Momentum and angular momentum reciprocity theorems for acoustic wave fields in an inhomogeneous fluid medium.

As a fundamental property obeyed by all linear physical systems, reciprocity theorems describe the symmetrical relation between the source and the field. Based on the governing equations in the frequency domain for time-harmonic acoustic waves propagating in an inhomogeneous fluid medium, the momentum and angular momentum reciprocity theorems for acoustic waves are derived, which involve two additional mutual force and torque densities due to inhomogeneities of fluid density and compressibility, respectively. A comprehensive analogous study is developed between acoustic and electromagnetic equations, quantities, and operators. Applying the corresponding principle, we also obtain the momentum and angular momentum reciprocity theorems for electromagnetic waves in an inhomogeneous medium. The extended acoustic theory can be utilized as a priori knowledge in practical applications involving acoustic waves in inhomogeneous media, such as acoustic emission, wave scattering, ultrasonic imaging, and object reconstruction.