An experiment in box materials

[Triticum Agricolam]

I see people asking all the time (sometimes here and a ton on facebook) "what's the best box material?". The two top choices are usually MDF or "birch" which each having its own group of advocates. I thought it would be useful to put some numbers to the debate and help everyone make more informed decisions.

The question:

The purpose of my experiment is to measure and compare the relative stiffness between materials. I have five materials I will be testing, all are considered to be 3/4" thick. They are as follows:

MDF - measured 3/4" thick - everyone knows what this is

baltic birch plywood - measured 11/16" thick - the gold standard of enclosure materials (IMHO) it has 13 even thickness plies, its expensive

Arauco plywood - measured 23/32" thick - its made from South American Radiata pine and has 7 even thickness plies, this stuff is middle of the road in terms of cost

Chinese "pine" plywood - measured 11/16" thick - they claim its pine, it looks a LOT like the usual cheap birch you get at box stores, this has 13 plies with some paper thin face veneers, its cheap

OSB - measured 13/16" thick - oriented strand board, super cheap, super ugly

Here is my testing apparatus, starting off with the Chinese pine:

And here it is with the weight applied:

Each board was cut to 3" wide at is at least 22" long. The supports holding the board up are 18" apart. I'm using a 25 lbs. bag of lead shot for weight and the apparatus just makes sure the weight gets applied to one point consistently between the samples. I'm using a dial indicator to measure board deflection in one-thousandths of an inch.

The results:

MDF - .067" of deflection

Chinese Pine - .0415" of deflection - 62% of MDF

Baltic Birch - .0315" of deflection - 47% of MDF

Arauco Ply - .041" of deflection - 62% of MDF

OSB - .023" of deflection - 34% of MDF

Conclusion:

The MDF is by far the least stiff material, which isn't very surprising, though it is certainly interesting to see just how much less stiff it is. Both the Chinese plywood and the Arauco performed significantly better than MDF and were almost identical to each other. The Baltic Birch was the stiffest of the plywoods, which is also what I expected. What I found really shocking was how stiff the OSB was. I know its intended as a plywood alternative, but its so cheap I didn't think it would perform THAT much better than the other plywoods did. The OSB does have a bit of an unfair advantage though being the thickest of the materials. Its 1/8" thicker than the BB and Chinese plywoods, which is pretty significant. Here is a picture showing the difference:

Hopefully this experiment will help people make more informed decisions about what materials to use. My goal here was not to prove the baltic birch plywood (or any other plywood) is the "best" material or than MDF is junk. There are plenty examples of all the different materials being used effectively. Also, there are a lot of factors that go into determining what the best material for a particular job will be and stiffness is just one of those factors, though I do consider it a pretty important one. Things like cost, surface finish, water resistance, ease of use, and weight all need to be considered as well.

Something else this experiment has given us to consider is how different materials should be implemented differently during design and construction. I would definitely approach how I was going to brace an enclosure made from MDF differently than I would one made from Baltic Birch. Since the BB plywood is significantly stiffer you could almost certainly get away with less intensive bracing. I'm working on another experiment right now comparing some different bracing strategies, I'll be posting it in the next day or two.

ETA: I just posted my thread on bracing, here it is: http://www.stevemeadedesigns.com/board/topic/206664-implementing-bracing/

[deathcards]

you should weigh the boards just to see the difference

Sweet test though

[Skullz]

Even though OSB is only 1/16" thicker it has it's strands in every direction compared to plywood which is in a north/south and east/west configuration, and why it is used as a substrate for floors. MDF has no strands of fiber running in any direction which is why the panel deflects as much as it does, but allows it to have a much greater density than the rest of the materials.

[Triticum Agricolam]

you should weigh the boards just to see the difference

Sweet test though

I have the weights written down. I'll post them later. The MDF is the heaviest and I think the arauco is the lightest.

[bsneon98]

thank you for this man.

when i just re did my whole trunk with baltic birch rather than mdf its a ton lighter and stronger,

It was hard to figure out just how much but it was significant.

[Triticum Agricolam]

you should weigh the boards just to see the difference

Sweet test though

Here are the weights:

Baltic Birch - 2.36 lbs/square foot

Arauco - 1.875 lbs/square foot

China Pine - 2.06 lbs/square foot

MDF - 2.92 lbs/square foot

OSB - 2.48 lbs/square foot

The high weight of MDF is one of the reasons I don't use it much any more.

[DLHgn]

I'm not sure what I want to use for my next build. I would like to use BB but I don't think I'm confident enough in my wood working skills to ensure I don't mess up a cut and waste wood considering that stuff is fairly expensive.

[audiofanaticz]

There is more then just how much a piece flexes that goes into it.

I would love to use nothing but Baltic, but the reality is not everyone is willing to pay that additional price tag for the material

[Krakin]

Isn't the whole point of using MDF is because it has enough deflection to be essentially an acoustically dead material?

[Triticum Agricolam]

Isn't the whole point of using MDF is because it has enough deflection to be essentially an acoustically dead material?

I think the main reason MDF get used is because "that's what other people do" and because its cheap, at least when it comes to vehicle subwoofer boxes.

For full range enclosures I know MDF is really well like because its well damped and "acoustically dead" like you said, but I'm not sure it gets that attribute because it deflects easily. This is outside my area of expertise.