mdf and birch

[jayhawker]

In a nut shell, need to know the difference in weight savings? 4x8 sheet of birch is lighter, but by how much?

Need to determine the weight savings in the box as the trunk is going to be loaded down with equipment. So what I am after here, how much weight savings can be had going from MDF to Birch. It will be a very standard box. This is goin in my Civic Hybrid 1 (hense the reasoning to use s lighter box material) with no fancy ports or anything. The box will be built later this week. Thanks in advance.

[DaveRulz]

I can't tell you what the weight difference will be, but I can say this: The birch that you are looking for is NOT available at your standard run of the mill lumber yard (Home Depot, Lowes, Etc). You need void free 14 ply baltic birch ply to compete with the rigidity of MDF. This plywood is very expensive, and you'll probably only be able to find it locally from specialty lumber yards or a cabinet maker.

Again, just because it's birch ply does not mean it is a suitable substitute for MDF.

[ItalynStylion]

Dave is referring to Baltic Birch when he says "void free". And yes it's harder to find. However. If you're doing a relatively simple box you should be fine with anything you get from HD or Lowes. If you fear any flexing just brace it with a piece or two.

[bfowler]

my local Lowes has 11ply which i have use on countless enclosures now. it has much less flex then mdf, less dense however. i would have to find some kind of scale, but i would guess about 40% lighter.


i went search for it for smiler reasons for my e36. after i was finsihed with box and amp racks the weight savings was significant

[Dryseals]

Finding a good "cabinet" grade plywood is easy. Check your local cabinet makers, they can tell you. Or just look in the phone book for a wood supply. I use a local supplier who carrys both 11 and 13 ply depending on what they can get and it's actually cheaper than the box stores. But from time to time the box stores have the same thing, just costs a little more. A sheet of MDF can weigh up to 100 lbs, but that depends on the core weight and application it was made for. Your plys are much less weight, about half.

MDF is not good for holding screws, but a ply is. The reason most like the MDF is it's easier to machine, rounding edges makes a nice smooth edge. I usually only use MDF for the front baffle because of machine quality, But for the box, a good grade mulitple ply wood is stronger and less flex and at the same time lighter.

When your looking at ply, the grading is A to D, D being the worse, but that's for the outer grades. So an AA grade means it has minimal blemishes both front and back.

For the inner cores, it's a whole new ball game, there are standards and ratings, but you have to know who made it to know what it is, but generally the more cores, the better chance you have it's going to have fewer voids.

I've been building cabinets and furniture for a long time, what you find out there today is going to be the chinese stuff, some good, some not so good.

[pcana11]

i am in a carpentry class in college, our teacher told us that the oak and birch at home depot is just simply layers of osb ply's glued together and a very thin piece of birch or whatever finish

[jayhawker]

Thanks for the replies guys. My buddy is measuring the trunk out tomorrow for the box. I still haven't determined which material I want but I am leaning toward the birch ply from Lowes. I "searched" on their site and it was less than $40.00 for a 4x8 sheet.

[nineball]

Originally Posted by bfowler

my local Lowes has 11ply which i have use on countless enclosures now. it has much less flex then mdf, less dense however. i would have to find some kind of scale, but i would guess about 40% lighter.

unless you are not properly bracing or using mdf that is too thin for your application you should not have flexing problems. my 12" type-x is being fed 1200rms in a box constructed from 3/4" mdf and i have no flexing issues.

[jayhawker]

This box will not be used for any SPL competitions or anything like that. I actually plan on using two XW12R woofers by Kenwood. I have two X1R amps that will power each woofer. Am I safe assuming there will be no flexing issues?

[bfowler]

Originally Posted by nineball

unless you are not properly bracing or using mdf that is too thin for your application you should not have flexing problems. my 12" type-x is being fed 1200rms in a box constructed from 3/4" mdf and i have no flexing issues.

it wasn't that i was having flex issues. i just noticed when i set the sheet over 2 sawhorses the middle would sag a bit. whereas the birch didn't. once it is cut down to box size pieces it is likely a mute point

[bikinpunk]

just a quick lesson in weight calculations here and how to use google to help you figure this stuff out. Note below where I say "to" in bold. This is to show you that google can convert things very easily for you if you're using mixed units. I use this stuff all the time. You can use onlineconversion.com, but IMO, google is faster. Especially when you consider you can write out an equation, give units, and tell google the units you want as the result.

mass = weight/gravity
Therefore...
Weight = mass * gravity



Furthermore...
mass = Volume * density

So, using the above mass/density equation and plugging into the weight equation you get
Weight = (Volume* Density)/Gravity

Now, ask yourself... what physical property of the two is going to change?
Not gravity... gravity is constant unless you're on another planet.
Not volume because you're buying the same sheet size in most cases (or, at least you can use this as your basis for differentiating the weight)
So, you're left with density. Density is the only thing that will change and effect weight between the two types.

