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

Abstract

Sound waves are reflected and absorbed by a passive side-branch device in a duct. The performance is limited at low frequencies if the cavity is compact. In this study, an electro-magnetic mechanism to enhance such low-frequency performance is examined. A common loudspeaker diaphragm, with its moving-coil immersed in the magnetic field, is used as a passive interface to the cavity, and a shunt analogue circuit is periodically connected and disconnected by a MOSFET. When the diaphragm is driven to vibrate by the incident wave, the reactive Lorentz force exerts extra acoustic impedance, which almost stops the diaphragm vibration if the shunt is close to a short circuit. The repeated transition between system damping of very low and high values scatters a significant portion of the incident sound energy to frequencies other than the source frequency. The peak energy scattering efficiency is 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. When the incident wave is random with a finite frequency band, scattering effect is found to offer a positive virtual mass, which counters the high system stiffness in the low frequencies.