Conventional loudspeakers work by moving a flat surface (diaphragm) back and forth using a combination of a wire coil and a permanent magnet.  This technology remains largely unchanged since its conception in 1924.  More recently, however, researchers from the Centre for Metamaterial Research and Innovation at the University of Exeter have been testing new devices for producing sounds, known as ‘thermophones’.  These devices are much simpler in their construction, consisting of only a thin film or wire connected to a power supply.  When the connected power supply operates on an alternating current, the electrically conductive film or wire is rapidly heated, experiencing fluctuations in surface temperature.  This heat is then dissipated as sound via the thermo-acoustic effect (i.e. the conversion of heat energy to sound energy).  Thermophones therefore provide a way to produce sound by modulating air temperature, similar to the rumble of thunder or the roar of a fire.


Diagram of a traditional loudspeaker

a)Diaphragm b)Voice coil c)Magnets

As thermophones have no moving parts and can be shrunk to millimetre scale sizes (through the use of metamaterials such as graphene), they allow for more robust sound-producing devices that are smaller, lighter and less intrusive when used in conjunction with other machines and objects.  Therefore, they hold great promises for applications such as photoacoustic imaging (using the photoacoustic effect of light exciting a material, causing it to produce sound), material fault detection and wireless power transfer.


Diagram of a thermophone loudspeaker

d)Thin film/wire to be heated

e)Wires for power delivery

f)Heat dissipated as soundwaves.