The importance of the middle layer
Usually the middle ply is the most overlooked when choosing the composition of a blade, but in my opinion it’s the most important one. Hardness is dictated by the whole composition, this means every layer has a contribution to this, and so does the thickness. However, the middle ply is the most important one when it comes to the amount, and type of feedback, we get in our hand. This happens because wood is an orthotropic material, meaning it has unique and independent properties in different directions. So, most of the strength will be along the grain, with little or no strength in the perpendicular direction. That’s why most blades use a crossed grain pattern, in order to achieve stability in all directions. This stability is what we usually call sweetspot, it basically means that the mechanical properties are more homogeneous along all directions of the blade face (more on that later). For example, a standard 5 ply blade will have a core and outer plies with the grain oriented vertically, while the middle plies will have the grain oriented horizontally. If all the layers were placed vertically, the blade would be very prone to warping.
The middle ply adds very little stiffness to the blade because the wood grain is at 90º, so it contributes more to the perceived hardness than the speed. So we can control the type of feedback by choosing a softer/harder or thinner/thicker layer. We can also introduce a composite layer, which is an isotropic material, so it will have important mechanical properties along all directions.