Carbon nanotubes rock—literally!

by Andrew Maynard on November 26, 2008

A nanotechnology fix for high-end audio addicts?

I’m sitting at my computer watching a surreal balletic movie—a sheet of highly aligned carbon nanotubes is being slowly stretched, then allowed to slowly contract.  In the background is a soundtrack of traditional-sounding Chinese music.

At least I think the soundtrack is over-dubbed, until I realize that the music is coming directly from the nanotube sheet itself, which has been attached to a small amplifier.

And it suddenly dawns on me that I am watching something rather special—perhaps the biggest breakthrough in loudspeaker technology in decades…

The movie comes out of the lab of Kaili Jiang at Tsinghua University in Beijing, and accompanies a paper recently published on-line in the journal Nano Letters (Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers, Xiao et al. DOI: 10.1021/nl802750z).

Jiang’s team has been perfecting a technique for growing “forests” of uniform carbon nanotubes on silicon wafers, and “drawing” them out into films just a few tens of nanometres thick.  These self-supporting films of closely aligned nanotubes have plenty of commercial potential, from transparent conducting surfaces to strong materials.  So it’s hard to imagine what prompted the research team hook one of their films up to an audio amplifier.

Yet at some point, some enterprising researcher took the initiative, and the music flowed…

Since the paper was posted on the Nano Letters website it has attracted a flood of web-chatter (for example, check out Michael Berger’s excellent coverage at Nanowerk, or the recent article in the Economist).  And much of it has focused on meshing the new technology with consumer electronics—phones, TV’s, laptop computers, even clothing that quite literally speaks volumes about the wearer (the nanotube films continue to work as an audio transducer when attached to fabric, and even when bent, stretched or otherwise manipulated).

But what caught my attention was the possibility here for high-end audio—not the aural mediocrity that consumers seem more than willing to put up with, but the drive to make sound production as real as… well, reality itself.

Surprisingly perhaps, nanotechnology hasn’t had much of a presence in the field of high-end audio so far—a sector that often adopts new technologies early on in the push to make ever-more exclusive, unique and audibly superior products.  The closest manufacturers have got to using nanotechnology (to my knowledge) are the rather superb 800D loudspeakers from Bowers and Wilkerson, that use micrometer-thin Chemical Vapour Deposition-formed diamond tweeters to produce some of the purest high notes possible (or so I’m told—the 800D’s are a tad above my pay grade!).

But the nanotube loudspeaker may change all that.  And here’s why:

Current loudspeaker technologies are the weak link in the audio chain—even the best loudspeakers distort the sound and add their own character to it to some degree.  And much of the problem lies in converting an electrical signal into a mechanical signal (an oscillating cone for instance), and thence into an acoustic signal.  Along the way, the purity of the sound is always compromised in some way.  And as a result, even a $20,000 pair of speakers will still have an audible influence on music heard through them.

The carbon nanotube film in contrast cuts out the mechanical stage, and makes it possible to convert an electrical signal directly into an audio signal.  Unlike any conventional loudspeaker I can think of, in a nanotube loudspeaker, there will be no moving parts.  This in itself would be a major leap forward in sound reproduction.  But there’s more.

In theory, the radiating surface of a nanotube loudspeaker could be made pretty large—meters square even—with every part of the surface generating sound in perfect coherence with every other part. This is a huge deal. It means having power and purity at the same time—something that is near-impossible with cheaper conventional loudspeakers, and barely possible with megabucks high-end models.

Add to this the electrical simplicity of the nanotube film (it presents a predominantly resistive load) and an ability to work over a very large frequency range, and you have a package this is looking very attractive, and might even have what it takes to make high quality digital speakers a practical reality—loudspeakers that take a digital signal from CD’s, DVD’s or computer audio files, and convert that signal directly into sound (without having to convert it into an analogue signal first—another weak link in the audio chain).

Even if these carbon nanotube films live up to just a fraction of their promise, this is a technology that should have any self-respecting audiophile addict in an ecstasy of anticipation.

Of course there are hurdles to overcome.  It’s unclear how quality control will affect sound quality, or how delicate the resulting transducers will be.  Some nifty electronics will also be needed to drive the nanotube loudspeakers to their maximum capability.

And then there is the question of safety.  It’s not clear whether the carbon nanotubes being used in Jiang’s lab match those that could cause health problems if inhaled, or whether there is any possibility of the films shedding potentially harmful carbon nanotubes.  But some judicious investigations on this front would seem in order—if only to assure people that listening to the finest quality high-end audio systems in the future will not end up being a terminally exhilarating experience.

There is also the cost of the carbon nanotubes themselves—which is still on the high end of expensive.

