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John O | May 2017

New method uses heat not vibrations to get graphene to make sounds


Researchers at the University of Exeter have made a breakthrough using graphene that could have wide-ranging effects on the audio and telecommunications industries, according to a report on the university’s website.

 


Researchers have combined speaker, amplifier and graphic equaliser into a chip the size
of a thumbnail. (University of Exeter)

 

The researchers combined a speaker, amplifier, and graphic equalizer into a chip the size of a thumbnail and, rather than using the mechanical process that typically creates sound, a thin layer of graphene was rapidly heated and cooled to produce sound waves.

 

The report explained, “Traditional speakers mechanically vibrate to produce sound, with a moving coil or membrane pushing the air around it back and forth. It is a bulky technology that has hardly changed in more than a century. This innovative new technique involves no moving parts. A layer of the atomically thin material graphene is rapidly heated and cooled by an alternating electric current, and transfer of this thermal variation to the air causes it to expand and contract, thereby generating sound waves.”

 

The team from Exeter became the first to show that heat not only can produce sound (thermoacoustics is not a new field), but that this process can also allow the mixing, amplification and equalizing of sound frequencies in the same small device.

 

“With graphene being almost completely transparent, the ability to produce complex sounds without physical movement could open up a new golden generation of audio-visual technologies, including mobile phone screens that transmit both pictures and sound,” the article continued.

 

Even the researchers were surpeised at the control that they were able to exert over the sound frequencies and it has opened up a number of applications that had yet to be considered, including ultrasound imaging.

 

“The known high strength and flexibility of graphene would allow intimate surface contact leading to much better imaging,” the article added. “Moreover, the fact that the acoustic devices the Exeter team has devised are simple and cheap make such concepts as intelligent bandages that monitor and treat patients directly a real possibility.”

 

As a feature from New Atlas explained, graphene in speakers is not a new concept, but typically the material has been used in the membrane that vibrates and produces sound.

 

“How good would a non-vibrating speaker sound?” the article asked. “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.”

 

The research was recently published in Scientific Reports. The abstract stated:

 

“The ability to generate, amplify, mix and modulate sound in one simple electronic device would open up a new world in acoustics. Here we show how to build such a device. It generates sound thermoacoustically by Joule heating in graphene.

 

A rich sonic palette is created by controlling the composition and flow of the electric current through the graphene. This includes frequency mixing (heterodyning), which results exclusively from the Joule mechanism. It also includes shaping of the sound spectrum by a dc current and modulating its amplitude with a transistor gate. We show that particular sounds are indicators of nonlinearity and can be used to quantify nonlinear contributions to the conduction.

 

“From our work, we expect to see novel uses of acoustics in metrology, sensing and signal processing. Together with the optical qualities of graphene, its acoustic capabilities should inspire the development of the first combined audio-visual nanotechnologies.”

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