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A prediction method for fan/compressor discrete-frequency tone generation and transmission is presented. The method is based on internal unsteady aerodynamics, coupled with an acoustic field described in terms of spinning modes. The sound-pressure levels generated by the rotor alone and the wake interaction between adjacent blade rows are calculated. The analysis of the sound propagation in the duct in the presence of axial and circumferential flows is described and incorporated in the prediction method. The analytical results are compared with experimental data and are shown to be in good agreement.

A study of various fan/compressor noise reduction methods is presented. The analytical treatment of the basic mechanisms of fan/compressor noise generation is described. The results are presented in parametric form and indicate the effects of fan/compressor design, number of blades, vane/blade ratio, aerodynamic parameters, and blade row spacing on pure tone noise reduction. These results are based on nonsteady aerodynamic treatment of wake and potential interaction effects and theoretical extensions of spinning mode theories.

The generation of multiple pure tones in Supereonic fans is discussed. The theoretical results of Kurasaka are reviewed and compared with experimental data obtained on a 36" diameter, 1550 ftlsec, 1.6 pressure ratio fan. Detailed measurements on bow shock locations taken with pressure transducers indicate that blade to blade discrepancies are the source 01 MPT eeneration. The paper presents Some experimental results on an attempt to reduce the shock strength, and subsequently the MPT's, through blade modifications. Other attempts at reducing the MPT's through wall treatment, high inlet flow mach number, acoustically treated splitters -are discussed. Experimental data is presented on the validity of these noise reduction methods.

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