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November 17th, 2008

Nanotube speakers – The next “big” thin thing

Published on November 17th, 2008 | By: Peter Wray

What’s transparent, stretchable, nearly weightless, flexible enough to be sown onto a shirt and plugs into your iPod? Maybe, soon, this may describe one of the best and most durable audio speakers you’ll ever own. A group of Chinese researchers have discovered that, by zapping an audio-frequency current through a carbon nanotube thin film, it can be made into a wispy-thin high-performance loudspeaker that emits sound in a wide-frequency range, operates without moving parts or magnets. Moreover, the phenomenon’s discoverers say the CTN loudspeaker can be bent into numerous shapes, mounted onto a variety of rigid and flexible insulating surfaces – including clothes – and, since it doesn’t vibrate, can even operate when torn, worn or partially damaged.

Details of the CTN loudspeakers have been reported in the Oct. 29 online edition of Nano Letters published by The American Chemical Society by KaiLi Jiang and Shoushan Fan, professors at Tsinghua University in Beijing, and their colleagues at Beijing Normal University. The researchers explain how they drew out a thin film from CTN arrays and, then, tested the thin film for acoustic properties. Finding that the nanotube speaker emitted sound, they wondered if it did so in a manner similar to conventional speakers. Testing it with a laser vibrometer, they learned it did not. The CTN thin film did not vibrate. “We attributed this to the thermoacoustic effect,” Jiang says in a Nov. 2 interview with Nanowerk, an online nanotechnology news vehicle. “The alternating current periodically heated the CNT thin films, resulting in a temperature oscillation. The temperature oscillation excites the pressure oscillation in the surrounding air, resulting in sound generation.”

Elaborating on this effect, via Newscientist.com, Shoushan Fan explains that when the team passed an alternating current through their CNT thin-film speaker, the film quickly switched back and forth between room temperature and 80°C. These swift temperature changes resulted in pressure oscillations in the air next to the thin-film. Fan reports the team found these thermally-induced pressure oscillations responsible for the sound emitted by the thin-film speaker. He notes that, when fully stretched, the nanotube speaker became transparent, causing the team to envision attaching it to the front of an LCD screen in place of standard speakers. Some of the more “exotic uses” they dreamed up for the speaker, he says, included the possibility stitching it into clothing to create a kind of “singing and speaking jacket.” Despite its many pluses, Jiang says the CNT loudspeaker has one negative – its output frequency doubles that of its input . This makes “the human voice sound strange when a commercial bipolar audio amplifier is used to drive the CNT thin-film loudspeaker,” he reveals in Nano Letters. He describes how this can be solved in a relatively simple manner, however, by adding a direct current bias I0 to the alternating current. Additional challenges will undoubtedly be encountered on the road to commercialization. At present, the researchers say they will focus on developing the CNT thin-film speakers into real products. But production of the material is relatively easy and should be inexpensive:

“One superaligned CNT array grown on a 4 in. silicon wafer can be totally converted to a continuous CNT thin film up to 10 cm wide and 60 m long, which can be further made into approximately 500 loudspeakers with a size of 10 cm by 10 cm! [The speakers] can be tailored into many shapes and mounted on a variety of insulating surfaces, such as room walls, ceilings, pillars, windows, flags, and clothes without area limitations. Furthermore, CNT thin films can also be made into small area devices, such as earphones and buzzers.”

Complete finding and several videos are published online.


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