 ports I already purchased)

If you decide to double brace, my only concern would be the weight of the enclosure. But technically the thicker the wall, the better.

Good luck, plz keep us updated

 

[Clay Autery]

Yes... That is my point... IF I DO double side, can I remove the lengthwise brace and one of the cross-wise braces to regain internal volume (to allow the reduction of height by 0.62").
Though I see your point about going to single layer... If I did that, I wouldn't have to worry about volume considerations at all... I could leave it exactly as planned and reduce all dimensions by 1.5"...
OR leave the bottom doubled (double bottom to flush mount driver and maintain box strength (and not screw up my volume calcs.
And IF I go for a single thickness design, I don't REALLY need the FULL longitudinal brace... I can delete the internal pieces, leaving the end pieces to break up the then 20" x 15" end panels into two 9.625" x 15" panels... which is actually a smaller panel area than in the Adire plan.
So, here are my options:
1) Current plan (NOT ACCEPTABLE)
2) Current plan with deleting long brace and one cross brace (ACCEPTABLE)
3) Remove outer layer (except double-baffle), use 4 cross braces, but only the two end fifths of the long brace. Adjust height to bring volume to desired figure. (ACCEPTABLE)
... or some, as yet, undetermined modification.
Please let me have your suggestions.... Patrick?

 

[Clay Autery]

Current "Coffee Table Sub" Plan View
Above is a link to the current plan view of my sub project (still working on it).
It still maintains the 50" x 20" internal LxW dimensions, but I've made the following modifications:
1) Deleted the outer layers of MDF on the sides, ends, and top. (Baffle will remain at 1.5"... double 0.75" MDF)
2) Deleted the interior portions of the longitudinal brace.
3) All bracing will be dadoed (1/4") into the sides and ends (no screws) for more glue area/stronger joint.
4) AMP Box is now set into the two center cross braces for support/rigidity of the single thickness box. The braces will be rabated (3/8") to accomodate the box and to tie the bottom/braces/box together as a unit.
5) Sides and Ends will be joined with a double rabate (3/8") to provide a multi-surface/plane glue joint for better strength and sealing properties. (no screws)
6) Added cutouts in the #1 cross-brace to reflect the 11" port tube pass-throughs... The tubes will be insulated from the brace with neoprene gaskets which will be glued in place on the brace.
Not shown...
The baffle/bottom will maintain its double thickness. The inner bottom will be an inset butt joint, but the outer bottom will be double rebated into the sides and ends (3/8") (no screws on edges, but will pre-drill for screws over cross braces... Each screw hole will take a shot of silicone caulking just prior to screw insertion... mostly for the heck of it).
Driver, amp, and port flairs will be flush mounted. Flairs and driver will be gasketed with a closed cell foam material for sealing/insulation from cabinet.
The top (single thickness) will be a simple butt joint reinforced along the edges and braces with silicone sealed pre-drilled screws.
Finishing: The entire cabinet will be finished in black (probably low-lustre enamel. The top surface and edges will be veneered in burl walnut (if I can find it).
I will adjust the internal depth to achieve the 214L Adire Alignment volume as dictated in the plans (have to re-do all of the volume calcualtions...
So... what do y'all think so far? Sorry i don't have time to do a 3D rendering of the sub... maybe later.

 

[Chris Tsutsui]

I think you'll be pleased with single walls. Keep in mind that it will more difficult to tune the placement of the sub due to its functionality as a table and foot print.

I just wouldn't go all out on a sub that might have only one location possibility.

To save time and $, I think an inner caulk seal would be all that's needed for the ports and driver. You will likly find it to be air tight without the use of a foam gasket. However, a foam gasket might help prevent port vibrations to be transfered to the cabinet.

May I ask why the ports are side firing while the driver is downfiring? I am just wondering if it will sound alright.

