Comparing Internal and External Standards in Voice Quality Judgments

Degree of hoarseness can be evaluated by judging the extent to which noise replaces the harmonic structure in the spectrogram of a sustained vowel. However, this visual method is subjective. The present study was undertaken to develop the harmonics-to-noise (H/N) ratio as an objective and quantitative evaluation of the degree of hoarseness. The computation is conceptually straightforward; 50 consecutive pitch periods of a sustained vowel /a/ are averaged; H is the energy of the averaged waveform, while N is the mean energy of the differences between the individual periods and averaged waveform. Recordings of 42 normal voices and 41 samples with varying degrees of hoarseness were analyzed. Two experts rated the spectrogram of each voice sample, based on the amount of noise relative to that of the harmonic component. The results showed a highly significant agreement (the rank correlation coefficient = 0.849) between H/N calculations and the subjective evaluations of the spectrograms. The H/N ratio also proved useful in quantitatively assessing the results of treatment for hoarseness.