Traditional speakers mechanically vibrate to produce sound, with a moving coil or membrane pushing the air around it back and forth. This bulky technology has hardly changed in more than a century.
The Exeter team’s technique involves no moving parts. Instead, a layer of graphene is rapidly heated and cooled by an alternating electric current, transfer of this thermal variation to the air causes it to expand and contract generating sound waves.
Though the conversion of heat into sound is not new, the researchers are the first to show that this simple process allows sound frequencies to be mixed together, amplified and equalised - all within the same millimetre-sized device. With graphene being almost completely transparent, the ability to produce complex sounds without physical movement could open up a new generation of audio-visual technologies, including mobile phone screens that transmit both pictures and sound.
“Thermoacoustics has been overlooked because it is regarded as such an inefficient process that it has no practical applications,” Explained Dr David Horsell, a senior lecturer in the Quantum Systems and Nanomaterials Group at Exeter. “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.”
Applications the team have in mind include ultrasound imaging, for use in hospitals and other medical facilities in the future. The high strength and flexibility of graphene would allow intimate surface contact leading to much better imaging. Moreover, the fact that the acoustic devices the Exeter team have devised are simple and cheap make such concepts as intelligent bandages that monitor and treat patients directly a real possibility.
Dr Horsell added: “The frequency mixing is key to new applications. The sound generating mechanism allows us to take two or more different sound sources and multiply them together. This leads to the efficient generation of ultrasound (and infrasound).
“However, the most exciting thing is that is 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 costly methods to do so.”