Make Some Noise for Graphene Aerogel Speakers

By Russ Swan on at

Audio is one of the hot battlegrounds in mobile technology, but no matter how good speakers have become they still operate on the century-old concept of making sound by moving a diaphragm to push air molecules around.

Manufacturers have been searching for ways to make a truly solid-state speaker good enough to power the next generation of devices, and they may have just found it with a mash-up of graphene and aerogel science.

Graphene is formed of sheets of pure carbon with a thickness of just one atom; it is the thinnest material in the known universe and holds much promise for future tech from wearable electronics to nano-sized drug-delivery systems. Aerogels, in contrast, are the lowest density solid materials ever made, so tenuous they barely weigh anything, and are sometimes called 'frozen smoke'.

Now scientists in Korea have cooked graphene sheets into an aerogel, and say they have solved the main problems in making thin, flat speakers that perform well and can be mass produced.

Their N-doped reduced graphene oxide aerogel, or N-rGOA, loudspeakers are tiny, less than a millimetre thick, and produce 'outstanding acoustic performance' with a much higher sound pressure level (SPL, or volume to you and me) for the power than any conventional speaker. They will be known as 'nargo' speakers. You heard it here first.

These devices create sound by exploiting the thermoacoustic effect, instead of a vibrating membrane. The driving signal is converted by the graphene aerogel into pulses of heat, causing the air molecules near the surface to expand and propagate into the surrounding space.

The resulting no-moving-parts design offers one huge advantage over conventional speakers – it needs no soundbox. The speakers can be attached to practically any surface, rigid or soft, flat or curved, and generate omnidirectional sound. A thin backing sheet of alumina exploits a neat reflection trick of constructive interference to double the volume of sound produced – it's a little like holding a candle in front of a mirror.

Two-stage manufacturing process cooks graphene into a 3D aerogel and can be scaled to industrial production (bottom right, scanning electron micrograph of the aerogel structure)

The entire assembly is just 0.75 millimetres thick, with prototype speaker panels measuring about a centimetre square.

A key factor is the speakers' ability to handle power – although they respond to low input voltages, they can also handle up to 40W of power. That would do serious ear damage if you held it close to your lug'oles.

Describing the invention in the American Chemical Society's journal Applied Materials and Interfaces [http://pubs.acs.org/doi/abs/10.1021/acsami.6b03618], the scientists from the Korea Advanced Institute of Science and Technology (KAIST) led by Choong Sun Kim, claim that although other graphene-based speakers have been built, including a couple using aerogel, these are the first to be suitable for mass production.

Their two-stage manufacturing process involves freeze-drying and chemical reduction, which is much cheaper than chemical vapour deposition and produces better results than solvothermal methods, they say.