Making the perfect sound absorbers using metamaterials

26 September 2022

Metamaterials are starting to live up to their promise with customisable sound absorbers making their way from theoretical principles to commercial products.

Metamaterials are engineered to have properties not found in naturally occurring materials. They usually do this by manipulating waves – like the electromagnetic waves that constitute light, or acoustic waves – in ways that normal materials cannot.

This has led to some incredible conceptual examples, including the ‘perfect lens’ and ‘invisibility cloaks’ that manipulate light in new ways, but these have not necessarily translated into useful products.

Work in the lab of Professor Ping Sheng from the Hong Kong University of Science and Technology, however, has made its way into the real world. From theoretical considerations of how to create the perfect sound absorbers, his students have spun out a company that now produces custom solutions for applications ranging from reducing ventilator noise to the world’s first metamaterial high-fidelity speaker.

The problem with conventional materials

For each circumstance where reducing sound is desired, there will be a different sound profile, or a spectrum of acoustic wavelengths. For conventional sound-absorbing materials, like foams or mineral wools, the absorption spectrum can only be adjusted by varying the thickness of the material. For low-frequency sounds, this can result in bulky materials.

But Sheng wondered: what is the absolute minimum thickness a material can be and still absorb the required sound? This is the ‘causality constraint’, and was the start of Sheng devising a new principle for acoustic metamaterial design.

While normal sound-absorbing materials get their properties from their composition, metamaterials get their properties from structure, meaning geometry is key. This can be achieved by designing structures like crystal types not found in nature, or by combining unconventional materials.

This had led to several new innovations in sound absorption, but most had been ‘narrow-band’, only absorbing a narrow spectrum of sound. This limited their usefulness.

Designing optimal metamaterials

Armed with the causality constraint, Sheng wanted to determine how to design ‘optimal’ acoustic metamaterials that absorbed the broadest band of sound in the minimum thickness.

“You can think of the thickness as an absorption ‘resource’,” says Sheng. “Once you know how much thickness resource you have to play with, the next question is what is the best way to allocate that thickness? One approach is to optimise the metamaterial design so the absorption spectrum matches the noise spectrum.”

There are several things to consider, including how many resonators are needed and in what arrangement to absorb the required spectrum and create the desired impedance. Sheng found that a mathematical ‘circle of consistency’ could be created to link all these parameters and optimise the design of the metamaterial.

In 2017, Sheng and colleagues published a paper in the journal Materials Horizons that demonstrated this principle and proved its effectiveness with several optimised metamaterials. Their first example combined 16 resonators, precisely spaced in accordance to designed resonance frequencies and folded to create a compact metamaterial that was 10.86 cm thick – incredibly close to the 10.36 cm minimal limit prescribed by the ‘causality constraint’. Above a frequency of around 400 Hz, up to a tested limit of 3000 Hz, the metamaterial had near-perfect absorption.

The team also demonstrated it’s possible to open a ‘window’ of sound, where certain frequencies are admitted rather than absorbed if needed. The design principle worked – metamaterials could be created for each sound absorption profile required.

Out of the lab

The principle proven, a team of Sheng’s students took on the challenge of creating a commercial product. They founded a company – Acoustics Metamaterials Group (AMG) – and have scaled up manufacturing while reducing costs.

One of their first applications was to reduce fan noise in ventilation ducts, providing sound absorption for the inner walls. The metamaterial performed much better than the conventional material that was previously used, cutting the noise by 26 decibels, as compared to 11 decibels for the conventional materials.

The group have even partnered with KEF to create the world’s first speakers incorporating a metamaterial, which absorbs 99% of the unwanted sound from the rear of the driver, eliminating distortion and providing purer sound.

And this is only the beginning. Sheng says: “Metamaterials are entering an exciting period right now as they transition from the lab to commercial applications. There are a small but growing number of companies that are allowing ideas from the lab to be tested out in real-world settings, and I can only see it expanding from here.”

One area Sheng is keen to expand into is underwater acoustic metamaterials. With French collaborators, he has recently published a paper that advances the idea of creating metamaterials made of mosaic pieces, to better match the impedance of water. “Underwater acoustics are less well studied, but we shouldn’t underestimate the importance of this area,” says Sheng. “After all, most of the area of the Earth is underwater.”