Towards altering sound frequency at will by a linear meta-layer with time-varying and quantized properties

Wave frequency is a critical parameter for applications ranging from human hearing, acoustic non-reciprocity, medical imaging to quantum of energy in matter. Frequency alteration holds the promise of breaking limits imposed by the physics laws such as Rayleigh’s criterion and Planck–Einstein relation. We introduce a linear mechanism to convert the wave frequency to any value at will by creating a digitally pre-defined, time-varying material property. The device is based on an electromagnetic diaphragm with a MOSFET-controlled shunt circuit. The measured ratio of acoustic impedance modulation is up to 45, much higher than nonlinearity-based techniques. A significant portion of the incoming source frequency is scattered to sidebands. We demonstrate the conversion of audible sounds to infrasound and ultrasound, respectively, and a monochromatic tone to white noise by a randomized MOSFET time sequence, raising the prospect of applications such as super-resolution imaging, deep sub-wavelength energy flow control, and encrypted underwater communication. Temporal modulation materials have been attracting attention thanks to their ability to break wave reciprocity, i.e., waves traveling from point A to B are identical to waves propagating from B to A. The authors present a linear temporal modulation device with a giant acoustic impedance modulation ratio and ability to linearly change wave frequency, operating in two quantized states when a shunt circuit is connected and disconnected by a MOSFET.