In this paper an experimental analysis is performed that demonstrates the aero acoustic instabilities of a class of rocket motors characterized by the presence of a cylindrical cavity formed due to a backward facing step and a submerged nozzle. The frequencies at which the pulsations occur are shown to depend on the upstream mean velocity, the upstream diameter and downstream length of such a geometry. Moreover, the frequencies at which possible pulsations occur are found to be contained within two discrete bands of Strouhal numbers. High amplitude pulsations will dominate at a particular band of Strouhal number depending on the downstream length to step height ratio. The amplitudes of such pulsations are affected by the step location within the configuration and also on the mean static pressure of the flow. Finally, certain frequencies of oscillations are shown to be suppressed if the inclination of the step is reduced. © 2003 by S. J. Shanbhogue, R. I. Sujith and S. R. Chakravarthy.