Metamaterials are defined as artificial materials with properties that do not exist in nature; these properties are due to structure and not material composition. The concept of metamaterials with negative properties in an electromagnetic wave was first proposed in the early 20th century but remained largely unexplored until recently. Similarly, the counterpart of electromagnetic metamaterials; i.e. acoustic metamaterials, have remained in a stage of infancy. There has been many different classes of acoustic metamaterials including locally resonant membrane-type, periodic resonators/scatters or sonic crystal type. These materials have shown unusual acoustic properties such as negative effective mass, negative modulus, bandgap, acoustic trapping and tunable band. Vigorous research has yielded membrane type acoustic metamaterials. These materials have broadened the realm of elastic wave characteristics achievable by photonic crystals. In a recent publication, a membrane-type acoustic metamaterial (MAM) model was reported to allow for a thinner structure. Technically, the reported MAM was formed by an elastic membrane fixed by a rigid frame, with a small weight at the center of the membrane for tuning the resonances.
Generally, membrane-type acoustic metamaterials are novel, lightweight and compact materials that can be used to isolate the sound at low frequencies beyond the limits of conventional materials. MAMs open up many possibilities for creating a new generation of acoustic materials and sensor devices. Studies on the eccentricity of the attached mass and ring masses have reveal that additional research is still required to achieve better acoustic performance at the low-frequency range with a wider frequency band. In light of this, researchers from the National University of Singapore: Dr. Zhenbo Lu, Dr. Siu-Kit Lau and Dr. Boo Cheong Khoo, in collaboration with Dr. Xiang Yu and Dr. Fangsen Cui at the Institute of High Performance Computing at A*STAR employed finite element simulations using multi-physics commercial software COMSOL with experimental validation to explore the acoustic-structural interaction of MAM with eccentric masses. Their work is currently published in the research journal, Applied Acoustics.
The research team sat the scheme to optimize the distribution of eccentric masses (such as thickness, weight, shape, the split number, the size of the ‘pocket’ and the size of the ring mass) to improve acoustic performance. It was anticipated that the mass added onto the membrane could change the membrane surface density, constrain membrane displacement and induce new anti-resonance modes. Overall, the gap between the splits, called a ‘pocket’, was examined to further explore the membrane-acoustic interactions in order to obtain a better understanding of the physical mechanism.
The authors reported that the acoustic performance of the MAM can be optimized through the optimization of the eccentric ‘masses’ distribution such as thicknesses, weight, shape, the number of splits, the size of the ‘pocket’ and the size of the ring mass. Additionally, it was observed that at high frequencies (200 Hz to 1000 Hz), a thicker membrane provided better acoustic performance, which was also based on the mass-law, while in the lower frequency range, (40 Hz to 300 Hz), the resonance peaks could only be changed through various mass configurations.
In summary, the study outlined the development of a finite element simulation model for the acoustic-structural interaction of the MAM with eccentric masses using multi-physics commercial software COMSOL. Remarkably, the study identified 4-split ring mass with the outer radius of 60 mm and the inner radius of 50 mm as the best mass configuration that could significantly increase the 5 dB-TL band by 1325% compared to a single membrane. In a statement to Advances in Engineering, Dr. Siu-Kit Lau, corresponding author, highlighted that their proposed structure could potentially be applied for low-frequency noise insulation, for example, as the outer layer of the windows in a building to prevent environmental low frequency noise propagating into the room – among other uses.
Zhenbo Lu, Xiang Yu, Siu-Kit Lau, Boo Cheong Khoo, Fangsen Cui. Membrane-type acoustic metamaterial with eccentric masses for broadband sound isolation. Applied Acoustics, volume 157 (2020) 107003