Equivalence of Eulerian and Lagrangian phonons: Sound pressure and two-fluid equations
In correspondence to the two different descriptions of hydrodynamic motion by Euler and by Lagrange, there are two natural ways of defining phonons and the corresponding classical sound waves. Eulerian phonons turn out to have a momentum nk (k = wave vector), whereas Lagrangian phonons carry no momentum. This might suggest that the two definitions lead to different physical effects. That this is not the case is shown in detailed discussions of two examples: the sound pressure and the derivation of two-fluid equations.
KeywordsAverage Velocity Sound Pressure Sound Wave Spatial Average Sound Field
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