Google:
Birch Plywood density = ~690 kg/m^3
MDF density = ~750kg/m^3

So, you can take a ratio of density: 690/750 = 0.92
Make sure, however, that you use the SAME UNITS when doing a ratio. Otherwise, you'll need to have google convert it for you.
Perecentage multiplication = 0.92 * 100 = 92%

So, the lesser (baltic birch) is 92% of the weight of MDF.

So, for fun, let's say you have a sheet that is 4ft x 4ft x 0.5".
Do some simple conversion for the thickness to get it into feet: 0.5"/12in/ft = 0.0416ft *note: you can bypass this step and just have google do the final conversion for you if you want*
So, type this into google: "4ft*4ft*0.0416ft to ft^3"
and get this : 0.6656 ft^3
So, your volume is 0.6656 ft^3


Now, as we saw above, weight = volume * density
You now know the volume. Multiply it by the density of either material.
Birch:
Google: "0.6656ft^3 * 690 kg/m^3 to lb"
Google gives you the weight of baltic as: 28.67 lbs

MDF:
Google: "0.6656ft^3 * 750 kg/m^3 to lb"
Google gives you the weight of baltic as: 31.16 lbs


As a sanity check, use the ratio we got above to double check this:
28.67/31.16 = 92%
Yep, checks out.

So, in conclusion, you can expect a weight savings of about 3 pounds using the baltic.


*Note: The densities I used may not be EXACTLY the same as what you buy in the store. But, the method for calculation is. You don't have to calculate the weight by the above method. You can simply go off the density ratio. If you know that the mdf sheet you're using is 10lbs, and you know the density ratio of the materials you're comparing, just multiply the 10lbs by whatever the ratio is, and you'll have your weight savings (or addition).*


Hope that helps you guys in the future. Google is good stuff.

[Dryseals]

Originally Posted by jayhawker

This box will not be used for any SPL competitions or anything like that. I actually plan on using two XW12R woofers by Kenwood. I have two X1R amps that will power each woofer. Am I safe assuming there will be no flexing issues?

The thing that causes flex is length from brace to brace. The shorter the distance between the higher strength and less chance for flex. I build furniture for kicks, using opposing grains for something like a table top gives it more strength. In Plys, the grains are opposed, MDF had no grain. Cut a four foot long 3 inch wide piece of the birch and the MDF and see which one flexes easier.

[bikinpunk]

Originally Posted by Dryseals

see which one flexes easier.

http://en.wikipedia.org/wiki/Yield_(engineering)

[Hispls]

Also if you're doing a low power application 5/8" MDF should be sufficient. If you have big sides use a couple pieces of wooden dowel to brace.

[Dryseals]

Originally Posted by bikinpunk

http://en.wikipedia.org/wiki/Yield_(engineering)

I'm not so sure "yeild engineering" has anything to do with the flex of the box. Maybe we should discuss linear elasticity, naw it don't work either.

If I was at work I could give the dynamics of MDF vs ply as per ASTM. The wiggle test is a simple way to show some one which material will be more prone to flex and need more internal bracing. People are more prone to use MDF because it's more uniform in construction, no inner yeilds to speak of. Plus it has a good dampening factor for speaker boxes. But for over all strength, it doesn't compare to ply.

[bikinpunk]

You may be very correct. My structures engineering is weak at best.
My point was to note the differences in yield and tensile strength of the two materials. I don't know where to find the properties for birch; I tried a rudimentary google hunt for it but didn't have much luck. Maybe your standards cover it?

[CulinaryGod]

Just to chime in, yes mdf machines better, yes it flexes easier in long runs, and yes it's heavier than "baltic" or "russian" birch. To clarify blatic birch is the only plywood that is truely birch all the way through. My suggestion is to use a combination core panel as it's kind of a combination deal - the main core is ply layers with a thin layer of MDF just beneath the top veneer. It's the best of both worlds - the MDF gives a dead flat surface for the top veneer to lay on and the inner ply layup reduces the overall sheet weight and increases the screw holding power. If you don't like the combo core try a nice sheet of veneer core, as I've found it's incredibly strong and light as the ply's are pretty thick and void free if you buy from the right panel company.

[89s rule]

mdf!

[Dryseals]

Originally Posted by bikinpunk

You may be very correct. My structures engineering is weak at best.
My point was to note the differences in yield and tensile strength of the two materials. I don't know where to find the properties for birch; I tried a rudimentary google hunt for it but didn't have much luck. Maybe your standards cover it?

The ASTM does cover differing plys, problem is getting everyone to be honest. My job is encompasses instrumentation, been at it for 30 plus years. I've worked at ply manufactures and MDF. Things change from batch to batch. If batch "A" is doing good, keep it running. If batch "B" looks off, blame it on the instruments being off and keep running. Only when the desired product is heavily monitored will the folks in charge stamp it correctly. The guy selling it can only go by what stamped on it.

The only fortunate part for me is that having been exposed to the real manufacturering process, I can tell what to look for when standing at the store. All the numbers stamped on the wood mean something. But it's not uncommon to see a grade "C" being stamped as an "A".

[jon w.]

please visit my website at whitledgedesigns.com. click on the audio system publicity link. part 5 of my series of articles is available as a free download. it has the answers to your questions.

best regards,
jon