A substructure FMBEM approach is developed for multi-domain sound field computations.The basic approach and process as well as advantages and disadvantages are introduced.Transmission loss of expansion chamber with extended inlet tube is calculated by using substructure FMBEM and conventional boundary element method(CBEM).Availability and accuracy of the approach are validated with experimental result.It indicates that,compared to the CBEM,substructure FMBEM reduces computational time and computational complexity obviously as the number of nodes is increased.