The dependence of photoacoustic (PA) spectra on the chopping frequency in t
he range of 10 to 180 Hz was studied in porous silicon (PS) samples of vary
ing thicknesses the PS layer. The enhancement of PA signals over those of b
ulk Si, reported in our previous work, is ascribed to the combined effects
of interstitial gas 'pressure and intrinsic change of absorption. Although
the enhancement seen in the long wavelengths, above 600 nm in the thick PS
samples, is mainly caused by the "pressure" effect, the enhancement in the
short wavelengths, below 600 nm, appears to be mainly due to the intrinsic
effects related to the quantum confinement effect because it increases as t
he porosity increases and seems to remain at higher chopping frequencies (f
= 180 Hz). We could also estimate the thermal diffusivity of the PS sample
s from the crossover frequency of the change of slope in the log PA signal
vs the log chopping frequency plot. The dependencies of the PA spectral edg
es on porosity and chopping frequency are shown and discussed in reference
to the double layered structure model proposed in our previous work.