Guitar Plate thickness is one of the most debated subjects in acoustic guitar building and the quest to find that "Magic Number" for the top/back thickness, that makes a great guitar. 

The magic number/set dimension method works perfectly for guitar factories. They can set-up the machine to their magic number and sand all their tops and backs to this magic dimension. This allows them to sand hundreds of tops/backs everyday.

The major issue with this method, is that wood is a very inconsistent material. Its weight and stiffness can have such a wide variance between each guitar plate. If we take three guitar tops and make them both the exact same dimension. Top one could be too thick to resonate efficiently. Top two could be too thin to withstand the pull of the strings, which would cause the top to belly only after a few years. Top three could be just perfect. But of because of this inconsistency, guitar companies would err on the side of caution, by making the tops thicker than they need be, to minimise warranty returns.

The Goal is to get each top/back to its optium dimension. To do this each one has to be assessed individually for stiffness and weight. If we do this for each plate, we can start to make a better sounding instrument, more consistently. Now, testing tops/back individually is not a new thing. There is documentation of violin makers tapping plates while building in the early 19th century and it was probabaly done many years before being documented. Then it was passed onto the spanish guitar makers. They would tap, listen and flex the top/back. The experienced maker would then know where to carve the top/back to make a better sounding instrument. This art, is known as "tap tuning" and is still carried out successfully today by many luthiers.

But it is an art and passing on something you hear to someone else, is very difficult. This skill/art often died with the individual luthier, as he was uable to successfully pass it on to his apprentice. Today we have machines that can measure parameters that we can actally see what is happening. With all this new technology we can now see the frequencies that the old Spanish guitar makers were hearing when they were tap tuning and it is much easier to repeat something you can see/measure, rather than something you hear. We can measure things like the thickness, density and most importantly, It's elestic properties (stiffness). Having this knowledge allows guitar makers like me, to consistently produce resonant guitars with a full colourful palette.

There are a lot of  guitar makers out there that swear by a specific target measurment or tap tuning or deflection testing and i don't think i have the right to say they are wrong to do so. I just have a preferred way of doing it. My preferred process for determining the thickness of a top/back is using "Young's Modulus".

I prefer this method as this takes into account the stiffness of the whole material (its long & cross grain stiffness). Young's Modulus is a mathematical formula that engineers use to calculate the elastic properties of a solid material or to put it simply: its measures the stiffness of a solid object.

The method i use, using Young's Modulus to test the stiffness of guitar plates is the brain child of engineer & luthier Trevor Gore and luthier Gerard Gilet. They spent many years proving their theory with mathematical equations, physics and then put all their theories into practice. They then documented all their findings in a fantastic 2 volume book called "Contempory Acoustic Guitar Design and Build". This book is probably the most in-depth guitar book i have ever read and goes into all the science and maths to proves all of their theroies and is a great read for any luthier who is looking to up his game, but not really directed towards the novice. 

Using Young's Modulus in this way to calculate the stiffness of a top/back gives me far greater repeatability in the stiffness from plate to plate compared to using a target dimension.