Thermoacoustic enhancement of photoacoustic spectroscopy: Theory and measurements of the signal to noise ratioa)

Abstract

Recent developments in thermoacoustics have shown that the quality factor, Q, of an acoustic resonator can be controlled by establishing a temperature gradient across properly positioned thermoacoustic elements. Quite separate from thermoacoustics, acoustic resonators are used in photoacoustic spectroscopy, where a laser beam, modulated at the acoustic resonance frequency, is partially absorbed, thereby producing sound. The photoacoustic signal is typically measured with a microphone, and is proportional to the laser power, to the absorption coefficient, and to the resonator Q, among other factors. The acoustic signal to noise ratio is proportional to Q1/2. Thermoacoustics can be used to enhance the signal to noise ratio of photoacoustic spectrometers by increasing Q. Measurements and theory are reported for the signal to noise ratio of a photoacoustic cell, with thermoacoustic enhancement, as a function of both resonator Q and the bandwidth of the lock-in amplifier. Regimes where thermoacoustic enhancement is useful are identified. © 1995 American Institute of Physics.