Nanotubes: Can we make speakers as thin as paper?

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.

The problem was that the sound wasn’t very loud, however, and the frequency response wasn’t up to reproducing speech. So the idea was shelved for almost a century.

It was revitalised in 2008, when a team in China found that they could extract thermoacoustic sound from a new material: a thin, transparent film made from microscopic tubes called carbon nanotubes (CNTs), aligned parallel to the plane of the film. These tiny tubes, whose walls are one atom thick and made from pure carbon, are highly robust, need very little heat input to warm them up, and are extremely good heat conductors – just what is needed, in other words, to finally put the idea of Arnold and Crandall into practice and create gossamer-thin loudspeakers.