In modern science, it is possible to engineer materials to have properties not found in naturally occurring materials. Innovations such as graphene and aerogel are examples of these, commonly known as ‘metamaterials’. In the field of building acoustics, material properties and geometry are utilised to control how sound behaves in and between rooms and buildings. One example of this is diffusers, which are employed to ‘even out’ reverberation in different locations in a space, by spreading sound energy evenly throughout the space. Since the 1970s, diffusers have remained largely unchanged, consisting of panels either with discrete troughs or wells of varying depths (such as Schroeder diffusers), or with contiguous but varying surface height. The resultant varying surface features of these panels mean sound waves hitting them are scattered away from the surface over a range of directions, rather than being directly reflected all in one direction. This results in a more uniform distribution of sound energy in a space, reducing unwanted acoustic effects such as room modes and flutter echo.
The current design of diffusers provides limitations on their use. To operate at lower frequencies, such as that of air traffic noise, some can be up to 700mm thick. This limits where the diffusers can be installed, increases their weight and material cost, and despite attempts to produce creative and visually appealing designs, diffusers can be difficult to incorporate into the aesthetic of some spaces. To remedy these issues, research has been conducted at the Collaborative Innovation Centre of Advanced Microstructures at Nanjing University in China into the use of metamaterials as diffusers. These so-called ‘metasurfaces’ have comparable performance to traditional diffusers, despite being up to an order of magnitude thinner. Where most Schroeder diffusers use wells, they instead utilise locally resonant ‘cells’ in the form of parallelepipeds (a ‘cube’ made of six parallelograms) with square openings in the top. The acoustic response of these cells can be tuned by changing the size of the opening. Such devices have been prototyped for testing using 3D plastic printing.
It is hoped that this new type of diffuser can soon be commercialised, providing more flexibility to designers, suppliers and their clients and also paving the way for increased future utilisation of metamaterials to improve building acoustics.