Acoustic switches allow for the manipulation of sound propagation by either permitting or preventing sound transmission through a barrier, depending on whether the acoustic waves meet a set of desired criteria. Typically, these switches require either mechanical change or active circuit components, both of which can be large energy drains. Zhang et al. developed a passive acoustic switch design that bypasses these complications to control acoustic waves.

Central to the authors’ device is an electromagnetic spring, a diaphragm which, when accelerated by high-frequency sound waves, pushes against a coil to drive current through a shunt circuit and induce spring-like acoustic impedance. Lower frequency sounds are suppressed by electric mass induced by a capacitor, which is simply air motion. Because each frequency region is governed by different physics, properly tuning this mass and the stiffness of the electromagnetic spring allows a user to select their desired frequency range to control.

“In our group, we always know that mechanics and electromagnetics are two parallel worlds, but we are pleasantly surprised that they are even on very similar scales of parameters, and the interaction can be so strong and fascinating,” said author Yumin Zhang. “Maxwell is fully united with Newton.”

Though they used an ordinary loudspeaker as a proof-of-concept of their method, the parameter range needs to be extended for real-world applications. Their first target will be passive earphones that temporarily allow the passage of sound without the use of complex digital signal processing for when a wearer is, for example, crossing the street. They also believe the technique can eventually be used for tunable building acoustics, allowing a concert hall to be easily converted into a convention center.

Source: “A tunable electromagnetic acoustic switch,” by Yumin Zhang, Chunqi Wang, and Lixi Huang, Applied Physics Letters (2020). The article can be accessed at