The characteristic behaviors of low-frequency modulations embedded in the vortex shedding process were studied experimentally with a trapezoidal cylinder situated in a low-speed wind tunnel. Hot-wire signals and smoke-wire visualization images were acquired simultaneously to examine the correlation between low-frequency modulations and the vortex formation length. Wavelet analysis was performed to extract the instantaneous properties from the raw signals obtained in the region upstream of the trapezoidal cylinder model. Results show that the variations of instantaneous vortex shedding frequency in time appear to be in negative correlation with the low-frequency modulations, that the cross-correlation coefficient can reach -0.5 in value. This substantiates that the low-frequency modulations observed are associated with the vortex shedding process.