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A Japanese patent filed in the late 1970’s described a method to reduce the blade passage tonal noise of fans by changing the relative angle between two sets of fan blades. The work presented here is an experimental investigation of this method. The experiment involved synchrophasing two axial fans in a duct to reduce the blade passage tonal noise. The experimental results showed that the sound pressure level at the blade passage frequency can be reduced by 10dB at the error microphone and in the free field by adjusting the relative angle between the two fans. It is also possible to reduce the sound pressure level of the harmonics of the blade passage frequency, however these do not necessarily occur at the same fan angle as the fundamental blade passage frequency.
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