Periodic flow noise reduction using flow-excited resonators: Theory and application

Sound reduction by placing resonators on the cut-off surfaces of centrifugal fans was modeled as an acoustic dipole that results from the interaction of a moving fluid with a solid boundary. Part of the moving boundary is replaced with the fluid entrained in the open mouth surface of the fluid excited resonator. By proper tuning of the resonator, a source can be generated in antiphase to those generated at the remaining solid boundaries, reducing radiated sound. To position the resonator, pressure distribution over the surface of the solid boundary must be known. To tune the resonator for maximum sound reduction, several geometrical parameters can be varied. However, since the choice of the mouth surface area and porosity (hole number and size) is limited by the requirement that the resonator response be made as large as possible, the most straightforward way to tune the resonator is by varying the length of its cavity.