Actually, I’m not sure this is such a barrier, judging by what people will pay for the last word in high-end audio equipment.  Take the pinnacle of the Bowers and Wilkerson loudspeaker range for example—the Nautilus loudspeaker.  A result of five years’ development with the goal being perfection—or as close as is humanly possible—a pair will set you back in excess of $60,000.  Yet people still buy them!

In this business, people aren’t going to be phased by paying a few thousand dollars extra in the quest for audio nirvana.  Which is why I think that high-end loudspeaker manufacturers will be taking this new technology seriously—and most likely will be paving the way for more accessible carbon nanotube audio products that really do rock—figuratively as well as literally.

And as a last word, if you are one of the millions who couldn’t care a jot about high-end audio, check out Jiang’s nanotube-enabled musical flag below. What it lacks in quality, it certainly makes up for in novelty!

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Technical stuff (added Nov 27 2008)

For the technically minded, Jiang’s team aren’t entirely certain how the nanotube film works as a loudspeaker, but they are pretty sure it is due to the thermoacoustic effect, where the audio signal leads to rapid heating ad cooling of the nanotubes, leading to the formation of soundwaves (Michael Berger’s Nanowerk article includes useful background on the effect, and an animation of what is most likely going on in the nanotube speaker can be found in the Jiang paper supplemental material).  The result is that electrical signals are converted directly into soundwaves, simultaneously at each point on the film, and without any moving parts.  And it is this that makes the nanotube film so robust, versatile, and potentially high quality.

One downside of the thermoacoustic effect is that the frequency in a normal alternating current signal  is doubled when converted into sound, leading to a rather disconcerting shift in pitch in what is heard.  The nanotubes are heated just as much when the incoming signal goes negative as when it goes positive, leading to the doubling.  The trick Jiang and colleagues used to overcome this was to bias the electrical signal, so it was always positive (essentially making this a “class A” speaker).

Intriguingly, this tendency to double the incoming frequency shouldn’t be a problem if the nanotube film is used as a digital speaker.  The idea here is that a stream of digital pulses is fed directly into the loudspeaker, and the conversion of the signal from digital to analogue starts as the electrcal input is transformed into an acoustic output.  Because the pulses in a digital stream are all of the same polarity (the signal is either on or off, rather than varying from positive to negaitive), there will be no frequency doubling.  And the frequency response of the nanotube film is wide enough to convert the pulses of a digitally sampled audio track into sound waves that will be interpreted as music or speech by the human ear.

Just one niggling question – will feeding a digital signal directly into the nanotube film cause a melt-down through excessive heating?  It’s a possibility, but it would be a great experiment to try!

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1 martin Winsemius December 17, 2008 at 4:12 pm

Your suggestion that thin film speakers could play digital signals directly demonstrates a misunderstanding of how digital audio signals are encoded and decoded. What you suggest could only possibly work from Pulse Width Modulated signals as are used in motor controllers. Audio is not encoded in PWM because it can not extract bit rate clock and also creates unwanted DC shifts during transmission.

Therefore audio encoding is done in a manner that allows clock extraction and compensates for DC shifts in the signal as transmitted. AES for broadcasters and SPDIF for consumers are the encoding schemes used. Both are a special form of Pulse Code Modulation that can not be directly filtered for play through any analog amplifier or speaker. PCM must be processed through a special Digital Signal Processor circuit for conversion to an analog form (or digital-pulses-filtered-to-analog signals such as Class D, I, T) that can drive a loudspeaker.

2 Andrew Maynard December 18, 2008 at 3:06 am

Martin,

Thanks for the clarification – in my haste I over-simplified! Of course, a typical 8-bit digital signal (or a compressed/encoded signal from a DVD/MP3 player etc) would not work here without conversion. As you point out, you would either need to convert this to a Pulse Width Modulated signal – or possibly a high sampling rate 1-bit bitstream.

But because you are still using a digital signal, it is in principle possible for lossless conversion in the digital domain from AES or SPDIF to something that can be amplified (very efficiently) and fed direct to the loudspeaker–with the D/A conversion occurring directly within the transducer.

3 Mak ajanovic February 21, 2012 at 4:21 pm

like Andrew said, you could use digital directly, especially if it were SACD 1bit sampling that DSD employs. That would be the most ideal thing. I want my nanotubes speakers now dammit! When are they gonna come out? At least give us some sexy nanotube headphones, although the heating might heat up your ears, but you wouldn’t need a lot of power so it might not be a problem.

4 Mike Barnum December 21, 2012 at 1:39 pm

Actually JVC makes a number of carbon nanotube headphones.

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