A thermal‐wave resonant cavity was constructed using a thin aluminum foil wall as the intensity‐modulated‐laser‐beam induced oscillator source opposite a pyroelectric polyvilidene fluoride wall acting as a signal transducer and cavity standing‐wave‐equivalent generator. It was shown that scanning the frequency of oscillation produces the fundamental and higher overtone resonant extrema albeit with increasingly attenuated amplitude—a characteristic of thermal‐wave behavior. Experimentally, scanning the cavity length produced a sharp lock‐in in‐phase resonance with simple linewidth dependencies on oscillation (chopping) frequency and intracavity gas thermal diffusivity. The thermal diffusivity of air at 294 K was measured with three significant figure accuracy: 0.211±0.004 cm2/s. The novel resonator can be used as a high‐resolution thermophysical property sensor of gaseous ambients. ©1995 American Institute of Physics.