Acoustic Casimir Pressure for Arbitrary Media
  In this paper we derive a general expression for the acoustic Casimir pressure between two parallel slabs made of arbitrary materials and whose acoustic reflection coefficients are not equal. The formalism is based on the calculation of the local density of modes using a Green's function approach.   The results for the Casimir acoustic pressure are generalized to a sphere/plate configuration using the proximity theorem 
J. Barcenas, L. Reyes, R. Esquivel-Sirvent