“…Moreover, an in-depth understanding of the natural mode frequencies and mode shapes of pressure oscillations in modern combustion systems (such as gas turbines and rocket engines) is critically important for safety reasons, as thermo-acoustic excitation is commonly observed, which may lead to strong discretefrequency oscillations and combustion instabilities (Davis, 2008;Dowling and Stow, 2003;Keller, 1995). While methods for both numerical and analytical solutions have been developed to obtain natural frequencies and mode shapes of acoustic resonance in cavities with different shapes (Blevins, 1979;Dowling, 1995;Dowling and Stow, 2003;Hong and Kim, 1995;Kim and Soedel, 1989;Lee, 2011;Mirsky, 1965aMirsky, , 1965b, experimental studies were mostly limited to natural frequencies based on local pressure measurement (Hickling et al, 1983;Payri et al, 2005;Stanković and Böhme, 1999). Only a few studies have been conducted to visualize the mode shapes in cavities using microphone measurements (Gregory et al, 2006;Mamede and Varoto, 2003) and Schlieren measurements (Chinnery and Humphrey, 1996).…”