Drum-like silencers using magnetic forces in a pressurized cavity

A feasibility study is carried out for utilizing magnetic force to yield a low-frequency shift of the transmission loss spectrum provided by a drum-like silencer consisting of two side-branch, rectangular cavities covered by ferromagnetic membranes. The results show that the transmission loss spectrum of the drum-like silencer is mainly controlled by the vibration of the first and second modes of the membrane. Three pairs of magnets are employed inside the cavity to promote the response of these modes. It is found that the magnetic force imposes both static and dynamic effects on the silencer. While the latter helps shift the effective region of the silencer towards lower frequencies, the former results in an increase of stiffness of the membrane which is detrimental to the operation of the silencer at low frequencies. Cavity pressurization is then proposed to neutralize the static effects of the magnetic force. A finite element model is developed to predict and optimize the performance of the proposed silencer with some of the results validated experimentally. The desired shift towards the lower frequency is validated although the silencer performance is still less than ideal due to both parametric and operational constraints of the rig.