The analysis of scalar wave propagation in 2D zonewise homogeneous media with vanishing initial and mixed boundary conditions is carried out. The problem is formulated in terms of time-dependent boundary integral equations, and then it is set in a weak form, based on a natural energy identity satisfied by the differential problem solution. Several numerical results have been obtained by means of the related energetic Galerkin boundary element method showing accuracy and stability of the method.A. AIMI, S. GAZZOLA AND C. GUARDASONI precise continuity and coerciveness properties [26,27]. Consequently, it can be discretized by unconditionally stable schemes with well-behaved stability constants even for large times.We remark that the idea of introducing a space-time weak formulation for the transient wave problem based on the energy identity is not new. In fact, in [16], it was exploited to get a satisfactory stability result for the acoustic wave equation with the aid of absorbing boundary condition. Unfortunately, the cases of Dirichlet and Neumann boundary conditions on a bounded time interval, as we have always considered, are much more difficult to be theoretically treated, and they have been analyzed in [26,27].The proposed procedure can be generalized to multi-domain wave propagation BIE problems as introduced in [28] within the limit of bi-domain cases. Here, the extension to n-domain case is proposed and further numerical simulations are presented and discussed: through comparisons with available (when possible) literature results [13,29], the stability and accuracy of the energetic approach are shown even in this framework. Plenty of examples, not easily findable in mathematical and even engineering papers on the subject, and numerical results included in this work illustrate the applicability and potentialities of this technique, even if theoretical analysis of stability and convergence for the multi-domain case is still to be done.H 1=2 .S/ :D fu 2 H 1=2 : i .U/ D uj @ i , i D 1, 2^ .U/ D u for some U 2 H 1 . /g, Figure 1. Example of bi-domain.