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Flexible, stretchable, nearly indestructible CNT loudspeakers

By Damir Beciri
6 October 2009

cnt-loudspeaker-1A thin film based on carbon nanotubes (CNT) could replace conventional magnetic loudspeakers. By applying an audio frequency current through the CNT, the loudspeaker can generate sound with wide frequency range, high sound pressure levels (SPL), and low total harmonic distortion (THD). The uniqueness of this advancement is that the films are flexible, stretchable, transparent, and can be tailored into many shapes and sizes, freestanding or placed on a variety of rigid or flexible insulating surfaces.

The technology is also a potential fit for extreme applications where conventional loudspeakers are likely to get damaged. “It will still emit sound if part of the film is broken, and it can be put on any flexible substrate, such as flags,” said Kaili Jiang, a professor at Tsinghua University in Beijing. He and other researchers from Tsinghua University and Beijing University created the thin CNT films by using two simple electrodes, and several films can be put together to make a large area loudspeaker. A cylindrical cage-like CNT thin film loudspeaker can also be shaped to emit sound in all directions. A sinusoidal voltage is applied across the two electrodes to create clear and loud tones.

Sound generation mechanism of the CNT film can be understood with the aid of a thermo-acoustic picture. The alternating current from the two electrodes periodically heats the CNT thin films, resulting in a temperature oscillation. This fluctuation excites the pressure to oscillate in the surrounding air, resulting in sound generation. The mechanism that produces the sound is not the mechanical movement of the film, but the thermal expansion and contraction of the air in the vicinity of the thin film.

When an alternating current passes through the CNT, the thin film will be heated for the duration of the positive and negative half-cycles. This results in a double frequency temperature oscillation, as well with a double frequency sound pressure.

The most promising property is the stretch-ability of the film. Using springs (which double up as electrodes), the film can be uniformly stretched up to a maximum of 200% of its original size and with this, the film increases in transparency. This see-through feature could enable the CNT film loudspeaker to be adapted to LCD modules. If the loudspeaker film is mounted on top of a LCD module, transmittance is reduced to 80%, with the view being only slightly darker than usual. However, by stretching the film or by administering a laser treatment, the transmittance increases to 95%.

The other important characteristic is the flexibility of the film which can be molded into any shape and placed onto a range of rigid or flexible insulating surfaces. The loudspeaker films can be placed on flags or even on clothes – to create a speaking or singing jacket. With all these incredible future applications, the project has many more hurdles to cross. The CNT would need to be freestanding and able to be developed into any shape or size.

There is no doubt that more and more applications using CNT films could be developed as time goes on.  The CNT thin films can also be made into small area devices such as earphones and buzzers. This technique might open up new applications and approaches to manufacturing loudspeakers and other acoustic devices. If you’re a sounded instrument player just imagine it had a coating of CNT speakers on it that could enhance the sound of your instrument. The film can also be applied to the iPod, placed on window glass, or even over paintings to make transparent loudspeakers.

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