Tuesday, September 10, 2013

Before it gets too previous, here is an earlier piece for BBC Future.


It’s time for one of those imagined futures which always miss the mark by a mile – you know, “Imagine setting off for work with your jet-pack…” But here we go anyway: imagine that photographs, newspapers and books speak, that you can play music out of your curtains, that food wrapping calls out “I’m nearly past my sell-by date!” OK, so perhaps it’s all a bit nightmarish rather than utopian, but the point is that some weird and wonderful things would be possible if a loudspeaker could be made as thin, light and flexible as a sheet of 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 a 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 I. B. 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 that are paced by the frequency of the a.c. current.

A sound wave in air corresponds to an oscillation of the air pressure. An ordinary loudspeaker generates those pressure waves 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 revitalized 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.

The Chinese team, led by Lin Xiao at Tsinghau University, showed that they could get their CNT films to emit sound. But that’s not the same as making a loudspeaker that will produce good-quality sound over the whole frequency range of human hearing, from a few tens of hertz (oscillations per second) to several thousand. So while the CNT speakers might have valuable applications such as sonar – they work perfectly well underwater – it isn’t yet clear if they can produce hifi-quality sound in your living room.

That’s what Barnard and colleagues have sought to assess. One of the factors determining the loudness of the devices is how efficiently heat can be transferred into the surrounding gas to induce pressure waves. This depends on how much the gas heats up for a given input of heat energy: a property called the heat capacity. A low heat capacity means that only a small energy input can create a big change in temperature, and thus in pressure. So the sound output can be improved by surrounding the CNT film with a gas that has a lower heat capacity than air, such as the inert gases helium, argon or xenon. 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.

Another way to improve the sound output is to make the surface area of the film bigger. That can be done without ending up with a carpet-sized device by stacking several sheets in layers. The Pennsylvania group has shown that this works: a four-layer speaker, for example, is significantly louder for the same power input.

All things considered, Barnard and colleagues conclude that “a high power CNT loudspeaker appears to be 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.

And there is already competition. For example, a small start-up British company called Novalia has created an interactive, touch-sensitive printed poster that can generate drum-kit sounds through vibrations of the paper itself. Curiously, that technology uses electrically conducting inks made from a pure-carbon material called graphene, which is basically the same stuff as the walls of carbon nanotubes but flattened into sheets. So one way or another, these forms of ‘nanocarbon’ look destined to make our isles full of noises.

Reference: A. R. Barnard et al., Journal of the Acoustical Society of America 134, EL280 (2013).

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