Back plate graduations

Refer to the section Graduations - top plate for all the information necessary to asses and graduate the plates.

Graduation patterns

a) bulls eye - thicker in the center in the for of a circle
b) spine - thicker along the centerline

For our violin, we will choose the most common bulls eye pattern. This is also the pattern Stradivari used, which is obvious from the average thicknesses in his back plates.

Thicknesses

In Fig. 1 you see the averages for 48 Stradivari back plates. You can distinctly see the bulls eye pattern used here with the thickest part right in the center of the plate. Again, these figures could be a little thicker. A good rule here is that the thickest part should be twice the thickness in the bouts.

Graduating

1. Rest the back on soft material to prevent dents.
   
2. Use a small thumb plane and start removing wood.
   
3. Keep checking the M5 frequency and the weight of the plate.
   
4. Create a table for your measurements, like the one below, to mark your progress.
   
   

Starting plate:

Mode 1	Mode 2	Mode 5	Plate weight
* Hz	* Hz	430 Hz	143 gr

Finished plate:

Mode 1	Mode 2	Mode 5	Plate weight
* Hz	* Hz	364 Hz	107 gr

Stiffness index: 14 177 072
(364 squared) * 107 = 14 177 072

Similar rules as of the front plate applies to M1 and M2 frequencies. M2 is a little bit under half of the M5. M1 half of the M2. M2 and M1 is dismissed there, because it can be hard to find on back plate. But the lower the M5, the more they can be heard. I had no problems to find and measure them as long as the M5 is under 440 Hz, but you got to know, what you are looking for. Back plate often tends to accentuate some strange high frequencies and some semimode vibrations. Some makers fine tune their plates to equal the M2 on back and front plate. Some fine tune M5 and M2 into exact octave. Some sources say that you should watch the M2 frequency and keep it above 160 Hz, or the violin sound will resemble viola. It seems M2 influences the timbre of your instrument.