Noise reduction of higher-order blade-passing frequency components in a centrifugal blower

The correlation analyses based on Lighthill's dipole noise theory to investigate the effective location of the noise source were utilized for the higher-order blade-passing frequency noise, abbreviated to BPF noise, radiated from a centrifugal blower. The extent of the effective noise source of 2nd- and 3rd-order BPF components was determined by measuring a cross-correlation function between the data of the acoustic pressure and the pressure fluctuation on the scroll surface. The effective noise source of these higher-order BPF components exists on the volute side surface of scroll relatively apart from the cut off apex, and varies irregularly with operating conditions of the blower. While the noise source of fundamental BPF component lies on the vicinity of cut off apex. Oscillating the volute side surface of the cut off, i.e. the effective noise source of higher-order BPF components, directly by piezoelectric actuators, sound pressure level can be reduced more than fifteen decibels, and discrete tone disappears completely from power spectra of the blower radiated noise.