In this synthesizer the harmonics for simulating musical-instrument timbre are calculated using interpolating functions and the avalilabe harmonics-data of current musical notes.
The manipulate routine could be improved using AudioGenerator
We do agree, calculating the harmonics this way for the guitar timbre is a crude approximation, however, if you have in mind experimenting with some particular frequencies, then you should find the real harmonics by using a real musical instrument and a spectrum analyzer.
I made the database of up to 21 harmonics for all the equal-temperament notes, however, I thought it was far enough to use 12 harmonics in this synthesizer.
I think, at this point, I must made the following remark: Even considering those delays in the Mathematica Play command, no other software allows you to do things like this, I think all this a great.
Mathematica is a truly powerful tool of liberation.