Mechanism of low-frequency spectral scattering by a side-branch electromagnetic device with switching shunt

Sound waves are reflected and absorbed by a passive side-branch device in a duct. The performance of such a configuration is limited for low frequencies and if the cavity is compact. In this study, an electro-magnetic mechanism to enhance such low-frequency performance is examined. A common loudspeaker diaphragm is used as a passive interface to the cavity, with its moving-coil immersed in the magnetic field, and a shunt analogue circuit is attached. When the diaphragm is driven by the incident wave, the reactive Lorentz force exerts extra acoustic impedance. A MOSFET switch is added to periodically toggle between the shunt-off state, which is made damping-free in this study, and the shunt-on state which can almost forbid the diaphragm to vibrate if the shunt is close to a short circuit. The repeated transitions scatter a significant portion of the incident sound energy to frequencies other than the incident, with its peak scattering efficiency found when the switching is twice the frequency of the incident. The sudden removal of the Lorentz force by MOSFET switch-off creates a boost in the diaphragm response which is otherwise suppressed by the cavity stiffness, leading to much enhanced sound reflection.