Abstract: Apparatus for reducing acoustic radiation which occurs during the operation of turbomachinery includes a shroud having a generally cylindrical inner surface and a coaxial rotor having a plurality of blades extending generally radially outwardly at equally spaced circumferential locations. The inner surface of the shroud is circumferentially contoured such that the tip clearance between each of said blades and the inner surface is caused to vary in a periodic manner upon rotation of said rotor. This creates a new periodic unsteady pressure field which is substantially equal to, and out of phase with, an existing periodic pressure field resulting from nonuniform inflow into the blades and results in the radiation from the turbomachinery of reduced blade rate frequency tones. The inner surface of the shroud may have a circumferential sinusoidal contour whose periodicity is an integral multiple of the number of the blades.