An experimental investigation of an open cavity with a length to depth ratio of six was conducted with and without passive suppression using a cylinder placed near the leading edge of the cavity. Five rod diameters were immersed at various gap heights in the approaching supersonic boundary layer where the freestream Mach number was 1.4. The study included surface pressure measurements and flowfield visualization using both Schlieren imaging and particle image velocimetry. The results indicate a

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... rod sized at least 40% of the boundary layer height and placed with it's top near the top of the boundary layer exhibited the most effective suppression configuration. In controlled cases the shear layer is spread and slightly lofted above the trailing edge of the cavity when an appropriately sized rod was placed at the cavity leading edge. Evidence of the disruption of the feedback mechanism well known to cause resonant tones were highlighted with correlation and coherence plots and further substantiated with particle image velocimetry and Schlieren images. The rod was able to reduce overall pressure fluctuations measured on the aftwall by nearly 35% and the peak tone reductions by 65%.