Acoustic simulation of sound propagation inside the vocal tract is a key element of speech research, especially for articulatory synthesis, which allows one to relate the physics of speech production to other fields of speech science, such as speech perception. Usual methods, such as the transmission line method, have a very low computational cost and perform relatively good up to 4-5 kHz, but are not satisfying above. Fully numerical 3D methods such as finite elements achieve the best accuracy, but have a very high computational cost. Better performances are achieved with the state of the art semi-analytical methods, but they cannot describe the vocal tract geometry as accurately as fully numerical methods (e.g. no possibility to take into account the curvature). This work proposes a new semi-analytical method that achieves a better description of the three-dimensional vocal-tract geometry while keeping the computational cost substantially lower than the fully numerical methods. It is a multimodal method which relies on two-dimensional finite elements to compute transverse modes and takes into account the curvature and the variations of crosssectional area. The comparison with finite element simulations shows that the same degree of accuracy (about 1% of difference in the resonance frequencies) is achieved with a computational cost about 10 times lower.