Mechanical properties of the vocal fold have been investigated by means of ultrasonic variant M mode. Vibrations of the vocal fold elicited by a single rectangular pulse were displayed on an oscilloscope. These oscillation waves were analysed by a wave analyser with fast Fourier transform, and resonance frequency, Q value, and damping ratio of the vocal fold in vivo and in vitro were obtained, respectively.
In the present study, the damping ratios were mainly discussed in various cases as follows; normal vocal folds in vivo, normal vocal folds of totally excised human larynges loaded with tension or a mass, vocal folds with unilateral lesions such as polyps, recurrent nerve paralysis and cancer.
Results
1) As phonation pitches were higher, the damping ratios of the vocal fold significantly decreased at phonation neutral gesture. In the excised larynges, as loaded tension stronger, damping ratio also decreased. These results suggested that damping ratio was correlated with the tension of the vocal fold.
2) In the vocal fold with polyp or cancer without vocal cord fixation, higher damping ratios were obtained when compared with healthy side. The damping ratios increased when the vocal folds of the excised larynges loaded with a mass. Mass effect of the vocal fold could be related to the damping ratio.
3) In the vocal fold with recurrent nerve paralysis, damping ratio increased and in the fixed vocal fold with cancer, damping ratio decreased. Stiffness of the vocal fold altered by these lesions changed the damping ratios.
4) The damping ratios of the normal vocal fold during quiet respiration ranging from 0.091 to 0.144 (mean value 0.116±0.014). The damping ratios of male (mean value 0.121±0.016) was slightly higher than that of female (mean value 0.108±0.006).
5) Lower resonance frequencies in comparison with healthy side were obtained in the vocal fold with polyp, cancer without fixation, and with recurrent nerve paralysis. In the case of fixed vocal fold with cancer, the resonance frequency was markedly higher. These results were nearly accordant with the data of the previous investigations obtained by continuous sinusoidal wave.
