Generalized theory of the photoacoustic effect in a multilayer material

In this work, a generalized expression of the photoacoustic ~PA! effect in a multilayer material is derived. This expression takes thermal and optical properties and geometry of a multilayer structure, as well as the thermal contact resistances between layers into consideration. In addition, a composite piston model consisting of a thermal piston and a mechanical piston of the PA effect is developed and interpreted from the viewpoint of thermodynamics. Mistakes occurring in the thermal piston model and in the composite piston model developed before are pointed out and corrected. It is also shown that the PA effect has an isochoric character for the thermal piston and a polytropic character with a polytropic factor of (221/g) for the mechanical piston. The theory developed in this work is in good agreement with the experimental results, and is applicable to a wide range of photoacoustic problems. © 1999 American Institute of Physics. @S0021-8979~99!05519-X#