Birch alternative

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.