Abstract 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.

中文翻译：

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声学和动态材料

摘要 结合可容许的背景流或介质、平衡定律、状态方程和微扰理论，推导出小幅度声扰动的控制方程。当背景介质在空间和时间上都可以变化时，这里使用这种标准方法，从而定义动态材料。当背景速度为零时，背景密度不会随时间变化，假设遵守通常的平衡定律（例如质量守恒）。当质量守恒被丢弃时，可能被增长模型所取代，时间相关的背景密度被允许，从而导致在某些动态材料中控制声学扰动的新方程。