Window Bracing vs Double Thickness

[mlstrass]

After nearly killing myself getting 2 17^3 net MDF enclosures out of the house I'm going to build a new one out of BB to save some weight and change the dimensions a little. They had to be near 350 pounds each 8O

Old enclosures were 1.5" thick, each housing 2 Tempests and had a simple 2" wide brace near the middle connecting the sides & front/back, so like a + sign. Never had any vibration issues, enclosure was rock solid.

So what are the pro's/con's of doing a 1.5" thick enclosure with minimal bracing vs a .75 thick enclosure with window bracing? I'm sure the 1.5" will be heavier, but the window bracing seems much harder to calculate displacement and a lot more cutting/routing. Any other things to consider?

Will be picking up the wood Saturday when I can get a friend with a pick up to help. This will be for 2 RL-P 15's, single 10" port, 34x24x46 dimensions.

[joecarrow]

The obvious question- why not build two enclosures with one driver each? That automatically cuts the weight you need to worry about when moving them, it reduces internal volume size, and therefore reduces any worried about potential standing waves at higher frequencies, and makes the largest panel half the size.

If you want them to fire in opposition for newtonian cancellation of forces, you could still connect the enclosures rigidly with clasps, clamps, or screws after placing the subs in their final position. If it's just for the sake of having a gigantic monolith... then talk to Kingpin or Crackyflipside

[mlstrass]

I've been building a LOT of car boxes lately and am getting burned out, so doing just one seems appealing. Also easier to fit a single 10" port vs an 8" if doing 2 boxes. Mike from SS did NOT feel that a single 6" port would be enough.

In the car audio world we do 12-16^2 of port per cubic foot of box, so obviously HT boxes are different, but he still said minimum of a single 8", so the length gets very long.

My plan is port out the top, subs downfiring...

[joecarrow]

I love thinking "outside of the box", so to speak. I don't build subs, so you can take this with a big grain of salt. what about letting the port stick out of the box? Just an idea.

There are some good sub people here. They might have some great input for you on this. Have you considered a sonosub?

[JonP]

Wow, now those were enourmous boxes!! Kinda wonder why so big? Most home drivers don't need quite that amount of volume, which is a good thing it sounds for your next one, and you not needing back and hernia surgery after the move! :W

I have used BB ply a few times, and I did some calculations... it's about 10-15% lighter than the MDF I compared it with. YMDFMV... Not huge, but it adds up. It is much stiffer though, and you should be able to get by with thinner amounts with decent bracing. I would agree that with a good bracing scheme and amount of... you can have a good, solid box with thinner materiel.

One thing that helps... it's the panels vibrating that you don't want, bracing supports them and divides them into smaller panels that vibrate at higher frequencies. They also do it at lower amplititude, as it's forced higher.

BUT.. this is a Sub, right? We have filtering to cut off everything above only 50-80-100-120Hz or so... So, it's less of a requirement to make a massively damped box, like you'ld want for a full range speaker. The box can still "ring" at some higher frequencies, but there's no "direct drive" from the speaker driver at, say 300, 500, 800Hz...

Another thought.. you might want to go dual box anyway... it's much better for overall bass distribution in a room to have two subs in different parts of the room. Eaiser to find and tune a good placement.

Anyway... It is a bit more work.. but think about the benefit of having maybe only that front panel a double one, the sides 3/4", when you move it? Oversizing a bit will take care of the loss of volume, and it's not all that hard to calculate fairly closely the final volume.

And, (in case you don't have it or somethng similar already) let me be the first to point you in the direction of WinISD, a free program that lets you play around with a driver in box dimensions, tunings, tell ya how long the port for a diameter and important here, will that airflow get too fast (chuff) with that diameter of port?

WinISD Pro

Hope it helps...

[mrogowski]

Personally, I would go with single thickness for sides and back, and double up on the baffle. Cut one full size piece for the center brace and use a 4" or 5" hole saw to punch some holes in it. That usually only takes about 5 minutes to do.

Going double thickness all the way around is a waste IMO.

Best,
Mark

[cjd]

Well, the Tumult (sealed) I built my folks is both double-wall birch ply and is braced such that no section of wall panel ends up being larger than about a foot square.

To move it, the driver is generally uninstalled...

Consider sonotube construction.

C

[mlstrass]

I've done a sonosub before, but want to do a box/boxes this time. Old enclosures were for 2 Tempests in each, that's why so big. New set up if for 2 RL-P's I'm taking out of the car.

Mark....box will be 24x34x46, subs firing down/port firing up, so I'd double up the top/bottom for sure. With a height of 46" would 2 window braces be enough, cutting the box roughly into 3rd's? I have no experience with them and have always built 1.5" walls and minimal bracing.

[ThomasW]

A single RL-p15" works fine with a 6" fully flared port or a 8" straight one. Either of these designs would be so loud at the onset of any chuffing, the chuffing couldn't be heard.

If you want low weight "box", build a tube sub then skin the outside with 1/2" ply over a lightweight wood frame.

[mlstrass]

Thanks for the feedback Thomas, nice to hear from someone with your level of experience.

I'd do a sono "if" I knew what the new house will be like, but I don't know if I'll have a dedicated HT room, so I'm trying to build something more room friendly just in case. Job transfer happening within the next 6 months.....ugh!

[joecarrow]

Thomas has a really good point there- curved walls are extremely rigid for this kind of thing. Think about things like pressure vessels- they all have curved walls. If you do the math, you might see quite a large reduction in enclosure mass.

[MOState]

Not sure how this affects the sound, but even doing something as simple as adding lateral support across the top and bottom(think floor joists) would do a great deal to stiffen it up. The volume would be easy to calculate, and it would have minimal cutting. If you were to use dimensional lumber it would even be possible to calculate the amount of deflection (flex) you would have (thats assuming there is something out there that would show the amount of force applied to the inside of the box). This of course comes from the principles of wood beam and joist design.

[mlstrass]

Well I've decided upon 2 boxes, 8.5^3 each, 19Hz tune. 1.5" thick all around with 2 window braces, dividing the box roughly into 3rds, and supporting the port. Port will be centered out the top, and sub offset firing out the bottom. About 5" off the ground and on casters. They'll be side by side in a corner about 8' from the listening position. Same place I had my old sonosub and LOTS of room gain. IB/dual Tempest boxes only had one place to put them and it turned out to be a -6dB null....

For now they'll only be getting 400watts each, so I hope that at least gets them moving decently.

Plan to start construction tomorrow night hopefully. Still finishing up a single 15" box for my car.

Thanks for all the feedback, it's greatly appreciated.

[---k---]

Make sure you take a look a the sticky thread with a link to Jon's published article on some speakers designed and built. He has some thoughts there on building a solid box. Maybe a little overkill for your project, but good perspective.