It is known that intense sound can be produced by electrically-powered nanotubes stretched into sheets, but researchers from University of Texas at Dallas (UT Dallas) have furthered this principle by arranging sheets of carbon nanotubes into "forests" which produce high-quality sound when struck with lasers. This is an exciting advancement in the field of acoustics since it is thought these forests could be used to form invisible wireless speakers that could be embedded into walls, windows, computer screens, cars - the list is endless.

The acoustic properties of electrically-powered nanotubes have been known for some time. Previous studies have shown that sheets of carbon nanotubes are able to produce sound when heated with alternating electrical current. However, the UT Dallas NanoTech research team has found that striking tones can be generated by vertical arrays of nanotube forests, which visually resemble black velvet. Furthermore, they discovered that high-quality sound can be generated when these forests are excited with laser light that is modulated in the acoustic frequency range.

“Nanotubes assemblies of various types are black and highly conductive,” said Dr Mikhail Kozlov, a research scientist and the study’s lead author. “Their dark, conductive surface can be effectively heated with laser light or electricity to induce variations in the pressure of the air around the nanotubes - which we perceive as sound. It’s called the photo- or thermo-acoustic effect, and it’s the same principle Alexander Graham Bell used to produce sound on the first telephone.”

No electrical contact with the nanotube speaker is required with laser excitation, so the speakers can be wireless and embedded into all sorts of materials. “Speakers made with carbon nanotube sheets are extremely thin, light and almost transparent,” Kozlov said. “They have no moving parts and can be attached to any surface, which makes the surface acoustically active. They can be concealed in television and computer screens, apartment walls, or in the windows of buildings and cars. The almost invisible strands form films that can ‘talk.’”

The speakers could also be used to cancel out sound utilizing the same principles that govern current sound-canceling technologies. Both sound effects could be achieved by the nanotube sheets on a very large scale.

Additionally, the study team took a number of capability measurements from the nanotube sheets to contribute to the growing knowledge base of characteristics and properties for use in future studies. Such measurements are important in new areas of research and serve as foundations for knowledge for further research in the field.

The study, co-authored by research scientist Dr Mikhail Kozlov, Carter Haines, senior physics major at UT Dallas, Dr Shaoli Fang, associate research professor, and research associates Dr Jiyoung Oh, Dr Marcio Lima was published in the Journal of Applied Physics. Haines is a former George A. Jeffrey NanoExplorer, who conducted research at the NanoTech Institute while in high school.

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