Active Control of Fan Noise in Ducts Using Magnetic Bearings

Abstract

Abstract : The objective of this project was to investigate global noise attenuation of narrow band fan noise in an air duct through the use of magnetic bearings. An axial flow fan creates tonal noise related to its rotational rate. Additional noise exists due to harmonics of this frequency as well as turbulent airflow. In addition to conventional brush bearings to support the fan shaft radially, this project used an active magnetic thrust bearing to control axial movement. The thrust bearing primarily functioned as an active sound control actuator. Active sound control is the method of achieving destructive interference of sound waves by outputting a secondary wave of equal amplitude and frequency, but 180 degrees out of phase with the primary wave. An error microphone was positioned in the duct to provide feedback to a Digital Signal Processor (DSP), which contained the active sound control program, while a performance microphone tested for global sound control at various points along the duct. Instead of using a secondary speaker, this project used the fan itself to collate the primary and secondary sound sources. Therefore, global sound control throughout the duct was theoretically possible. This project demonstrated this global control of noise experimentally.