Graphene Enables Flat Speakers for Mobile Audio Systems

Realizing the promise of over a century of thermoacoustic-based speakers, a simple process means the technology could find its way into portable speakers

2 min read
Graphene speaker
Photo: Choong Sun Kim/KAIST

Nanomaterials have been responsible for all sorts of innovation of audio speaker designs. We’ve seen magnetic nanoparticles used to eliminate the need for a magnet in speakers.  Carbon nanotubes have also been demonstrated to produce sound with heat. While these designs have been innovative, they were developed to demonstrate the capabilities of nanomaterials rather than to produce a piece of audio equipment.

Now, researchers at Korea Advanced Institute of Science and Technology (KAIST) have developed a new speaker design specifically targeted for the mobile audio market that draws its capabilities from nanomaterials. The KAIST researchers have used graphene to produce a speaker that does not require an acoustic box to produce sound.

In research described in the journal ACS Applied Materials & Interfaces, the researchers used graphene in a relatively simple process that yielded the long-elusive thermoacoustic speaker. Thermoacoustics is based on the century-old idea that sound can be produced by the rapid heating and cooling of a material instead of through vibrations.

While graphene has previously been shown to enable thermoacoustics (and carbon nanotubes have even been used previously to create thermoacoustic speakers), what sets the KAIST researchers’ work apart is the ease with which the graphene-based speakers are fabricated. The simple, two-step process, they say, will make commercial applications more likely. 

They started by first freeze-drying a solution of graphene oxide flakes. They then reduced and doped the oxidized graphene to improve its electrical properties. (The process does not require any templates to complete the fabrication.) The end result: an N-doped, three-dimensional, reduced graphene oxide aerogel (N-rGOA) that is freestanding.

The final aerogel sound element has a porous macroscopic structure can be easily modulated. The speaker the KAIST researchers produced consists of an array of 16 of the aerogels; it operates on 40 watts of power and produce a sound quality comparable to that of other graphene-based sound systems.

The researchers believe that because of the simplicity of their fabrication method, speakers can be mass-produced for use in mobile devices and other applications. As you can see in the video below, the fact that speakers are flat and don’t vibrate means that can be placed against walls and even curved surfaces.

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Now that recorded sound has become ubiquitous, we hardly think about it. From our smartphones, smart speakers, TVs, radios, disc players, and car sound systems, it’s an enduring and enjoyable presence in our lives. In 2017, a survey by the polling firm Nielsen suggested that some 90 percent of the U.S. population listens to music regularly and that, on average, they do so 32 hours per week.

Behind this free-flowing pleasure are enormous industries applying technology to the long-standing goal of reproducing sound with the greatest possible realism. From Edison’s phonograph and the horn speakers of the 1880s, successive generations of engineers in pursuit of this ideal invented and exploited countless technologies: triode vacuum tubes, dynamic loudspeakers, magnetic phonograph cartridges, solid-state amplifier circuits in scores of different topologies, electrostatic speakers, optical discs, stereo, and surround sound. And over the past five decades, digital technologies, like audio compression and streaming, have transformed the music industry.

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