Objective evaluation of degree of hoarseness and voice quality
Practica Otologica Kyoto 79(7): 1149-1166
A spectrogram of a normal sustained vowel, taken with a narrow filter, shows well-developed harmonics as equally spread horizontal stripes, while that of a hoarse voice presents the noise components as a cloudy shadow between harmonics. A small laboratory computer was used to evaluate hoarseness objectively. Three pitch periods extracted from the voice waveform were analized at up to 5.5KHz harmonic frequency by Fourier Series Expansion. Since it offers a dispersal spectrum with good frequency resolution, the acoustic energy of the harmonics is calculated separately from that of the noise components. This study was carried out in 28 normal subjects and 30 patients. The ratio of harmonics to noise (Ra) of the normal group ranged from 15.0 to 23.5dB, and in 95% of normal cases the Ra value ranged from 15.4 to 23.3dB. With this criterion, pathologic voices could easily be differentiated from normal. The Ra value also showed a definite correlation with the auditory impression and the visual impression of the spectrogram, suggesting that this is a useful tool in quantitative evaluation of the degree of hoarseness. Furthermore, the relation between the distribution of the noise energy and the voice quality in each case was investigated. The ratio of harmonics to low frequency noise components below 1.38KHz (RL) corresponded well with the R factor (roughness) of the auditory impression, while that of harmonics to high frequency noise components above 1.38KHz (RH) correlated well with the B factor (breathiness). More detailed investigations of noise energy would probably increase the success of differentiation among various laryngeal diseases.