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.