A device for improving the effectiveness of the acoustic treatment(s) in an aircraft propulsion unit that includes a pipe (36) into which a gas stream flows, whereby the pipe (36) is delimited by two approximately concentric inside walls (40) and outside walls (42), wherein the device includes at least one partition (38, 38′) that extends over at least one portion of the length of the pipe (36) between the inside and outside walls (40, 42) and in that the profile of at least one wall (44, 46) of the at least one partition (38, 38′) is defined so as to increase the number of reflections of an acoustic beam on at least one of the inside and/or outside walls (40, 42) of the pipe and/or on at least one of the walls (44, 46) of the at least one partition (38, 38′).