The exhaust plume produced by a planar, multistream supersonic nozzle is studied with and without the effect of an aft deck and sidewalls. Measurements encompass static wall pressure of the internal flow, shadowgraphy images of the exhaust plume, and far-field acoustics for a range of pressure ratios. An experimentally validated Reynolds averaged Navier–Stokes model of the internal flow is then used to reveal how the aft deck and sidewalls allow the expanding gas to persist longer, thereby extending the location of the separation shock. Where the far-field sound is concerned, the aft deck and sidewall attachments cause a noticeable reduction in overall sound pressure levels by as much as 5 dB at shallow angles and sideline observer positions. Many of the tones associated with screech and broadband shock associated noise are also absent with the addition of the aft deck and sidewalls.