Spectrogram of a typical audio record of 3 seconds. Experimental conditions are identical to Figure 2. Frequency components are plotted as a function of time. The ladder of intensity goes from dark blue (no frequency component) to yellow (important frequency component). The amplitude modulation is represented by the alternation of bright (yellow) and dark zones. Oscillations of the harmonics of the main frequency (~ 800 Hz) with the same periodicity as the amplitude modulation are also visible.