What might be, if any, some good numerical approximation functions to the ISO 226 family of human perceptual equal loudness curves? Is there a polynomial fit of reasonably low degree?
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:
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.