Official Luthiers Forum!

Recently, when I thicknessed a flat top for a jumbo, using a hand plane, I used a method of assessing the top stiffness, as described by several builders, where I shook the top and observed how floppy it was and listened to the "wap wap wap" similar to sheet metal.

Would this work with a carved top? Should there be a noticeable change in stiffness when shaking the plate when reaching the optimum thickess? Or would the graduation etc. make the top more like a braced flat top, and not really respond to the shaking? Or is there simply no comparison?

The reason I ask is that while I thicknessed several sitka flat tops, when I reached a specific thickness, different on each top, the stiffness change was so drastic it was almost a religious experience. But when thicknessing the arch top the changes were noticeable, but very subtle.

The arch itself adds a ton of stiffness to the plate, so no, you can't judge it the same way you would a flat top. Pressing-down at the bridge area to gauge its "springback" and stiffness can be useful, as is tapping it, but careful measuring is the best way to go. Especially on your first few tries! If you attempt to make a carved plate "move" like a flat top, you're going to go wayyyyy too thin!

Thanks grumpy, that makes sense.