Xiao et al., Nano Letters, online edition (2008)

Silent no more. Ultrathin films made from aligned carbon nanotubes (left) blare as loudly and clearly as conventional speakers.

Big Noise From Little Tubes

Staff Writer

Last year brought news of Lilliputian radios made from whiskerlike carbon nanotubes. Now, researchers in China have added ultrathin loudspeakers to go with them. The devices, made from transparent and flexible carbon nanotube films, don't require any of the bulky magnets and sound cones of conventional speakers. So they could lead to a new generation of nearly invisible, flat speakers that can be integrated into everything from ceilings and walls to clothing and curtains.

Carbon nanotubes have long been hailed for their strength and high conductivity. But their ability to generate sound came as a surprise. Conventional speakers use magnets to move a thin diaphragm, generating pressure waves in the air that we register as sound. The new nanospeakers have none of these accoutrements. Instead, the device developed by physicist Kaili Jiang of Tsinghua University in Beijing is just a thin film of nanotubes. Last year, Kaili and his colleagues placed electrodes on opposite ends of a 10-centimeter-wide film, switched on an electric current, and were flabbergasted when it made noise.

In a study that will be published in the 10 December issue of Nano Letters, the scientists report that changes in the current cause the air surrounding the nanotubes to rapidly heat and cool, which in turn produces pressure waves and sound--no magnets or diaphragm required. This "thermoacoustic" effect was first reported more than 100 years ago by researchers experimenting with thin metal foils. But early efforts to make use of it never went anywhere because it was so weak. Sound from the carbon nanotubes is 260 times louder, because the nanotubes are better at converting electricity to heat, Kaili says.

The Chinese team capitalized on its find to create paper-thin loudspeakers of various sizes and shapes--from a few square centimeters to as large as a sheet of notebook paper--that can blare as loudly as conventional speakers with extremely low distortion and high sound quality (see videos below). Kaili says his team is currently working to commercialize the technology.

The new work is "very clever stuff," says John Rogers, a materials scientist and nanotube radio builder at the University of Illinois, Urbana-Champaign. Ray Baughman, a materials scientist and carbon nanotube film pioneer at the University of Texas, Dallas, adds that the simplicity of the new speakers is "a big plus" for paving the way toward potential applications, including speaking clothing that could aid the physically or mentally impaired.

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