Selection criteria for microphones used in pulsed nonresonant gas-phase photoacoustics

Abstract

Theoretical and experimental investigations aimed at optimizing the detection performance in nonresonant pulsed photoacoustic (PA) studies in the gas phase by selecting microphones with appropriate characteristics are presented. An analytical function is introduced that allows the simulation of the measured signal-to-noise-ratio (SNR) for given microphone specifications. The crucial parameters for maximum SNR are the microphone responsivity, its bandwidth, and noise characteristics. Good agreement between calculated and measured PA signal shapes and their fast Fourier transform spectra is obtained. Comparisons of experimental data recorded in a gas mixture of 100 ppm ethylene buffered in synthetic air with various condenser and electret microphones with different characteristics confirm our theoretical predictions. The peak amplitude of the recorded microphone signal increases with decreasing laser beam radius rb. However, for standard microphones with a bandwidth of ≈20 kHz, there is no reason to use rb<0.5 mm because the spectral composition of the signal shape for frequencies <20 kHz remains unchanged. A large microphone bandwidth of a few 100 kHz is advantageous for resolving the temporal evolution of the PA signal but it can be disadvantageous for trace gas monitoring due to an enhanced noise level. Our simulation permits the selection of the most appropriate microphone for maximum SNR under given experimental conditions. Finally, an analysis on the pressure dependence of the responsivity of a selected microphone is presented that is of interest when performing trace gas measurements at reduced pressure to enhance the detection selectivity.