Natural Sound Speech Synthesis by 3D Acoustic Model Based on MRI Measurement of Speech Production System

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This research aims to find in detail the acoustic effects of fine morphological construction of the nasal cavity. Several kinds of numerical acoustic tube models were constructed on a computer from measurements of the nasal cavity by Magnetic Resonance Imaging technology (MRI). Using FEM (finite element method), we calculated acoustic pressure, particle velocity, acoustic intensity, and acoustic transfer function of the simulated nasal tracts.The simulated nasal tract is essentially an acoustic tube which is similar in some aspect to human nasal tract from the soft palate to the nostrils, between which there are a bifurcation of left and right nasal passages and several pairs of the sinuses. A elliptic model was made as to be straight tube having elliptic cross section in which an areas and coircumference were identical with the actual dimensions, respectively. A realistic model was made as to be bent and complicated in cross section as in actual nasal tract. The maxillary sinus and th … More e frontal sinus, were also introduced. In addition, the acoustic effcts of narrowing the nasal tract cross section, and inserting something obstacles at somewhere in the nasal tract were examined.From the results, it is found that not only the conventional area function of the nasal tract which represents cross sectional areas of the nasal tract along with axial locations, but the cross-section shape, the bifurcation, the left-right asymmetry, and the bending of the nasal tract also affect the acoustic characteristics. We showed a limit of approximation ability of straight-elliptic tube models. The left-right asymmetry of the nasal tract always produce pole-zero pairs in transfer function. The complicated shape and the bending of the nasal tract produces zeros of the transfer function by a circle like flow of acoustic intensity in the nasal cavity. It is confirmed that the sinus cavity itself acts as a Helmholtz resonator and produces a pole-zero pair at nearby its resonance frequency in the transfer function. However, there are actually several number of sinus cavities in both sides of the nasal tract, and they are not symmetrical, therefore, they produce fairly complex acoustic property below 2000Hz. The narrowing of one side of the nasal tract causes a severer left-right asymmetry and produces a complexity in transfer characters. Finally, it is shown that any obstacle inserted in the nasal tract may produce a branching flow of acoustic wave in the nasal tract and may cause some pole-zero in the transfer function. Less