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Our photoacoustic cells are carefully optimized for maximum signal amplification and noise suppression. They are milled into a stainless steel block, which guaranties their robustness. They are operated in a differential mode, i.e. from two identical acoustic resonator tubes only one is illuminated by the laser light. Therefore, by measuring the microphone signals of the resonators differentially, external electrical and acoustic noises, as well as noise generated by the flowing gas are largely eliminated without reducing the amplitude of the PA signal. This PA signal is maximized by modulating the laser at a frequency corresponding to the first longitudinal resonance of the acoustic resonator, i.e. at about 4000 Hz (at 25 °C and atmospheric pressure). The quality factor of this resonance is about 10. Acoustic filters, which are optimized to efficiently suppress outside and flow noises at the resonance frequency of the cell, are attached to the ends of these resonators.