Equal loudness curves: any good numerical approximation equations?
I believe that the actual standard (access to which of course must be purchased) specifies all the mathematics. I did find this one MATLAB file in file exchange:

ISO 226 Equal-Loudness-Level Contour Signal

If you're going to use it in a commercial applications though, the distribution license won't allow you to do so. Still, a good start if you don't have the standard handy.