[bloak]

Ah, the string spacing cancels out, doesn't it? I'd expect the frequency to be proportional to sqrt(t/d)/a, where t is the string tension in Newtons, d is the linear density of the strings in kg/m, and a is the diameter of the racquet head (assuming they're all the same shape and measured in the same way, which probably isn't true, unfortunately).

[zhacker]

yeah. Spot on. For the app, I don't take into account the density, but racket head is taken into account. Prominent shapes are oval and isometric, so I have separate co-efficients for those, which were learned using regression. One challenge that I haven't figured out yet is when 2 different types of strings are used. It seems, some advanced players use different strings at different tensions for the horizontal and vertical strings. Like 32x28lbs or other similar configurations.