A CT scan of a Stradivarius suggests that its critical difference from other violins is the density of its wood:
Link (Photo: Brendan McDermid of Reuters)
Using an adaptation of a computer program developed to calculate lung densities in people with emphysema, they were able to analyze the physical properties of violins without risking damage to instruments worth millions of dollars.
They found no significant differences between the median densities of the modern and the antique violins but did discover far less variation between wood grains of early and late growth in the old ones.
Link (Photo: Brendan McDermid of Reuters)