I measured the beat signal and VCO feedback signal to see a peak from cavities' mode of vibration, there are 4 peaks from 1kHz to 20kHz.

We expect to see  the cavities' resonance around 10k Hz with high Q, say 10^6. I measured the beat signal and VCO feed back to double check my results.

There are 4 peaks, 5.98 kHz, 10.28 kHz, 12.8 kHz, and 13.85 kHz, that coincide. However, the peaks do not look sharp (Q is too low), and the FWHM

of peaks at 5.98kHz and 13.85kHz might be too broad to be the cavities' mode. So 10.28 and 12.8 seem to be good candidates.