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The ability to use the finite-element method to solve numerically the frequency-dependent coupled equations of the photoacoustic–thermal effect is demonstrated. Both solids and fluids are simulated by the same set of equations with temperature and displacement as variables. The main interest of this formulation lies in its great flexibility to deal with mixed fluid–solid systems. As a first application, we consider the influence of thermoacoustic coupling on the pressure in a photoacoustic cell. We show that with increasing frequency, a transition from an isothermal to an adiabatic process occurs. Subsequently, results obtained from a numerical simulation of the photoacoustic cell, which includes the effect of a residual volume, are in good agreement with existing experimental data.

A pulsed photothermoelastic technique for nondestructive testing of layered materials is described. A Fourier-transform signal analysis is presented for a quantitative characterization of subsurface defects. Discrepancies between the experimental results and the data obtained from the thin-plate analytical theory are interpreted through a finite-element numerical model. The effect of the vibrational properties of the whole structure is shown to be particularly important on the spectral features of the thermoelastic signal for small defects.

It has been previously shown that ultrasonic absorption in a solid material can be detected and, in some cases, measured by monitoring, with an infrared detector, the variation of temperature at the surface of the sample resulting from the periodic modulation of the ultrasonic intensity. Thermal-wave vignetting effects, which are caused by the finite ultrasonic beam size and the finite area viewed by the detector, and effects caused by the ultrasonic standing-wave pattern are analyzed in this paper by a theoretical derivation and by experiment. Satisfactory agreement is found between both.

Strong absorption bands of photoacoustic Fourier-transform spectra are often truncated relative to weaker bands owing to signal saturation. To correct this problem, we propose processing both the phase and the amplitude information in the photoacoustic signal. Under certain conditions, easily fulfilled in typical experiments, we are able to calculate the absolute absorption spectrum from the photoacoustic data. Experimental results are given for asbestos fibers (chrysotile).

Separation of surface absorption from bulk absorption by using the phase of the photothermal signal is shown on samples of SiO deposited on CaF2. The bulk signal obtained with these samples shows good agreement with the signal obtained for uncoated CaF2 samples.