Do I need bracing?

[goj12]

Hey guys, I'm about to make a box for 2 sundown sd2s, and I was wondering if I need bracing if I'm already going to double up on the front panel so it'll be 1.5in thick. The box will be about 4.5ft3, so is bracing necessary with a double front panel? If it is, would it matter where I would put one? If I have to, is putting one down vertically thats 4 by .75in in the middle between my subs ok?

[goj12]

one more question, what do you guys think of drilling holes in my box for my speaker wire, running them through it, and then putting sealant around them instead of using cup terminals?

[planet10]

What are you building the box out of? A box that large, even if built with quality ply will require bracing to keep it from ballooning. With MDF you'll need much more.

A sealed hole with wires coming out is fine, not as elegant as a cup/binding posts but better sounding.

dave

[goj12]

It's all from mdf, so what about two braces with the dimensions I said earlier vertically and front to back at the middle, will that and the 1.5in front panel be good enough?

[planet10]

For MDF it needs to be almost 2x as thick as quality ply, so you'd have a stiffer/better box if built from 1/2" ply. There should probably not be any unbraced span greater than 8" (can be a long rectangle 8" wide). The braces should reach across the box to the other side. You can greatly reduce the amount of mechanically transmitted energy to the box by mounting the 2 drivers push-push with a rigid connection between them.

dave

[goj12]

thanks, but would I have any serious problems with just the front baffle and a couple braces? I already got one sheet of mdf and I'd rather not put out more money for another sheet since I'm a little tight on cash now. Also what do you mean by mounting them push-push?

[Djim]

Originally Posted by planet10

For MDF it needs to be almost 2x as thick as quality ply, so you'd have a stiffer/better box if built from 1/2" ply.

Hi Dave,
Can you back up your statement with any data?

[planet10]

Did some googling, MDF is typically 3450 MPa, Baltic Birch 9100 MPa (bigger is better), which, in this case, gives an even grater advantage to BB than the GM supplied number of 18mm BB ~ 1 1/4" MDF

Attached is a more generic table from a textbook. You can see that BB is at the top end of the scale.

dave

[Djim]

Thanks Dan for explaining.

In my view, conclusions based on material characteristics charts are not very useful for this subject. Charts of those properties are based on 'contact vibration' and panels in free field. They don’t represent acoustic properties from a loudspeaker enclosure.

In a study from Jim Moriyasu (AudioXPress, February 2002) for an 8” subwoofer, he tested the same design with different blank panels to construct several enclosures. He measured a 1 to 2dB lower level of primary resonances for MDF compared to plywood. In a study for studio loudspeakers from Any Munro (Munro Acoustics) for Dynaudio Acoustics/Professional (not published but shown at an AES convention) the conclusion was 22mm MDF had similar properties to high quality 25mm (18-layer) Scandinavian plywood for the Dynaudio ABES studio subwoofer. In several AES papers you can find studies as part of research for subwoofers/loudspeakers in which they 'prefer' the acoustic properties of MDF. 99% of all top range studio monitors use MDF. Using an economic factor for their choice would be strange for 50K+ Dollar handcrafted systems.

Anyway, studies may have shown an 1 or 2dB better damping values for MDF, this number becomes even less when proper (supported) braces are used. For that reason you can even build high quality enclosures from (fine) particle board if you know what you are doing. But lets face it, how serious and sexy would your design be received if you are using particle board? When you ask a critic if he ever properly constructed a particle board enclosure and compared it one on one with ply or MDF the answer is... In my view the only motivation why particle board has a bad reputation is because all cheap and bad constructed encloses use it. It doesn't matter much what you are using; mdf, ply or even particle board. However, I do agree with Dan that (supported) braces are not an option...

[planet10]

It has been 10 years since i read Jim's article, at what frequencies did the back panel resonate? The Q and frequency of the resonace are important, level is meaningless without those.

As to the others there are counter-examples to each & every one of them.

On a sub, as long as you push the (potential) resonances above the operating bandwidth they will not get excited and be, in practice, non-existent (a pretty easy task -- stiff, light material makes it easier to get there) then what you have to worry about is ballooning. Stiffness reigns supreme there.

Can the Munro paper be had?

dave