A tunable electromagnetic acoustic switch

An acoustic switch permits or forbids sound transmission through a partition, and its performance is governed by the stiffness and mass laws at low and high frequencies, respectively. The mechanism of artificial mass and stiffness, either positive or negative, is required to break these laws; all are demonstrated experimentally in this study. The switch consists of a suspended diaphragm with electric moving coil and a magnetic field, shunted by an essentially passive analog circuit. We show that electrically mediated damping is extremely large, and its mechanism as a powerful wave stopper can be very broadband, which contrasts with most resonance-based devices in the literature. We also show that a serial shunt capacitor introduces a mechanical mass that softens the diaphragm spring at low frequencies, while a shunt inductance is an electromagnetic spring that pacifies mechanical inertia at high frequencies. By manipulating the dynamic mass, stiffness, and damping electronically to enhance or defy the mass law and stiffness law, a switch effective in over one octave and working at a deep subwavelength scale is realized, and the maximum switch ratio is as high as 28 dB. As circuits can be miniaturized and easily tuned, these illustrated physics point to a versatile tool for digital control of sound waves.