Analysis of acoustic properties of the nasal tract using 3-D FEM

In order to examine the acoustic effects of the complicated morphological construction of the nasal tract, we have analyzed acoustic models constructed according to measurements by magnetic resonance imaging (MRI). A prototype model was made to be as similar as possible with the actual nasal tract, which is asymmetrical in the left and right passages and has complicated cross-sectional shapes. Sound pressure, particle velocity and sound intensity in the model were calculated by the finite element method (FEM). Several modifications were applied to the shape of the prototype model in order to learn what acoustical effects are produced by the following modifications: (1) models having an elliptic shape, (2) models with and without a pair of maxillary sinuses, (3) a left-right symmetry model in which one passage is modified to be identical with the other passage, and (4) models having narrowed or blocked passages. The results show that the poles and zeros are produced and shifted by a mutual branching effect caused by the left-right asymmetry of the nasal passages and an additional side-branch effect of the sinus cavities. Those effects are also produced by complicated cross-sectional shapes of the nasal tract in the 3 kHz region. The reduced cross-section in the narrowed-passage models causes a shift of the pole and zero frequency and weakens the mutual side-branch effect of the left and right passages when either of them are excessively narrowed.