Birch ply vs. MDF for sub woofer enclosures
Written by Orca
Tuesday, 20 March 2007
There has been a lot of talk recently about using birch ply to construct sub woofer boxes as an alternative to MDF.
But why bother? Birch ply is expensive…
Birch ply is a stronger and stiffer material than MDF. A birch ply box will flex less than a MDF constructed box. I’ll let someone else explain why box flex is undesirable.
There are many grades of birch ply and even more grades of plywood available. Far too many to be listed here. Care must be taken when purchasing your plywood.
In general:-
Birch faced ply: -
This is plywood faced with a birch wood veneer. The internal laminates are minimum grade spruce and birch in alternate layers.
Birch ply:-
All laminates are birch. Face layers are also birch. Standard number of plies for 18mm thickness = 13
MDF:-
Chips about 20mm in length from either softwoods or hardwoods are thermally softened before being past through a machine which mechanically refines the chips into bundles of fibres or individual fibres. These fibres are mixed with a synthetic resin then pressed into a sheet. The fibres are randomly arranged within the material. This mat is then pressed again whilst subjected to elevated temperatures and pressures to cure the resin and produce a sheet of the desired thickness.
Mechanical properties:-
In laymen’s terms, strength is an indication of how much load a material will withstand before failure and stiffness is how much the material will flex, or deform, when subjected to a given load.
We are not really interested in the ultimate strength of our sub boxes as they seldom fail, so the material strength is of little use. However, we are interested in how much a box will flex. Therefore, we will concentrate on the stiffness of sub box materials.
Eb = Modulus of elasticity in bending.
Eb describes how stiff a material is under bending. The higher the number, the stiffer the material is and the less it will flex. It allows us to easily compare the stiffness of different materials.
The following properties are all based on 18mm sheet materials:
Birch faced ply:-
Eb = 2700 N/mm^2 perpendicular to grain
4600 N/mm^2 parallel to grain
Density = 11.6 kg/m^2
Birch ply:-
Eb = 3400 N/mm^2 perpendicular to grain
4600 N/mm^2 parallel to grain
Density = 12.4 kg/m^2
MDF:-
Eb = 2200 N/mm^2
Density = 10.8 kg/m^2
It can be seen from the above, that the plywood properties are directional depending on the orientation of the grain. For the purposes of this analysis, we can assume the stiffest properties as sub box panels are generally supported on all four edges. MDF is a homgeneous material and the mechanical properties are identical in any direction.
Comparing the Eb values given above it can be seen that the birch based plywood is over twice as stiff as the MDF. Therefore, if two identically sized sub boxes were fabricated, one from 18mm MDF and the other from 18mm birch ply, the MDF box would flex twice as much as the birch ply box.
That’s quite an improvement from simply using a different material for your enclosure.
Thickness vs. Material stiffness:-
Birch ply is expensive and MDF is relatively cheap.
It’s often asked, ‘Do I use 18mm Birch ply or 25mm MDF for my new sub box?’
As discussed above, birch ply is a stiffer material than MDF. It’s just over twice as stiff. However, the thickness of the material has a far greater effect on the overall stiffness than the actual material stiffness.
Example:-
It is common to ‘double up’ on the baffle thickness of a sub box from 18mm to 36mm.
You’d imagine that by doubling the thickness, you’d double the stiffness and effectively half the flex of the baffle. However, this couldn’t be further from the truth…
By doubling the thickness from 18mm to 36mm you in fact increase the stiffness by a factor of 8. The 36mm baffle will be 8 times stiffer than the single 18mm baffle.
This is derived from the second moment of area of the section given by Ixx = (bd^3)/12
Where d is the depth, or thickness in this example. The thickness value is cubed to arrive at the section stiffness.
For the full increase in stiffness to be exploited, the baffles should be glued together so the two layers act together in unison under load.
A double 18mm baffle is 8 times stiffer than a single 18mm baffle.
A triple 18mm baffle is 27 times stiffer than a single 18mm baffle.
A quadruple 18mm baffle is 64 times stiffer than a single 18mm baffle.
And so on….
By simply increasing the thickness of the material the stiffness overall is increased dramatically. Remember that birch ply is twice as stiff as MDF. However, the original question was ‘Do I use 25mm MDF or 18mm birch ply for my box?’
In this case, the increased thickness of the 25mm MDF will far outweigh the increased material stiffness of the 18mm birch ply. The panels of the 25mm MDF box will be 28% stiffer overall than the 18mm birch ply box.
In general, the thickness of the material is king to minimise flex. The thickness will increase the stiffness far more than a stiffer material would. However, by doubling the thickness you also double the weight of the box.
For ultimate performance, the stiffer birch material should be used in multiple layers. You will then get the best of both worlds. But it is worth keeping in mind that double the layers of material and the weight of the box will also double.
As always in car audio we are faced with a compromise. Box stiffness vs. weight vs. cost.
The choice is yours.
But for someone constructing a simple 18mm thick enclosure, Birch ply will be twice as stiff and only a fraction heavier than MDF. I know which one I’d go for and I'd double the baffle too whilst I was at it.
http://www.talkaudio.co.uk/index.php?op ... &Itemid=26
MDF with proper bracing does the job for me
