You can launch sound in a crystal at a frequency so high that the human ear can't hear it, and make that sound run in circles, like a train on a loop track. Engineers took a thin film of silicon nitride, deposited it on a lithium niobate substrate, and carved waveguides for sound waves into it. The result is an acoustic chip the size of a fingernail, in which waves at about a billion oscillations per second travel with almost no loss.
The key component is a ring resonator — an acoustic 'loop' where a wave can circulate for a very long time. Based on it, they built an oscillator: a device that outputs an electrical signal at a strictly constant frequency, like a metronome that never drifts. The frequency is 1 gigahertz — a billion oscillations per second. The precision is high: the phase noise (frequency jitter) at a 100 kHz offset is -159 dBc/Hz: that's on par with the best quartz oscillators, but in a package hundreds of times smaller.
Such acoustic chips open the door to quantum devices where sound serves as a mediator between superconducting qubits and photons. A sound wave in a crystal can store quantum information longer than an electrical signal in a wire, and transfer it without heating. Perhaps future quantum computers will hum at frequencies we can't hear, but which control their computations.
A frequency of 1 GHz is a million times higher than the highest sound a human can hear (about 20 kHz). If such a wave traveled through air, its wavelength would be less than a third of a millimeter.
In Arthur C. Clarke's novel 'Light of Other Days,' wormholes transmit sound through a crystal lattice — similar to how sound waves travel through silicon nitride in this work.