Sound waves – oscillating pressure and velocity fields – bear a striking resemblance to pistons. They are ‘virtual’ pistons, since they do not require any moving parts, or lubricated close-tolerances, with intricate and often expensive construction. As such, they may be made to perform useful work, much like any mechanical piston.

Heat engines and pumps perform most of the energy conversion in modern society, be it electricity production, air-conditioning or refrigeration. Replacing these machines with their acoustic analogs represents an exciting prospect – such machines hold the promise of simple, cheap construction, requiring no exotic materials or harmful fluids.

Converting heat into a pressure wave is quite like the thunder that follows a lightning bolt – a rapid expansion of air that propagates as sound. In the presence of a locally large enough temperature gradient, sound is generated spontaneously and may be sustained as long as the required heat is supplied.

Here you can see one of our engines, in which heat is supplied at the bottom where it converts into sound waves, and the rest of the chamber (resonator) acts as a gas spring to control the frequency of oscillations. Such a device can generate extremely intense sound waves (think of putting your ear next to a jet engine), which can be used to power a small acoustic refrigerator.

These acoustic instabilities are studied in an attempt to gain better knowledge of how they may be generated controllably. In particular, we are looking for ways to reduce the temperature gradient at which these oscillations are excited, so that low-grade heat may be used as the energy source – for example, using simple rooftop solar collectors.