

That’s what is envisaged in a study by Andrew Barnard and colleagues at the Pennsylvania State University. They have revisited an idea nearly one hundred years old, and sounding decidedly steampunk: the thermophone or thermoacoustic loudspeaker, in which sound is generated by the effect of a material rapidly oscillating between hot and cold. In 1917 Harold Arnold and IB Crandall of the American Telephone and Telegraph Company and Western Electric Company showed that they could create sound by simultaneously passing alternating and direct currents through a very thin platinum foil. This heats up the foil, and the heat is conducted into the air surrounding it in pulses paced by the frequency of the AC current.

Sound waves are created by oscillations in air pressure, and an ordinary loudspeaker generates these via a mechanical vibration of a membrane. But air pressure is also altered when the air gets hotter or cooler. So the thermal oscillations of Arnold and Crandall’s platinum film also generated a sound wave – without any of the cumbersome, heavy electromagnets used to excite vibrations in conventional speakers, or indeed without moving parts at all.



It’s a gas, gas, gas


That’s what Barnard and colleagues have sought to assess. One of the ways in which  to improve sound output is to surround the CNT film with a gas that has a lower heat capacity than air, such as the inert gases helium, argon or xenon, as this means a lower input of energy can create a big change in temperature, and therefore pressure. Xiao’s team has already demonstrated this effect, but Barnard and colleagues now show that it offers perhaps the best avenue for improving the performance of these devices. To transmit the acoustic vibrations of the inert gas to the air beyond, so that we can hear the results, one would separate the gas and air with a flexible membrane.


All things considered, Barnard and colleagues conclude that a high power CNT loudspeaker is feasible, but it won’t be simple. The CNT films will probably need to be enclosed and immersed in xenon, for example, which would pose serious challenges for making robust “wearable” speakers.

