Comparison of vocal tract transfer functions calculated using one-dimensional and three-dimensional acoustic simulation methods

Acoustic characteristics of the vocal tract have been investigated extensively in the literature using a onedimensional (1D) acoustic simulation method. Because the 1D method assumes plane wave propagation only, it is recognized to be valid only in the low frequency region (below about 4 or 5 kHz). Recently, a three-dimensional (3D) acoustic simulation method was developed, to obtain more precise acoustic characteristics of the vocal tract. In the present study, from a male’s vocal tract shapes, transfer functions were calculated using the 1D and 3D methods and compared with each other to evaluate the valid frequency range of the 1D method. As a result, when acoustic effects of the piriform fossae were considered in the 1D method, the transfer functions agreed with each other up to 7 kHz (ignoring small dips). The 3D method showed that a deep dip was generated at around 8 kHz by the transverse resonance mode in the pharynx. Above this dip frequency, the transfer functions disagreed with each other. Thus, the 1D method is valid up to 7 kHz for this subject. Because this subject has a relatively large vocal tract, in general the upper limit of the valid frequency range could exceed 8 kHz.