Abstract: A photoacoustic spectrometer that is intensity-modulated, laser-driven and with a calculable cell constant. The axially symmetrical photoacoustic spectrometer combines first-principles models of acoustic wave propagation with high-resolution spectroscopic measurements, and takes into account molecular relaxation. The spectrometer includes a duct and two chambers disposed at the end of the duct. Inlet and exit tubes, which are disposed in substantially the location of acoustic pressure nodes, permit the gas, gaseous mixture or aerosol to enter and exit the spectrometer. The absolute response of the spectrometer may be modeled and measured. A detailed theoretical analysis of the system and its predicted response may be predicted as a function of gas properties, resonance frequency and sample energy transfer relaxation rates.