The current experiments investigate the acoustic characteristics of an idealized, supersonic rectangular nozzle with aft deck used for three-stream engines. Since the second stream for these jet configurations mix with the bulk flow far before the nozzle's exit plane, a completely mixed first and second stream is assumed. Thus, this jet is represented as two unmixed exhaust flows that come from a three-stream engine. The nozzle is configured as a rectangular single expansion ramp (SERN) convergent/divergent (CD) nozzle with a variable speed bypass (3rd) stream. By varying the Mach number in the 3rd stream, we are able to alter the flow towards a state of “lower noise” while causing minimal effects to the mass flow rate of the jet. The experiments were conducted using far-field microphone arrays, aft deck pressure measurements, schlieren imaging, and investigates different aft deck lengths and far-field locations.