 

[Clay Autery]

So... do you think the bracing scheme will be satisfactory with the reduced wall thickness?
Placement is set... If the response curve is too ugly, I'll have to EQ it... (BFD?)
What do you mean by not going all out because of placement restrictions... It has to be built right or no amount of tuning will make it work...
The gaskets are not a big deal... I have like 30 kinds of foam on hand... and a couple of extra bucks to buy a new one is nothing compared to the hundreds invested in wood, driver, amp, et al... I don't mind paying for the right materials.... which reminds me... I need to pick a glue for this project.... hmmm long work time and good sealing properties...
Oh... and the gaskets also make it much easier/cleaner to remove the driver, etc if need be...
Ports are side firing to make sure there is no restriction around the port exit... With the flair and 11" tubes, a 15" internal depth makes bottom firing an iffy proposition... I'll have to wait to see what the exact overall dims are for the FP3... I would PREFER to have the ports bottom firing of course.... I would re-arrange the driver/port location in that instance probably...
I asked Dan W. if the side firing ports would have ill effects and he said, "No problems..."
I guess this design isn't very interesting, as it appears that only the two of us are talking about it... Maybe the finished product will generate some aditional interest...

 

[Chris Tsutsui]

What if you made a sub box that can serve as a table or a tower. If you want it to act as a coffee table you then put a table top on top of it so it rests firmly in place. Then if you want to optimize your HT you then lift the table top off and tilt the sub so it's front firing and move it to the pre-marked optimal location.

I just thought it would be nice to move the sub around to tune it. Which reminds me I have to fix a 70-100hz spike in my HT before company comes over and tune my tempest this weekend. I have an EQ I'm gonna have to play with along with placement.

I'd like to hear somebody else say it's ok for the bracing but I think it will work.

I've seen a coffee table sub before and it should sound fine, just a couple questions come to mind:

How will you hide the subwoofer cable?

How will you easily tune/turn on the amp.. lift the 200lb box on its side or lay down and crawl under the table?

Maybe making your plate amp a separate will allow an easier access? The PE plate amp gets hot with a tempest so cooling issues come up as well. You'll have a power cable to trip on.

Maybe you can use a rug to cover the AC cable and sub cable but there'd still be a buldge. Will the AC cable be long enough to reach an outlet?

 

[Jeff Rosz]

hello,

is the amp box a set in stone feature of your sub. if my math was correct it is half the volume you need to get rid of. braces dont have to be sheet mdf with 4" holes. something as simple as a 2x2 can be used and hog less volume. how bout shelf bracing (looks like windows of a french door)? go back and look at the lastest sub patrick sun built for his friend. hope this helps.

 

[Clay Autery]

I'll have to give that convertible idea some thought...
Hiding sub/power cables: Don't really know how I'll do it...
Plate amp access: Phot of amp face and touch memory... Did it with a helicopter cockpit, can do it with a sub... You don't mess with it much after it's set anyway.... OR, install the remote version form PE...
Cable length is not a problem... I can just make custom ones... But, I am thinking more and more about hiding the cable... Originally, I was planning to put the components in the back left side of the room, but now I am considering a more conventional location beside the RPTV up front (possibly with an IR repeater so they can be curtained off in the bay window "alcove" along with the RPTV... another project)
Now, I am actually considering dropping the idea altogether and simply building it as a standard downfiring sub from the plans and place it on the opposite side of the RPTV (which would still be between the mains, but would get it to the front of the room...)
I don't know... I'm all confused now...
Well at least I have some time to think about it... I still have the rest of the room painting, replace wall plugs/plates, H-D Duette shades, carpet, component flexy rack, and 6 drawer DVD storage cabinet to complete before I start on the Tempest sub, 281s, and LCC... I just wanted to get a start on the planning...

 

[Clay Autery]

Jeff,

Actually, no... The AMP box is not set in stone... Volume is not really a big concern at this point... Removing the outer layer on the sides/ends fixed that problem, but the placement might indeed cause cooling issues... I don't know.

I'll go take another look at it... Thanks..