Active source cancellation of the blade tone fundamental and harmonics in centrifugal fans

In previous papers it has been shown that the aerodynamic noise of centrifugal fans can be controlled with active source cancellation methods. In particular, cancellation of a fan's blade tone fundamental was achieved by replacing a section of the fan's cut-off where the aerodynamic blade tones are generated with two appropriately driven secondary sources. In the present set of experiments, the method is extended to include cancellation of the harmonics of the blade tone. A special case is also examined where the frequency of the blade tone harmonic lies just above the cut-on frequency of the first higher order mode of the fan ducting. Results indicate that active control is effective at the harmonics of the blade tones as well as the fundamental for a variety of fan loading conditions and duct terminations. Further evidence has also been obtained with global measurements of the fan's radiated sound that supports the idea that the active control mechanism involves a local alteration of the aerodynamic source pressures.