Computational analysis is used to investigate the aeroacoustics characteristics of supersonic jets with design Mach numbers of 1.8. To account for nozzle expansion from over to optimal, the nozzle pressure ratio ranged from 2.75 to 5.75. For nozzle pressure ratios of 2.75 and 3.75, the shadowgraph results show complex shock cell structures as well as the Mach stem. However, once the NPR is significantly greater than highly over-expanded, the Mach stem disappears from the jet plume. Furthermore, even at the design operating condition, a convergent-divergent supersonic nozzle exhibits a throat-induced shock, but the shock from the nozzle exit dissipates when it works at the design NPR. Unsteady numerical shadowgraph is used to identify propagating sound waves emanating from the jet. Acquired time-series acoustic pressure data from the six different microphone locations are utilized to calculate the sound pressure variations across the frequency scale. Processed SPL values show that when the jet expands for 2.75 and 3.75, its magnitude increases at 90 and 115˚ where shock-associated noise tends to dominate. But for the angles from 30 to 75˚ magnitude of the variation is smaller than the other microphone locations. Once the operating NPR is close to the design value, the sound pressure level values increase for the angles of 30 to 75˚ where mixing noise dominates, but for the upstream angle, SPL magnitude is notably lesser than the other angles. Hence it can be inferred that highly over-expanded flow tends to emit more intense shock noise than the mixing noise, but the opposite is true for the operating nozzle pressure ratios 4.75 and 5.75.