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

Theoretical and experimental investigations aimed at optimizing the detecti on performance in nonresonant pulsed photoacoustic (PA) studies in the gas phase by selecting microphones with appropriate characteristics are present ed. An analytical function is introduced that allows the simulation of the measured signal-to-noise-ratio (SNR) for given microphone specifications. T he 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 obta ined. Comparisons of experimental data recorded in a gas mixture of 100 ppm ethylene buffered in synthetic air with various condenser and electret mic rophones with different characteristics confirm our theoretical predictions . The peak amplitude of the recorded microphone signal increases with decre asing laser beam radius r(b). However, for standard microphones with a band width of approximate to 20 kHz, there is no reason to use r(b) < 0.5 mm bec ause the spectral composition of the signal shape for frequencies < 20 kHz remains unchanged. A large microphone bandwidth of a few 100 kHz is advanta geous for resolving the temporal evolution of the PA signal but it can be d isadvantageous for trace gas monitoring due to an enhanced noise level. Our simulation permits the selection of the most appropriate microphone for ma ximum SNR under given experimental conditions. Finally, an analysis on the pressure dependence of the responsivity of a selected microphone is present ed that is of interest when performing trace gas measurements at reduced pr essure to enhance the detection selectivity. (C) 1999 American Institute of Physics. [S0034-6748(99)02312-6].