Several methods of reducing the noise of centrifugal fans are discussed, most of which were aimed at a lower blade passage frequency level because tonal noise components are of particular annoyance. The various methods are (1) increasing the cutoff clearance, (2) increasing the radius of curvature of the cutoff edge, (3) angle of inclination between impeller blades and cutoff edge, (4) staggering the blades of double inlet or double row impellers, (5) wire meshes mounted along the inner and outer circumference of the radial blade row, (6) irregular blade spacing, (7) mismatch between the acoustic impedances of fan and duct system, (8) triangular guide belt around the impeller, (9) rectangular fan casing, (10) circular fan casing, (11) acoustically optimized fan casing, (12) optimum annular clearance between impeller eye and intake nozzle, (13) acoustical lining of the interior of the fan casing, (14) using minima of the fan acoustic radiation efficiency, and (15) resonators at the cutoff.

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