the frequency space, which approximates the harmonic difference between two frequencies. The result should be low for frequencies that are close harmonies of each other, and high for frequencies that aren't. Principle Mathematical Formulation Notes are perfectly harmonious with themselves hd(a, a) = 0 Notes exactly an octave apart are equivalent hd(a, 2a) = 0 independent of direction (commutative) hd(a, b) = hd(b, a) constant for any interval - independent under transposition hd(a, b) = hd(ra, rb)