The history of using nanomaterials such as magnetic nanoparticles or carbon nanotubes in audio speakers has mainly been to demonstrate the capabilities of these materials rather than to yield speakers that will actually be listened to. That changed last year when South Korean researchers used graphene to produce a speaker that does not require an acoustic box to produce sound.
Now researchers from the University of Exeter in the UK are turning again to graphene to make a speaker that produces sound thermoacoustically. Instead of depending on vibrations of a material inside of an acoustic box, thermoacoustics leverages a century-old idea that sound can be produced by the rapid heating and cooling of a material. Where the Exeter researchers’ work departs from that of their Korean counterparts is that this newest device serves not only as a speaker, but also as an amplifier and graphic equalizer—all on a thumbnail-size chip.
In research described in the journal Scientific Reports, the Exeter scientists were able to demonstrate that when graphene is rapidly heated and cooled by an alternating electric current, it transfers those thermal variations to the surrounding air. The air expands and contracts, thereby generating sound waves. The key to the device’s multipurpose capability is controlling the electrical current going into the graphene.
“Thermoacoustics (conversion of heat into sound) has been overlooked because it is regarded as such an inefficient process that it has no practical applications,” explained David Horsell, a senior lecturer at Exeter and lead author of the paper, in a press release. “We looked instead at the way the sound is actually produced and found that by controlling the electrical current through the graphene we could not only produce sound but could change its volume and specify how each frequency component is amplified. Such amplification and control opens up a range of real-world applications we had not envisaged.”
While mobile devices are the first applications to come to mind for such a device, other uses are being considered. Because graphene is almost completely transparent, the researchers envision that this technology could be used to transmit both pictures and sound. Because of this capability, the researchers believe that it could be used for ultrasound imaging in medical applications.
Based on graphene’s strength and flexibility, the Exeter scientists believe that an ultrasound device enabled with this graphene chip could offer better imaging because of better contact with the patient. Also, because the device is relatively cheap to produce, the Exeter team believes that it could someday be used in real-time patient monitoring applications such as intelligent bandages.
Until these more speculative applications come to fruition, Horsell sees a real and immediate use for the technology in the telecommunications industry. Noting the technology’s frequency mixing capability, Horsell added, “The most exciting thing is that it does this trick of multiplication in a remarkably simple and controllable way. This could have a real impact in the telecommunications industry, which needs to combine signals this way but currently uses rather complex and, therefore, costly methods to do so.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.