A theoretical study on altering the frequency of high-order modes using a time-varying wall property

Abstract

Noise radiation by aero-engines features high circumferential modes and high frequencies. It is rather difficult to suppress such high-order modes near their cut-on frequencies. Delicate designs are needed. When the engine operation conditions change, a design good for one set of parameters may not be optimal for another. This study explores a new path: an electromagnetic diaphragm with a MOSFET-controlled shunt circuit can be used to convert the wave frequency to any value at will by creating a digitally pre-defined time-varying material property. When properly organized, the incident sound energy is dispersed to several frequencies, some of which are below the cutoff frequency. In this way, it is showed that part of the energy of high-order modes can be reduced. This method creatively utilizes electroacoustic coupling to alter frequencies and control the propagation of high-order modes. Additionally, the time-variation is made on a passive basis in the sense that it requires no sensor input and no energy input to the actuator, thus ensuring stability and potential application in engineering.