Generalized theory of the photoacoustic effect

TLDR

The study extends the photoacoustic theory to incorporate mechanical vibration of the sample. The authors solve coupled thermal and acoustic wave equations in the sample and surrounding gas, showing that acoustic coupling can significantly alter the gas pressure signal. The fully coupled analysis is accurately reproduced by a composite‑piston model that superimposes gas boundary layer motion on sample surface vibration, yielding simple algebraic expressions applicable to many cases.

Abstract

The theory of the photoacoustic effect is extended to include the contribution of mechanical vibration of the sample. Coupled equations for thermal and acoustic waves are solved in both sample and gas. It is shown that the pressure signal in the gas may be significantly affected by acoustic coupling in the sample, and experimental confirmation of this extended theory is given. The results of the fully coupled treatment are shown to be accurately reproduced by an extension of the Rosencwaig piston model: the pistonlike motion of the gas boundary layer adjoining the sample is superimposed on the mechanical vibration of the sample surface to give a composite piston displacement which then produces the pressure signal in the gas. The composite-piston model provides relatively simple algebraic results applicable to many cases of physical interest.