Harmonic distortion may be characterised by the proportion of energy of a sinusoidal signal transferred to the harmonics. Differential time scaling resulting from the spectral warping transform allows the fundamental and harmonics to be separated, and thus measured separately. Two spectral warping transforms for distortion measurement are compared: the standard all-pass mapping, and a piecewise linear mapping. Both are shown to be effective at measuring distortion, although the piecewise linear mapping is computationally less expensive.