Bridge pressure on a bass top would be deadly if not for the arch, which provides strength while still allowing the vibrating plate to be relatively thin. The soundpost pressing on the back does not take advantage of the back's arch because the back is convex, not concave (in other words, the pressure from the post works against the back's arch). I install a "soundpost pad" in my roundbacks, so I can carve them to a thickness that vibrates nicely without the soundpost bulging or breaking the back. I have experimented with many sizes and shapes.
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