Sonic Block Silencing for Axial and Screw Type Compressors Free

The sonic block concept has proven to be an extremely effective method of reducing compressor radiated noise. This type of silencer is based upon the classical fact that when a fluid is traveling in a direction opposite to and at a speed equal or greater than the velocity of propagation of a disturbance in that fluid, then the disturbance cannot be transmitted upstream through the fluid. Since the principle of operation involves only direction and relative speeds, its acoustical performance is independent of frequency. The amount of noise reduction is limited only by (1) suppressor wall noise radiation; (2) noise escape through boundary layer at the sonic block; and (3) fluid acceleration noise. This paper describes sonic block designs for turbojet engine axial compressors and small industrial screw type units. Actual noise reductions are presented for a 225 lb/sec axial flow compressor and a 2 lb/sec Lysholm type unit. In the case of the high speed axial flow unit, high frequency blade noise was submerged below the levels of background noise. Noise reductions, ranging from 28 48 db, were measured from 20–10 000 cps for the screw compressor.