This paper describes mean flow measurements of supersonic jets with and without noise reduction devices to understand the flow physics behind noise reduction. Pitot pressure measurements are conducted for single over-expanded supersonic axisymmetric and rectangular jets from 22.5-mm-equivalent-exit-diameter convergent-divergent nozzles. Hard-wall corrugations and fluid inserts are examined as noise reduction devices and the differences in acoustic benefits for each device are discussed based on the obtained mean flow profiles. The experimental results suggest that the development of the shear layer is a key factor in understanding the changes in turbulent mixing noise in the peak noise direction. Additionally, the results indicate that changes in the shock cell structure strongly control the characteristics of the broadband shock-associated noise. Moreover, even similar noise reduction devices such as hard-wall corrugations and distributed blowing in the form of fluid inserts are found to cause significant differences in noise and the flow fields. It is shown how the interaction between the main jet and the flow induced by the noise reduction devices depends strongly on both the main jet conditions and the flow structure induced by noise reduction devices.