Velocity, acoustic stress dependence

Wang and Lee [1] define the so-called Langevin and Rayleigh radiation pressures, respectively, as the mean excess pressures that either depend upon the sound wave only (i. e., with C = 0), or on the sound wave together with a constraint which determines the constant C that contributes to the pressure. The concept of the radiation pressure enables the calculation of forces acting upon material surfaces, such as an interface between two fluids or the surface of a particle or a drop in a sound field. Strictly speaking, one should use the acoustic radiation stress tensor n to calculate such forces. However, in many situations, such as when the surface is rigid or when the velocity at a surface is normal to that surface, it is convenient to use the radiation pressure rather than the full stress. 

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