THERMOACOUSTIC ENHANCEMENT OF PHOTOACOUSTIC-SPECTROSCOPY - THEORY ANDMEASUREMENTS OF THE SIGNAL-TO-NOISE RATIO

Recent developments in thermoacoustics have shown that the quality fac tor, Q, of an acoustic resonator can be controlled by establishing a t emperature gradient across properly positioned thermoacoustic elements . Quite separate from thermoacoustics, acoustic resonators are used in photoacoustic spectroscopy, where a laser beam, modulated at the acou stic resonance frequency, is partially absorbed, thereby producing sou nd. 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 n oise ratio is proportional to Q(1/2). Thermoacoustics can be used to e nhance the signal to noise ratio of photoacoustic spectrometers by inc reasing Q. Measurements and theory are reported for the signal to nois e ratio of a photoacoustic cell, with thermoacoustic enhancement, as a function of both resonator Q and the bandwidth of the lock-in amplifi er. Regimes where thermoacoustic enhancement is useful are identified. (C) 1995 American Institute of Physics.