A web of silence

Materials World magazine
1 Dec 2017

Ellis Davies reports on a new spider-web inspired sound control method using acoustic metamaterials. 

According to the European Environment Agency, 20% of Europe's population is exposed to noise levels deemed unacceptable – above 55dB – by health experts because of road traffic. This results in around 8 million people suffering from sleep disturbance because of ambient noise, which can lead to hospitalisation and even premature death from hypertension or cardiovascular diseases. A sound control structure made of aluminium, developed by a collaboration of researchers from the Universities of Torino and Trento, Italy, Le Havre, France, Queen Mary University of London, UK, and the Italian Space Agency, could help to minimise these effects using the structure of a spider’s web. 

Low frequency sounds, present in road traffic, are characterised by long wavelengths. To shield these acoustic waves, thick and heavy expensive structures are often needed, which can be impractical for many applications. The metamaterial structures developed by the team are very thin, but can still effectively reduce and manipulate low-frequency sounds. 

An acoustic metamaterial is an artificially engineered periodic assembly of repeating patterns with a structure that allows the manipulation of acoustic waves, and can therefore attenuate or reflect sound. The study uses labyrinthine acoustic metamaterials, which coil up waves into channels of much smaller width/size than that of the wavelength of the waves. 

The materials needs to have a large impedance (opposition to acoustic flow) mismatch with air, which requires a material with a density and speed of sound that differ greatly from those in air. ‘This allows most of the wave energy to be reflected from the sidewalls and eliminates air-structure interaction,’ explained Nicola Pugno, Professor of Solid and Structural Mechanics at the University of Trento and part-time Professor of Materials Science at the Queen Mary University of London. ‘Since a large variety of engineering materials satisfy this requirement, one of the benefits of labyrinthine metamaterials is that they can be made from a large variety of materials.’

The development is a variation on labyrinthine metamaterials structures –  made by assembling long and relatively thin curved panels of the desired material to create labyrinthine channels throughout the structure – studied previously by the group. In the latest study, the team proposed the introduction of a square frame around the circular channels, similar to the structure of a natural spider web. This geometry enables additional control over wave dynamics and adds tunability to the structural attenuation performance.

'The interplay of attenuation effects near the walls, and the curved geometry of the channels, causes wave paths to extend compared to non-curved channels, and the amplitude of sound significantly decreases. Therefore, unwanted noise can be substantially reduced,’ Pugno said.  

Attenuating characteristics can also be tuned to a desired operating frequency range. The labyrinthine channels can be designed with varied width and length, or the edge cavities with different sizes, which will calibrate the operating frequencies to desired values and the overall performance. 

The main application of labyrinthine metamaterials is noise reduction, but they can also be used for sound wave guiding and for amplifying sound by enhanced reflection in open-air theatres or large concert halls.