The impact of cavity geometry on the source of acoustic resonance (Helmholtz or quarter-wave) for synthetic jet type cavities is presented. The cavity resonance was measured through externally excited microphone measurements. It was found that, for pancake-shaped cavities, the Helmholtz resonance equation was inadequate (off by more than 130%) at predicting the acoustic cavity resonances associated with synthetic jet actuation, whereas a two-dimensional quarter-wave resonance was accurate to 15%. The changes in the geometry (cavity diameter, cavity height, and orifice length) could alter the cavity resonance by up to 50%, and a finite element solver was accurate at predicting this resonance in all cases. With better knowledge of the phenomena governing the acoustic resonance, prediction of the cavity resonance can become more accurate and improvements to current prediction tools can be made.