Reciprocity and propagation in space–time acoustic materials

Generalized reciprocity relations for space–time-dependent acoustic materials are considered. The form of the resulting propagation equations depends on the original constitutive equations and thermodynamic relationships. A specific example of a space–time acoustic fluid is developed and it is found that in this case, the propagation is reciprocal, in the sense that the Green’s function is self-adjoint. The physical behavior is different from conventional pressure-acoustics however and, for the spatially varying case, propagating waves must have associated vorticity even in the absence of viscosity. [© Crown copyright (2024), Dstl. This material is licensed under the terms of the Open Government Licence except where otherwise stated. To view this licence, visit http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3 or write to the Information Policy Team, TheNational Archives, Kew, London TW9 4DU, or email: psi@nationalarchives.gsi.gov.uk.]