Abstract The generation of aerodynamic sound by the interaction of flow at a Reynolds number of 150 and a Mach number of 0.2 with a rigid square cylinder attached to a rigid thin flat plate is numerically investigated. When the length of the plate is varied from L = 0.5 D to 6 D , where D is the side length of the square cylinder, the results can be grouped into three distinct regimes. For the first regime ( L ≲ D ), the sound levels decrease with increasing plate length. A 3 dB sound reduction is obtained when the length of the plate is D. For the second regime ( 1.25 D ≲ L ≲ 4.75 D ), the sound levels increase with increasing plate length. For the third regime ( 5 D ≲ L ≲ 6 D ), the sound levels decrease as the length of the plate increases but the levels are higher than for the other regimes. Results also show that the lift fluctuation is the dominant sound source. These acoustic results can be explained in terms of the fluid mechanics occurring in the near wake of the cylinder.