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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. In this theoretical study, we explore a new approach of high order mode noise control by converting the sound frequency to below the cut-off frequency of the duct. An electromagnetic diaphragm with a MOSFET-controlled shunt circuit is used to create time-varying acoustic impedance on the wall. When properly organized, the incident sound energy is dispersed to side-band frequencies, some of which are below the cut-off frequency. In this way, it is found that part of energy of high-order modes is absorbed by the electrical sink in the shunt circuit. The time-variation is made on passive basis in the sense that it requires no sensor input and no energy input for actuation, thus ensuring stability and potential application in engineering. In this study, parametric studies are conducted to obtain the best sound energy dispersion efficiency.
Huang et al. (Fri,) studied this question.