Refining harmonic measures H1 and H2 are very sensitive to frequency of F1, which limits vowel comparisons Inverse filtering recovers the voice source, but is not always practical Iseli & Alwan (2004), Iseli, Shue & Alwan (2006) provide corrections for higher formant frequencies and BWs H1*-H2*, H2*-H4*, H1*-A3*