Toward a new generation of paper-thin loudspeakers

Nov 17, 2008
This paper-thin cylinder -- composed of carbon nanotubes -- emits sound in all directions. Credit: American Chemical Society

In research that may redefine ear buds, earphones, stereo loudspeakers, and other devices for producing sound, researchers in China are reporting development of flexible loudspeakers thinner than paper that might be inserted into the ears with an index finger or attached to clothing, walls, or windows. Their report on what may be the world's thinnest loudspeakers, made from transparent carbon nanotube films, is scheduled for the December 10 issue of ACS' Nano Letters.

Kaili Jiang, Shoushan Fan, and colleagues note that most of today's loudspeakers are relatively bulky, complex, and inflexible, consisting of a permanent magnet fixed to a voice coil and a cone. To meet the growing demand for smaller speakers for portable digital consumer electronics devices, manufacturers need new technology, they say.

The scientists describe the development of super-thin carbon nanotube (CNT) films — 1/1,000th the width of a single human hair — that are capable of transmitting music and other sounds. In laboratory tests, the researchers mounted a thin CNT film onto two electrodes to form a simple loudspeaker. The speaker produced sound with the same excellent quality as conventional loudspeakers, but without magnets and moving components, the researchers say. They also demonstrated that the flexible film could be used just as effectively to play music from an iPod and while pasted to a flexible, waving flag (please see accompanying video).

"These CNT thin film loudspeakers are transparent, flexible, and stretchable, which 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 limitations. Furthermore, CNT thin films can also be made into small area devices, such as earphones and buzzers. There is no doubt that more and more applications will be developed as time goes on. This technique might open new applications of and approaches to manufacturing loudspeakers and other acoustic devices."

Citation: "Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers" dx.doi.org/10.1021/nl802750z

Source: American Chemical Society

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Ivan2
3 / 5 (1) Nov 17, 2008
When I didn't have an amp, sometimes I would go to the lobby of my building and touch the 11 foot glass with the tip of the guitar. It acted as a sort of amplifier -very low volume. The vibration was spread all over the glass and therefore the sound was heavy with reverb but it was a pleasant sound. Could this flat-sheet amplification also become (carbon-tube?) technology later on?

(Don't get too tech: I don't know the first thing about amps or loudspeakers, though I have blown a few.)
DGBEACH
4.3 / 5 (3) Nov 17, 2008
No one said there was any amplification occurring in this material. The carbon-tube sheet contracts (put simply) when an electric current is passed through it, seemingly at the same rate as the current (thus the high fidelity of the sound), and returns to its original shape once the current stops, like a regular magnet-based speaker does.
Years ago electrostatic speakers were all the rage, being thin-filmed ribbons stretched within a statically charged (high voltage) baffled box. The ionized air would actually get moved by the audio signal flowing through the ribbons, recreating the original sounds recorded, giving the same results as the above-mentioned carbon-tube sheets.
...except they've managed to do the same thing without the dangerous voltages ...congratulations!

And btw, as an acoustic-bass player I can definitely relate to your window story, though with my instrument the wooden floor works even better :)
Agisman
5 / 5 (4) Nov 17, 2008
This has been described here and elsewhere as a thermoacoustic device. The CNT heats up and cools down with the change of signal level, causing a pressure oscillation in the surrounding air. They claim a high output sound level but then they skirt around the details. The measurements are made at 5cm instead of typical 1m and input power is 4.5W instead of 1W. At human vocal ranges, they claim 100dB at 5cm with 4.5W input. This should put their actual sensitivity down to around ~60db/W at 1m which is a more appropriate figure of merit. Quite a bit lower than conventional speakers which can go higher than 110dB/1w/1m. Remembering that dB is a logarithmic scale.

This is in contrast to the operational mode of electrostatic speakers. Electrostatics have a charged sheet (like Mylar) between two electrode sheets. By changing the voltage between front and back, the membrane oscillates. The only way to get appreciable force (and SPL) is to use high voltages. An electrostatic speaker should never ionize the air during normal operation. Sound is just generated by moving a diaphragm back and forth as usual.

This CNT speaker has some neat applications but it will be difficult to make high quality sound sources on monitors and TVs with. Ignoring the poor low frequency response, the dipole nature of this speaker is problematic. While it's not exactly a dipole but more of an 'omnipolar' source, the back wave reflected off a plane could cause phase shifts reducing output and adding distortion. I'm curious and excited to see what applications this ends up used for.
physpuppy
not rated yet Nov 17, 2008
... I would go to the lobby of my building and touch the 11 foot glass with the tip of the guitar. It acted as a sort of amplifier -very low volume...


The strings of a piano don't make much sound - thats why it has a soundboard which picks up the vibrations from the string and by resonance passively amplifies it.
sgeek
not rated yet Nov 18, 2008
Ivan2, the glass is acting as a resonator rather than an amplifier. It's not amplifying the sound energy - rather, it's transmitting that sound energy (which would otherwise just be dissipated as heat).
DGBEACH
not rated yet Nov 18, 2008
This is in contrast to the operational mode of electrostatic speakers. Electrostatics have a charged sheet (like Mylar) between two electrode sheets. By changing the voltage between front and back, the membrane oscillates. The only way to get appreciable force (and SPL) is to use high voltages. An electrostatic speaker should never ionize the air during normal operation. Sound is just generated by moving a diaphragm back and forth as usual.

...you're right! As the quebecois say, "I guess I'll go to bed more intelligent tonight"...although it's usually said in a dialect of French which most Frenchmen find objectionable -:)