The equations of acoustics are based on the general equations of fluid dynamics: conservation of mass, momentum, energy and closed by the appropriate constitutive equation defining the thermodynamic state. The use of a perturbation ansatz, which decomposes the physical quantities density, pressure and velocity into mean, incompressible fluctuating and compressible fluctuating ones, allows to derive linearized acoustic conservation equations and its state equation. Thereby, we derive acoustic wave equations both for homogeneous and inhomogeneous media, and the equations model both vibrational- and flow-induced sound generation and its propagation.
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