Development of an Acoustic Simulation Method during Phonation of the Japanese Vowel /a/ by the Boundary Element Method

Abstract

The purpose of the present study was to establish the method for an acoustic simulation of a vocal tract created from CT data during phonation of the Japanese vowel /a/ and to verify the validity of the simulation. Material and methods. The subjects were 15 healthy adults (8 males, 7 females). The vocal tract model was created from CT data acquired during sustained phonation of the Japanese vowel /a/. After conversion to a mesh model for analysis, a wave acoustic analysis was performed with a boundary element method. The wall and the bottom of the vocal tract model were regarded as a rigid wall and a nonrigid wall, respectively. The acoustic medium was set to 37°C, and a point sound source was set in the place corresponding to the vocal cord as a sound source. The first and second formant frequencies (F1 and F2) were calculated. For 1 of the 15 subjects, the range from the upper end of the frontal sinus to the tracheal bifurcation was scanned, and 2 models were created: model 1 included the range from the frontal sinus to the tracheal bifurcation; and model 2 included the range from the frontal sinus to the glottis and added a virtually extended trachea by 12 cm cylindrically. F1 and F2 calculated from models 1 and 2 were compared. To evaluate the validity of the present simulation, F1 and F2 calculated from the simulation were compared with those of the actual voice and the sound generated using a solid model and a whistle-type artificial larynx. To judge the validity, the vowel formant frequency discrimination threshold reported in the past was used as a criterion. Namely, the relative discrimination thresholds (%), dividing 4F by F, where F was the formant frequency calculated from the simulation, and 4F was the difference between F and the formant frequency of the actual voice and the sound generated using the solid model and artificial larynx, were obtained. Results. F1 and F2 calculated from models 1 and 2 were similar. Therefore, to reduce the exposure dose, the remaining 14 subjects were scanned from the upper end of the frontal sinus to the glottis, and model 2 with the trachea extended by 12 cm virtually was used for the simulation. The averages of the relative discrimination thresholds against F1 and F2 calculated from the actual voice were 5.9% and 4.6%, respectively. The averages of the relative discrimination thresholds against F1 and F2 calculated from the sound generated by using the solid model and the artificial larynx were 4.1% and 3.7%, respectively. Conclusions. The Japanese vowel /a/ could be simulated with high validity for the vocal tract models created from the CT data during phonation of /a/ using the boundary element method.

An Acoustic Simulation Method of the Japanese Vowels /i/ and /u/ by Using the Boundary Element Method

“…In the verification using actual voice, the relative discrimination thresholds were within 9%, except for one of the six cases for F1 of /i/ and two of the six cases for F2 of /u/. No significant difference was found between the numbers of participants in whom the relative discrimination thresholds were 9% or less in the present simulations of /i/ and /u/ or the previous simulation of /a/ [9]. Similar to the results of the previous simulation of /a/, adjusting the length of the virtual cylindrical extension of the lower part of the glottis resulted in relative discrimination thresholds of less than 9%.…”

“…By using the boundary element method in the same way as for the Japanese vowel /a/ and applying the same parameter settings [9], an acoustic simulation could be stably performed for the Japanese vowels /i/ and /u/. In a previous study [14], F1 and F2 of /i/ ranged from 250 to 500 Hz and from 2000 to 3000 Hz, respectively, and those of /u/ ranged from 300 to 500 Hz and from 1000 to 2000 Hz, respectively; the formant frequencies obtained from the present simulation were almost within these ranges.…”

“…A previous study that conducted a sound analysis of the Japanese vowel /a/ [9] set the sound source as a point in a place corresponding to the vocal cords, modified the sound by reflection, absorption, and interference on the wall of the vocal tract, and used the boundary element method to analyze the sound that radiated from the mouth and nostrils to the outside space. As compared with the Japanese vowel /a/, /i/ and /u/ are narrow vowels, and the volume of the oral cavity is slightly narrower, but not extremely so.…”

“…Acoustic analysis was performed at a range from 1 to 3000 Hz for /i/ and from 1 to 1500 Hz for /u/ at 1 Hz intervals using WAON (Cybernet). Next, the boundary element method and the Kirchhoff-Helmholtz integral equation were used for discretization and to represent the wave equation for sound propagation, respectively, the details of which have been described elsewhere [9].…”

“…Therefore, we were able to use a model of a wide range of areas including the paranasal sinuses, oral cavity, pharynx, larynx, and airway. The model was validated and found to be able to perform simulations with sufficient accuracy [9].…”