The finite-difference time-domain (FDTD) method has been widely used for vocal tract acoustic modelling due to its simplicity and low computational cost. Nevertheless, the method suffers from high discretization error while approximating realistic vocal tract geometries using orthogonal grid elements. Alternatively, simplified vocal tract shapes having regular contours can be used for articulatory models. These geometries can be generated from one-dimensional (1D) area functions, which approximate vocal tracts as concatenated tubes with different cross-sections. To this aim, we modify an existing 3D FDTD model for faster acoustic simulation and synthesize five English vowels with various simplified vocal tract shapes. We implement six geometrical shapes for each vowel, consisting of circular, elliptical and square cross-sections with centric and eccentric tube segment configurations. Vowel transfer functions obtained from these FDTD simulations are compared with a highly accurate finite element (FE) scheme. The acoustic formants of the FDTD model agree well with the corresponding FEM approach for most vowels. The influence of vocal tracts with different geometry approximations remains insignificant for frequencies below 5 kHz. However, vocal tracts with elliptical or eccentric configurations have produced higher-order acoustic modes. This paper characterizes the acoustic properties of simplified vocal tract shapes using the 3D FDTD scheme.