Figure 6.

(a) Lock-in filtered microphone signal (integration time of 1.14 s) while sweeping the LED modulation frequency over the third harmonic of the resonator at different NO${}_2$ concentrations. A reading is taken every 0.75 s. The resonance frequency shift is due to the dilution of NO${}_2$ in synthetic air. (b) Mean value of the three highest points of the sweep, together with their standard deviation ($\pm 1\sigma$) and the linear correlation.