Membrane-type acoustic metamaterials have been studied extensively over the past decade [1
] because they could exhibit good acoustical performance at below 1 kHz while being compact and lightweight. To date, the literature provides many different designs—ranging from simple to complex constructions—that may be considered as a benchmark for further development. This class of acoustic metamaterial is usually realised by adhering the edges of a pretensioned membrane to a rigid frame. To pretension the membrane, a mechanism could be designed to stretch the membrane edges uniformly over a prescribed distance [1
]. Alternatively, epoxy could be applied along the membrane edges in which stretching is achieved by thermal curing [4
]. Evidently, to produce the specimen in large quantities, the assembly process would likely require a huge amount of time and effort. This issue may be addressed by considering either a honeycomb panel [13
] or a perforated panel [14
] as the frame. The acoustical performance is characterised solely by the resonant behaviour of the membrane in which the frequency bandwidths of good acoustical performance are achieved at around the anti-resonance frequencies, while the frequency bandwidths of poor acoustical performance are obtained at around the resonance frequencies. By varying the dimensions, the material properties, and the pretension of the membrane, the frequency bandwidths of good acoustical performance could be adjusted to coincide with the frequency range of the unwanted noise. Consequently, noise control is achieved. The membrane could also be attached with a highly dense platelet to introduce additional tuning parameters. The list of tuning parameters may be extended. For example, an additional support structure could be implemented to suppress specific eigenmodes of the membrane [8
], or a perforated membrane could be considered to introduce the effect of a Helmholtz resonator [2
Knowing that membrane-type acoustic metamaterials could potentially be considered in the industry for noise control, it is imperative that the finalised small-scale designs are extendable. Of course, it would be ideal if the large-scale design could exhibit the same acoustical performance achieved during the small-scale study. Considering how typical designs are realised, manufacturing challenges may be faced during the scaling process. These challenges, for example, include stress relaxation and stress uniformity of the membrane, and spatial consistency of the platelet on the membrane. Moreover, if numerous platelets were considered, the scaling process would be laborious. Huang et al. [15
] reported that the challenges could be faced even for small-scale designs. Hence, avenues should be explored to address the challenges and improve the scalability of membrane-type acoustic metamaterials.
Recently, numerous works [16
] have shown that a plastic film can be considered in place of the elastic membrane to eliminate the challenges pertaining to stress relaxation and stress uniformity. Since there is negligible pretension in the film, the membrane-type acoustic metamaterial is transformed into another class of acoustic metamaterial known as the plate-type acoustic metamaterial. Despite this transformation, the acoustical performance is still characterised solely by the resonant behaviour of the film. Again, if a platelet was considered to modify the resonant behaviour, the challenge relating to spatial consistency would remain unresolved. To address this challenge, Ang et al. [21
] proposed that the design could adopt a double-leaf configuration with an internal tonraum resonator (i.e., two cavities connected via an orifice) so that the objective of having more tuning parameters could still be met without attaching platelets.
Over the past five years, researchers [1
] have shown growing interest in large-scale membrane-type acoustic metamaterials despite the aforementioned challenges. However, large-scale plate-type acoustic metamaterials have yet to be studied extensively, evident from the literature in which limited works [20
] can be found. Varanasi et al. [20
] proposed a large-scale design (1.22 × 1.22 m2
) that had close resemblance to a plate-type acoustic metamaterial. The design was fabricated from a continuous acrylic panel in which the unit cells were realised by varying the thickness at different regions of the panel. Although numerous configurations were considered, none had dissimilar unit cells on the same panel to study whether they could collectively contribute to the acoustical performance of the panel. Ang et al. [26
] filled in the research gap with a large-scale design (994 × 994 mm2
) that adopted the concept of modularity for customisable acoustical performance. The proof-of-concept design—also referred to as the meta-panel—was realised by slotting specimens with two different configurations (without and with resonator) into the periodic cutouts on a host structure. By varying the distribution percentage of the specimens on the host structure, the acoustical performance of the meta-panel was shown to be collectively contributed by the acoustical performance of each specimen configuration. Additionally, the acoustical performance of the meta-panel was found to be insensitive to how the specimens were oriented or arranged on the host structure. The findings suggest that the meta-panel could potentially be considered in the industry for noise control.
This study aims to justify whether the meta-panel could indeed be viable for noise control in the industry by investigating two selected meta-panel configurations reported previously [26
] in greater detail. This investigation compares the acoustical performance with that of two commercially available noise barriers and evaluates the acoustical performance in three different noise environments—traffic noise, aircraft flyby noise, and construction noise. Section 2
presents the design details of the two meta-panel configurations and the experimental methods. Section 3
presents the results. Section 4
discusses how the findings could provide avenues for future research. Section 5
concludes the study based on the observations.
Naturally, being a proof-of-concept, it is no doubt that further development and optimisation are necessary to arrive at a meta-panel design suitable for the noise environment of interest. This section discusses some of the avenues for future research.
The influence of different weather conditions on the meta-panel has yet to be considered. In Singapore, the weather conditions include heavy rain, strong wind, and hot sun. If the meta-panel is considered for outdoor applications—as a noise barrier, for example—the specimens and the host structure must be designed with adequate structural integrity. A protective feature may be required to prevent the films from excessive deflection, which could alter the resonant behaviour of the films and influence the characteristic of the meta-panel. For example, the protective feature could simply be a continuous panel fastened to each side of the meta-panel to enclose the specimens. Consequently, the overall acoustical performance of the meta-panel may also be improved based on the understanding of the mass law. The protective feature could also serve to prevent surface damage of the films and dust accumulation on the films. In actual environments, the former could be caused by foreign objects, while the latter is inevitable. If both factors are not considered, the durability of the meta-panel will be affected, not to mention its reliability in noise mitigation.
The concept of modularity could provide the meta-panel with versatility and aesthetics. By having a library of uniquely designed specimens, the meta-panel could be configured such that its acoustical performance would be optimised for the noise environment of interest. The specimens could also be colour-coded to categorise them according to their acoustical performance. Since the acoustical performance of the meta-panel was previously shown [26
] to be insensitive to the distribution and the arrangement of the specimens, both factors could be varied to form a pixel art, providing aesthetics to the meta-panel.
Being a design that is highly customisable for different noise environments is no doubt advantageous especially in terms of implementation cost. As the host structure does not need to be replaced during the change in configuration, the cost incurred from the customisation should only be dependent on the cost of producing new specimens. In this study, the production cost of each specimen was approximately S$4. This amount may be reduced further if the specimens are produced in large quantities via conventional manufacturing techniques.
Lastly, as the scope of this study was limited to experimental evaluation, it is essential to develop a numerical model of the meta-panel so that the desired acoustical performance could be predicted prior to fabrication. Consequently, high computational power will be expected.
To reiterate, the purpose of this study was to justify whether the aluminium and the wooden meta-panels solely occupied by specimens with resonator could indeed be viable for noise control in the industry. By comparing the STL with that of two commercially available noise barriers, the meta-panels were shown to be superior at low frequencies (80–500 Hz). This superiority was substantiated when the STC and the OITC were compared. The meta-panels were then evaluated in three different noise environments, showing that they could provide an average noise reduction of 27.4 and 22.7 dB, respectively, at 80–400 Hz. In conclusion, the meta-panels were justified to be viable for low-frequency (80–500 Hz) noise control in the industry. Being a proof-of-concept design, there is a large room for improvement to obtain a meta-panel that is lightweight and yet has good acoustical performance at low-frequency range (<500 Hz), which is the frequency content of most problematic noises.