Typical Fourier transform. In this case, the experimental conditions were: N = 6, m = 0.3 g, l = 2.7 mm, e = 2.4 mm, f_rot = 3.7 Hz and R = 9.8 cm. Four identifiable zones (a), (b), (c), (d). Only (c) and (d) are audible as showed by the PSD (Power Spectral Density) integral. (a) Is the macroscopic sound produced by the balloon pushing air during its movement. (b) Is the sound components ranging from 30 to 80 Hz. (c) Is the first eigenfrequency of the membrane. (d) Is the frequency produced by the nut rolling and hitting the membrane. Harmonics are also produced.