Electret transducers for use in optoacoustic spectroscopy

• Published in: The Journal of the Acoustical Society of America Vol. 70; no. S1; p. S58
• Main Authors: Farrow, L. A., Richton, R. E.
• Format: Journal Article
• Language: English
• Published: 01-11-1981
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.2018947

The optoacoustic effect is based on the conversion of electromagnetic to sound energy, which is then detected by a microphone whose output is amplified. The resultant signal is predicted theoretically using a classical stretched drum type electret microphone as the transducer in a cylindrical optoacoustic cell filled with gas which absorbs single frequency laser radiation. These calculations simulate the conditions under which a series of experiments on chemical kinetics were performed; the experimental results are shown to be in good agreement with the predictions at gas pressures ≳ 0.5 Torr. Below 0.5 Torr, the microphone equation breaks down because the mean free path of the gas becomes comparable with the microphone electrode separation. This breakdown is observed experimentally at low gas pressure in the cell. Based on these considerations it is possible to use the optoacoustic effect to measure gas phase collisional relaxation rates and rate constants for homogeneous gas phase chemical reactions. Since some of the gases encountered are corrosive, gold is substituted for the usual aluminum evaporated onto the polymer to form the electret microphone diaphragm material.