With the thickness of the back carved to nominal values, it is time to tune the plate. We want the plate to vibrate freely but we also want it stiff enough to provide carrying power. These two goals are at cross purposes so we have an optimization problem.
Using the work of Carleen Hutchins, I sprinkle tea leaves on the plate and vibrate it with a signal generator and amplifier. I look for two resonate frequencies, F5 and F3. I examine the pattern formed by the tea leaves. From the pattern and the resonate frequency, I can tell where to remove more wood. I also weigh the plate and calculate a number that is proportional to the stiffness of the wood. Finally, I also look at the range of frequencies at which the leaves still vibrate. I find that as I get closer to the optimal thickness, the leaves will only vibrate over a range of a few Hertz for a given resonate frequency. That is when it is time to stop removing wood.
Here is a picture of the pattern I get for F5. This is also called the ring mode for obvious reasons.
Here is a picture of F3.
Here is the finished back.
Now, on to the front.