Researchers from the Korea Advanced Institute of Science and Technology (KAIST) have found a way to mass produce ultra-thin graphene thermoacoustic speakers.

Thermoacoustic speakers generate sound waves from temperature fluctuations by rapidly heating and cooling conducting materials. Unlike conventional voice-coil speakers, thermoacoustic speakers do not rely on vibrations to produce sound, and thus do not need bulky acoustic boxes to keep complicated mechanical parts for sound production. They also generate good quality sound in all directions, enabling them to be placed on any surface including curved ones without cancelling out sounds generated from opposite sides.

The KAIST team, led by Professors Jung-Woo Choi and Byung Jin Cho of the School of Electrical Engineering and Professor Sang Ouk Kim of the Material Science and Engineering Department, published their research results in the journal Applied Materials & Interfaces.

KAIST thermoacoustic speaker 01 Figure 1: A thermoacoustic loudspeaker consisted of an array of 16 3D graphene aerogels. (Source: KAIST)

Based on a two-step, template-free fabrication method that involved freeze-drying a solution of graphene oxide flakes and the reduction/doping of oxidised graphene to improve electrical properties, the research team produced a N-doped, three-dimensional (3D), reduced graphene oxide aerogel (N-rGOA) with a porous macroscopic structure that permitted easy modulation for many potential applications.

KAIST thermoacoustic speaker 02 Figure 2: Two-step fabrication process of 3D reduced graphene oxide aerogel using freeze-drying and reduction/doping. (Source: KAIST)

Using 3D graphene aerogels, the team succeeded in fabricating an array of loudspeakers that were able to withstand over 40W input power and that showed excellent sound pressure level (SPL), comparable to those of previously reported 2D and 3D graphene loudspeakers.

"Thermoacoustic speakers have a higher efficiency when conducting materials have a smaller heat capacity. Nanomaterials such as graphene are an ideal candidate for conductors, but they require a substrate to support their extremely thinness. The substrate’s tendency to lose heat lowers the speakers’ efficiency," said Choong Sun Kim, the lead author of the research paper and a doctoral student in the School of Electrical Engineering at KAIST.