The noise level of consumer products is a growing concern for manufacturers since customers' product sound quality expectations are increasing. The product that motivated the present work was a "bladeless" fan in which the rotor and stator module were located in the base of the unit. Direct intensity measurements were used to characterize the fan's acoustical output, which was found to radiate primarily from the air inlet beneath the rotor in the base unit. The noise control design concept explored here is based on the dipole characteristic of axial fans. It has been shown previously that when only one side of the fan is connected to the exterior sound field, the radiation is monopole-like. But, by exposing both sides of the dipole to the exterior field, the radiation efficiency of the source is reduced, and hence the sound power is reduced compared to a situation in which only one side of the fan connects with the exterior field. That concept is applicable not just to the bladeless fan investigated in this work, but could also find application in other ducted fan systems. The monopole-to-dipole source conversion approach is demonstrated here both numerically and experimentally based on a simplified model of the fan base unit.
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