What controls the sweet spot in all-wood blades?

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What factors affect the location and size of sweet spot for all-wood blades?

Is it:
-Shape of the paddle head?
-Whether the handle is hollow?
-Quality of the wood plies?
-something else?

Essentially, I am asking:
1) Why is the "normal" location of the sweet spot just north of the center of the blade? (Is it because that's just how paddles behave, or because the players wanted it that way and the manufacturers were able to change the location?)

2) What do manufacturers do to keep the sweet spot consistent across different blades?

I read that carbon blades generally have bigger sweet spots, but I'm more interested in all-wood blades because the smaller sweet spot helps people learn to be more accurate with their strokes.
 
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Since there is little written about table tennis technology compared to other sports, it is wise to consult what has be determined by sports such as golf or baseball. In this case base ball
https://www.acs.psu.edu/drussell/bats/sweetspot.html
The key points made with the bat document is that the sweet spots are where points on the bat where the impact is not felt as much and also it is a point that is close to the first to the nodes of the first two modes of vibration.

I don't see where carbon, glue, or quality of the wood has anything to do with the sweet spot because it seems to be based on size and mass. The glue, wood density and carbon can change the mass though but this is indirect. Carbon and the type of wood can affect the COR and damping ( how fast the vibration attenuates ).
 
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The sweet spot is basically stiffness. The way tt blades are shaped, the maximum stiffness is located in alignment with the handle. However, wood is an orthotropic material, meaning that the mechanical properties are not equal in all directions. The maximum stiffness is along the grain. As such, you have to use perpendicular layers, in order to enralge the sweetspot. That's why composite blades have a larger sweetspot, because carbon more homogeneous than wood.

So, what controls the sweetspot? Everything... You achieve a certain longitudinal stiffness by choosing the materials, their mass, and their relative distance (talking about core and outer plies). More longitudinal stiffness means a bigger sweetspot in that direction. Then you "expand" the sweetspot with the perpendicular layers.
 
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