Acoustics and dynamic materials

摘要

The governing equations for small-amplitude acoustic disturbances are derived by combining an admissible background flow or medium, balance laws, an equation of state and perturbation theory. This standard approach is used here when the background medium can vary in both space and time, thus defining a dynamic material. When the background velocity is zero, the background density cannot vary with time, assuming that the usual balance laws (such as conservation of mass) are obeyed. When mass conservation is discarded, perhaps replaced by a growth model, a time-dependent background density is permitted, leading to new equations governing acoustic disturbances in certain dynamic materials. (C) 2020 Elsevier Ltd. All rights reserved.