IntroductionRecently the time-domain finite-element method (TD-FEM) is becoming an efficient wave-based numerical method for wideband room acoustics simulation up to several kilohertz frequencies, with the drastic advancement of computer technology [1,2]. However, a cost-efficient simulation up to high frequencies using the method is still considered to be a difficult task, due to the high computational cost. A dispersion error, which is an inherent discretization error coming from spatial and time discretizations of a computational domain, is one of the reasons for the high computational cost. Because of the error, a spatial discretization requirement known as a rule of thumb is imposed in a mesh generation process to yield reliable results. Also, a time discretization error must be considered in a time-domain analysis. These requirements engender the solution of large-scale problems with many degrees of freedom (DOF) and many time steps for a simulation at high frequencies.Many dispersion reduction methods have been developed to increase the computational efficiency [1][2][3][4][5][6]. The authors have also proposed some dispersion-reduced TD-FEM based on an implicit method to solve the large-scale problems efficiently [1,2]. Also, there exists an explicit TD-FEM with a simple dispersion reduction technique called modified integration rules (MIR) [5], in which fourth-order accuracy with respect to the dispersion error can be obtained for the idealized case using square or cubic finite elements (FEs). This explicit method is very attractive for realizing an efficient room acoustics simulation because it does not need a solution of a linear system of equations at each time step. However, as presented in the literature [5], the explicit method does not consider a dissipation term for treating an absorption at boundaries, which is important for room acoustics simulation.When a sound field inside a room with an finite impedance boundary is analyzed using this explicit method, a time derivative of sound pressure related to the dissipation term has to be approximated by a less-accurate backward difference even though time derivatives of other physical quantities are approximated by a second-order accurate