The invention concerns an afterbody for a mixed-flow turbojet engine having a central axis (LL), comprising a lobed mixer (6), having alternating hot lobes (12) projecting into the secondary flow (F2) and cold lobes (13) penetrating into the primary flow (F1), and a nozzle (1) comprising, on the trailing edge (14) of same, longitudinal indentations (15) defining a crown of noise-reducing chevrons (7), characterised in that, at a predefined abscissa (X5) on the central axis (LL) downstream from the lobed mixer (6), the inner wall (2) of the nozzle (1) has a neck where the surface area of the transverse passage section of a flow into the nozzle passes through a minimum, and in that, downstream from this predefined abscissa (X5), the radius of the inner wall (2) of the nozzle (1) varies between the indentations (15) and the chevrons (7) so as to produce, in the flow, in the vicinity of said crown of chevrons (7), azimuth fluctuations of the Mach number. It also concerns a method for designing such an afterbody that comprises setting the azimuth of the lobed mixer (6) and of the chevrons (7).
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