2. Wave expansion discretisation scheme
The wave expansion method (WEM) was first proposed by Caruthers et al.[8], and was more recently used by Ruiz and Rice [10] and Barrera Rolla and Rice [15] to investigate sound propagation in quiescent media. The discretisation scheme may be used for solving linearised time-harmonic propagation equations. The WEM is a physically-based numerical scheme, in that it uses fundamental solutions of the wave operator. As such, it has a very low dispersion error compared to those associated with other numerical schemes [8]. As a result, it is a highly efficient numerical procedure requiring only two-to-three points per wavelength to obtain accurate solutions. This discretisation is perhaps optimal, as it is valid down to the Nyquist limit of two points per wavelength. The value of the unknown at each discrete point in the domain is related to the values at a selected set of neighbouring points by plane-wave functions. The WEM represents a local interpolation formula to The obtain the field values in the domain.