Let's build a custom Subwoofer for Carlman's basement theater

[mgalusha]

You need to update the drawing Rich, it should have BAM on the sub... BigAssedMother... 

[bpape]

I think you're going to have a hard time like that.  It's going to be pretty tough to get it braced and solid enough to not have that driver just shake it apart.

Bryan

[richidoo]

Good Idea Mike. Finally it has a name!

Thanks Bryan. The largest unsupported span is about 12 inches, from vertical brace to rear corner, which are supported in two directions, so effective max unsupported span is about 9 inches. All surfaces are double ply MDF glued and stapled every 2 inches except right near baffle where 2 layers just won't fit and I made some 4" wide reliefs at the center of the driver. Do you have some suggestions how to improve the strength?  I could easily add more vertical braces, in fact I had 4 in there, but I thought 6 inch max unsupported span was overkill. I was also thinking about using threaded rods to pretension the box, I could use a few of those without increasing the box size too much, but there is plenty of room to extend the length for more bracing. There isn't room for thicker top/bottom walls or anything more near the driver. Maybe a layer of FG on the seams, but that won't help much with stiffness. I was also thinking of epoxying on some 1/8 aluminum sheet in critical spots, if needed. Or even making the baffle from aluminum, but that's probably $200 for custom CNC. tia for any ideas
Rich

[bpape]

It's not the baffle itself that I'm concerned with.  I'm worried about where the angle meets the top of the riser.  The kind of force that driver is going to exert is just asking to sheer that angle right off.  If the driver is capable of being mounted vertically (cone up), I'd likely just do that.  Having seating on top of it won't stop much of anything at the very low frequencies.  Or, is the depth from cone to back of magnet too big to mount that way?

Bryan

[Carlman]

I'm not nuts about it pointing straight up.  I think it'd be awesome for a few minutes, though.   It would be firing directly into someone's rear end...

I know low bass is considered omnidirectional but the bass won't be distributed evenly for the audience if one person is sitting on the sub and another person is 7' farther.  There will be a distinct imbalance that will be impossible to cure doing that.

Pointing the sub to the rear of the room gives it a chance to pressurize the area behind the seats first.

I like the design as-is... It works for me and looks pretty rigid with the bracing you've proposed.  Looking forward to putting it all together!

[richidoo]

It's not the baffle itself that I'm concerned with.  I'm worried about where the angle meets the top of the riser.  The kind of force that driver is going to exert is just asking to sheer that angle right off.  If the driver is capable of being mounted vertically (cone up), I'd likely just do that.  Having seating on top of it won't stop much of anything at the very low frequencies.  Or, is the depth from cone to back of magnet too big to mount that way?

Bryan

Thanks Bryan, I see your concern. I weaved the edges together at the angle, and put that 2x4 beam in there to support the angle in the center. The angle is also woven into the side walls. But I could put an aluminum angle on the outside, spanning the width. Whatya think about that? You're worried about the flexing of the joint under pressure, right?

Vertical driver in a simple box would have been much easier, but the motor is 8 inch diameter, so angling the driver vertical makes the top of the motor about 13" off the ground, so none of the driver would fit under the deck. Tilted, I get about 2/3 under the deck. I agree it is gonna be a highly stressed structure. We'll have to see if we can break it after a few beers. I'll bring the ear protection. haha   Then we can build another square box to go in the attached utility room.

[bpape]

Yeah - I'm worried about having anything other than a direct brace in the direction of the driver movement.  Pushing at an angle like that I think it will eventually fail.  Maybe a brace that's parallel to the driver axis that extends from the baffle at that angle all the way down to the joists on the floor? 

Bryan

[richidoo]

OK, I see what you mean. There is plenty of room either side of the driver, so it should be easy enough, and it will take some of the fore/aft vibration off that joint. Good idea, thanks

[richidoo]

I bought a jigsaw for the subwoofer project, and I saw the platform today too. We decided to make a rectangular box and shoot it straight upward from the top of the platform. With Carl's Piegas solid down <40Hz, and OB subwoofers up front down to 25Hz, this sub will crossed in very low so it shouldn't matter which way it points. We'll brace the hell out of it, should be quite solid. We are supposed to knock it out next week. I hope to have the square parts cut ahead of time.

The only challenge is making a thick walled box, while leaving an inch of clearance between bottom of box and back of motor for cooling air to the voice coils.

[Carlman]

If need be, I can remove the carpet, subfloor, and my membrane stuff to get at least another 1.25" under the platform.  Then the sub will rest directly on the concrete which I think might be better anyway for isolation purposes.
I'm not sure how much room is under the seats I'm getting but it should be at least a couple of inches.
Here's a link to the seats.  About 1:15 into the video you can see the back and it looks like there will be plenty of room to have some protrusion out of the top.

OK.. off to bed.. everyone have a great weekend... I'm excited about getting this sub built, Rich.. thanks again for all the help!

[richidoo]

I redesigned the sub with Bryan's concerns and time considerations in mind. It's now rectangular and pointing straight up. The chair over it will be in great demand.

Since crossover will be very low ~40Hz, the chair above and the direction of pointing should have very little influence, at least on sonic performance elsewhere in the room. The person sitting in the chair above was always gonna get a little something extra for their ticket price.

Comments please? We plan to build it on Thursday.
Thanks
Rich

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[richidoo]

Finally some reality behind one of these "Let's Build a..." projects! Feels good to finally make something again.  We had a blast, and nobody got stapled.

The walls are 2 laters 3/4" MDF laminated with Titebond1, then Carl stood on the top while I stapled every 6 inches or so over the whole surface. He moved his foot for each staple, so there should should be few voids if any. We forgot to count blade kerf for one of the cuts, doing the fraction math by pencil on the board and making the same mistake 3x. funny!

After 6+ solid hours, the box is almost finished. Still left to do is mount driver with blind nuts, cap screws (and red loctite?) Then attach the top with glue, staples and screws to the support studs.

This box is absolutely solid, and huge... Can't wait to hear it.



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[stereofool]

Way cool!!! Carl-in-a-box .

[Carlman]

What a great day, thanks so much to Rich for thinking it through and making it a fun project.
Without some planning this might've been a real disaster filled with stop and go head scratching.  Instead we got it 90% complete in a day... and towards the end we were both smiling.. and 1 of us being somewhat silly.. as shown in the pix ..

The box seem solid and well-supported.  I've never seen anyone build something so exacting out of wood.  It has very tight tolerances throughout, especially for a garage-assembled project.

Thanks again and looking forward to hearing it soon... The projector won't be here mid-May so we have to wait a while.. I'm thinking it'll all be worth it.

[richidoo]

Hell with the projector, we can just listen to the soundtracks!!!   
Do you have Pearl Harbor?