  The acoustic wave equation is solved in time domain with a boundary element
formulation. The time discretisation is performed with the generalised
convolution quadrature method and for the spatial approximation standard lowest
order elements are used. Collocation and Galerkin methods are applied. In the
interest of increasing the efficiency of the boundary element method, a
low-rank approximation such as the adaptive cross approximation (ACA) is
carried out. We discuss about a generalisation of the ACA to approximate a
three-dimensional array of data, i.e., usual boundary element matrices at
several complex frequencies. This method is used within the generalised
convolution quadrature (CQ) method to obtain a real time domain formulation.
The behaviour of the proposed method is studied with three examples, a unit
cube, a unit cube with a reentrant corner, and a unit ball. The properties of
the method are preserved in the data sparse representation and a significant
reduction in storage is obtained.

Numerical Analysis;Numerical Analysis