Abstract: Effects of cavity length-depth ratios (L/D) on aeroacoustic characteristics of cavities were discussed at Mach numbers of 0.6 and 1.5 by analyzing sound pressure level (SPL) distributions and sound pressure frequency spectrum (SPFS) inside the cavities. The results indicate that there is a transformation that the cavity flow type changes from open to transitional and closed cavity flow when L/D is increased from 7, 8 to 9, 12 and 15, which also leads to changes in SPL distributions and SPFS characteristics inside the cavities. When L/D is 7 and 8, some flow oscillation frequencies exist because the shear-layer, expansion or shock waves have insignificant influences on sound feedback mechanism, which indicate self-sustained oscillations appear inside the cavities. However, there are few oscillation frequencies and flow self-sustained oscillations inside the cavities with length-depth ratios of 12 and 15 because the shear-layer, expansion or shock waves weaken the self-sustained oscillation energy and disturb the sound feedback mechanism. Generally, increase in L/D leads to reduction in flow oscillation frequencies and decrease in SPL in the oscillation frequencies.