Reduction of centrifugal fan noise by use of resonators

The subject of this paper centers around a method by which an acoustic resonator can be used to reduce the aerodynamic noise generated by turbomachinery. In the study reported, the casing of a small, centrifugal blower was modified by replacing the cutoff of the scroll with the mouth of a quarter‐wavelength resonator. The mouth of the resonator was constructed from a series of perforated plates with the same curvature as the cutoff to preserve the original geometry of the casing. Tuning of the resonator was achieved by changing the length via a moveable end plug. The noise measurements were made in an anechoically terminated outlet duct at nearly a free delivery operating condition of the blower. With appropriate tuning of the resonator, reductions in the blade passing frequency tones of up to 29 dB were observed with corresponding overall sound pressure levels reductions of up to 7 dBA. Parameters which influenced the bandwidth of the resonator response were the porosity and hole size of the resonator mouth and the flow velocity near the cutoff region. Throughout the tests, the aerodynamic performance of the blower was unaffected by the addition of the resonator to the casing.