Figure 18.

(a) Stability diagram for the $h=2$ mm and $h=1$ mm deep ethanol layers showing two subharmonic Faraday tongues. The oscillation wave frequency is $f_F=50$ Hz, half of the vertical vibration frequency $f=100$ Hz. (b) Experimental optical spectra for seven gradually increasing (from bottom to top) amplitudes of the vibration signal with $f=100$ Hz. The depth of the ethanol layer is $h=2$ mm. All spectra are vertically offset by 100 dB. The three lowest spectra are dominated by the vibration frequency $f=100$ Hz and its higher-order harmonics $nf$ ($n=2,3,\cdots$) that appear due to nonlinear acoustic effects in ethanol. The fourth and so on spectra are dominated by the Faraday wave at $f_F=50$ Hz and its higher-order harmonics. Note that all peaks have a characteristic triangular shape (see the main text). The equal spacing between them allows using the spectra as AFCs. Reproduced from [24] with permission of SPIE and the authors of the publication.