Acoustic resonance of a cylindrical cell in an optoacoustic spectrometer

Abstract

The optoacoustic spectrometer is an instrument with which one can study the absorption of light by a sample of gas. Thermal energy provided by a periodically interrupted laser beam is converted to an acoustic signal of the same frequency, and the amplitude of this signal often is increased through operation at an acoustic resonance frequency of the sample cell. The construction of one such spectrometer and its use to study the variation of acoustical properties of a cylindrical cell with ambient pressure are described. Measurements have been made with the monochromatic light produced by a tunable dye laser, with atmospheric oxygen as the optically absorbing gas, and over the pressure range 0.05–1.0 atm. The acoustic “Q” for the 4.1‐kHz first azimuthal mode of the 5‐cm‐diam cylindrical sample cell is found to decrease by less than 10% (from approximately 100) as the pressure is reduced from 1 to 0.5 atm, and then to fall off much more rapidly. The position of the commercial 1‐in. condenser microphone on the cylindrical cell wall is shown to have a significant effect on the cell resonance frequency. [Work supported, in part, by NSF.]