Pretty much every speaker works with a membrane that physically vibrates, distorting the air in specific patterns to generate sound waves. But now, researchers at the University of Exeter have developed a speaker that doesn't need to mechanically vibrate at all. The key to this potentially ground-breaking speaker is – what else? – graphene, which is heated and cooled with carefully controlled electric currents to create sound waves.

Incredibly light, thin and strong, graphene has been showing up in speakers for a few years now, but it's usually in the form of those aforementioned membranes. Its lightness means it takes much less energy to get that surface vibrating, making for smaller and more energy efficient speakers.

But taking the moving parts out of the equation entirely could improve size and efficiency even further, to the point where the Exeter team's graphene-based speaker is a single chip no bigger than a human thumbnail. Inside that tiny device is a speaker, amplifier and graphic equalizer, all in one.

The new speaker works through thermoacoustics, which involves converting heat into sound. The process hasn't found many practical applications yet, but given graphene's laundry list of impressive properties, it's not surprising that this is the material that may start to change that. Graphene is a great electrical conductor, and by sending very specific pulses of electricity through it, the material can be quickly heated and cooled in sequence. That affects the air around the membrane, which expands and contracts to generate sound waves.

How good would a non-vibrating speaker sound? The team says the device can create "a rich sonic palette," through careful control of how and where the electricity flows through the graphene. The heating and cooling cycle also allows the speaker to mix, amplify and equalize multiple sound frequencies at the same time, which could boost its output above the range of human hearing and open up ultrasound applications.

"Thermoacoustics has been overlooked because it is regarded as such an inefficient process that it has no practical applications," says David Horsell, lead author of the study. "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."

Those applications include making ultrasound devices for medical imaging smaller, more efficient and able to return images of a higher resolution. The technique could also improve the frequency-mixing technology used for radio and phone signals.

"The sound generating mechanism allows us to take two or more different sound sources and multiply them together," says Horsell. "However, 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."

The research was published in the journal Scientific Reports.