Experimental studies are carried out on an axisymmetric cylindrical base body for six freestream Mach numbers between 0.54 to 1.41. Unsteady pressure is measured on the base surface using high-frequency response Kulite pressure transducers. The effect of passive flow control devices on the mean base pressure and the unsteady characteristics of base pressure has been studied. A blunt base, a conventional cavity device and three different ventilated cavity devices have been tested along with four different rounded base lip devices. A total of 20 different base geometric modifications are tested at six freestream Mach numbers resulting in 120 test cases. The cavity devices improve the base pressure as compared to the blunt base case. Among all the cases considered, a maximum increase of 8.6% in the base pressure coefficient is noticed for the Normal Ventilated Cavity device as compared to the blunt base case for freestream Mach number of 1.22. The power spectral density of base pressure fluctuations revealed the dominant peaks on the base surface. The shear layer flapping frequency for all the cases have been found and the Strouhal number based on base diameter varies between 0.2 to 0.27. In the presence of cavity devices, dominant peaks are observed in the range of 2 kHz to 8 kHz which can be attributed to the vigorous action within the recirculation bubble. Maximum reduction in base pressure fluctuation is observed for the Normal & Inclined Ventilated Cavity device configuration test cases.