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Advance through nanotechnology

Scientists in Germany and the US are using nanotechnology to work towards 'the invisible speaker'. Simon Duff reports

Unusual and innovative loudspeaker design continues apace with a new groundbreaking ultra-thin technology made possible by the use of carbon nanotubes (CNTs). Research scientists from the Fraunhofer Institute for Manufacturing, Engineering and Automation (IPA) in Stuttgart have prototyped the 200 micron diameter speaker (about twice as thick as a human hair) and demonstrated it in February at the Nano Tech 2010 Show in Tokyo, Japan.

Using CNTs, a loudspeaker can be made without the need for components such as magnets and piezoelectric devices, or any moving parts, enabling them to be lightweight and placed in a variety of different locations.

The demonstration model shown measured 8cm x 6cm and was made by applying a conductive coating onto a PET (polyethylene terephthalate) film, which contained CNTs. When a 12V direct voltage is applied to it and the current is switched on and off rapidly, it generates an audio signal corresponding to the frequency of the switching. “The principle of the sound generation is not produced through vibration but thermoacoustically,” comments Ivica Kolaric, department manager of Fraunhofer’s process engineering of functional materials department.

“The CNTs generate heat and expand when the current is on, and they cool down and shrink when the current is off.”

It is this expansion and contraction of the film that moves the air molecules, creating the resulting sound. The demo model, run off a transistor amp developed by Fraunhofer – although it can work off a conventional amp – has a frequency response of 200Hz-20kHz so, in keeping with conventional audio systems, a woofer is required for accurate low-frequency production.

Carbon nanotubes are molecular-scale tubes of graphite carbon with outstanding properties that have caused researchers and companies to consider using them in several fields. For example, because they have the highest strength-to-weight ratio of any known material, researchers at NASA are combining CNTs with other materials, creating composites that can be used to build lightweight spacecraft.

Research is also being carried out into weaving them into clothes to create stab-proof and bullet-proof clothing, as well as in medicine as a possible gene delivery vehicle to destroy cancer cells. A nanoradio, a radio receiver consisting of a single nanotube, 1/10,000 the diameter of a human hair, was demonstrated in 2007 by physicists at the University of California, Berkeley, and in 2008 it was shown that a sheet of nanotubes could operate as a loudspeaker if an alternating current is applied.

The major benefit of the CNT speaker is its extraordinary thinness, allowing it to be mounted on a number of flat surfaces such as wallpaper or advertising boards. However, researchers at Fraunhofer have also speculated that if fewer nanotubes could be used in the film, it would be possible to make it completely transparent.

This would enable them to be used in a variety of different locations, including on large areas such as carpets and walls, or on computer monitor screens and windows. Kolaric also believes that the speakers could have sensors in them allowing users to control them just by pointing a finger.

CNTs are also under investigation by scientists at the University of Texas, Dallas. A team there has published a study in the Journal of Applied Physics expanding on CNT capability and found that sound can be generated by vertical arrays of nanotubes, they call forests, which resemble black velvet. The team discovered that high-quality sound could be generated when nanotube sheets or forests are struck with laser light that is modulated, or ‘altered’, in the acoustic frequency range.

“Nanotube assemblies of various types are black and highly conductive,” comments 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.”

With laser excitation, no electrical contact with the nanotube speaker is required, making the speakers wireless. In addition to filling a room with sound from invisible speakers, nanotube speakers could easily cancel sound from the noisiest neighbour or dim the roar of traffic rushing past a neighbourhood, using the same principles as current sound-canceling technologies.

Development of Fraunhofer’s CNT speaker started in 2000 and to date 20 have been made, although plans for commercial manufacturing are still some three years off. Engineers at the institute believe that the speakers will be ideal for a variety of market places, including the automotive, home appliances, next-generation OLED (organic light-emitting diode) TVs and other consumer goods sectors.

At present, marketing and demonstration is carried out at conferences and exhibitions. Kolaric is both ambitious and determined. She concludes by saying: “Our vision is to be a leading developer of thin-film sound panels in an overall loudspeaker market that analysts at the Electronics Industry Market Research and Knowledge Network in California are predicting to grow to $4.2 billion (€3.2 billion) this year.”