Generation of photoacoustic sound for gases in an open environment

Abstract

Photoacoustics is the generation of acoustic waves due to unsteady heating from a light source. It was discovered over 119 years ago and is commonly used for trace gas detection and molecular spectroscopy when the photoactive gas or gas mixture is contained in a chamber. However, photoacoustic phenomena are less well characterized in an open environment. In this presentation, scaling laws for radiated photoacoustic sound pressure are developed from dimensional analysis and verified with experiments using a carbon dioxide laser tuned to 10.6 μm and small plumes of sulfur hexafluoride, a photoactive tracer gas. In addition, a forced wave equation that includes laser-gas heating is developed to predict the generation and propagation of photoacoustic sound. Using this forced wave equation, the shape and amplitude of photoacoustic sound pulses are determined based on the shape of the tracer-gas plume and the extent of laser-beam absorption through the Lambert–Beer law. The model results are compared to measured photoacoustic signal pulses and show favorable agreement. [Work supported by Ford Motor Company.]