左右の梨状窩の音響的相互作用

摘要

The center-frequencies of the two spectral dips derived from bilateral piriform fossae disagree with the resonance frequencies of each piriform fossa alone. This discrepancy is due to acoustic interactions between the bilateral piriform fossae. To examine the interactions, in the present study a simple vocal tract model including piriform fossae was developed, and the pressure distribution patterns at the two dip frequencies were calculated by the finite-difference time-domain method. Simulation results showed that at the lower dip frequency, the bilateral piriform fossae resonated with reverse phase, while at the higher dip frequency they resonated in-phase. This implied that the two dips were generated by the normal vibration modes of a coupled two-oscillator system. Analytic modeling of the system successfully explained the relationship between the dip frequencies on the transfer function and the resonance frequencies of each piriform fossa alone.%声道伝達関数における左右の梨状窩に由来する2つの零点の周波数は，どちらの梨状窩の共鳴周波数とも一致しない．これは左右の梨状窩の相互作用によって零点の周波数が決定されるからである．本稿では，この相互作用を検討するために，声道を単純な形状でモデル化し，時間領域差分法を用いて零点の周波数における音圧分布を計算した．その結果，低い周波数の零点では，左右の梨状窩が逆相で振動し，高い周波数の零点では同相で振動していることが明らかになった．これは，梨状窩の零点が，左右の梨状窩を2つの振動子とする達成振動系の基準振動により生ずることを示唆する．そこで，声道を達成振動系でモデル化したところ，梨状窩の共鳴周波数と伝達関数における零点の周波数の関係を明らかにすることが出来た．