materials Article Pneumatically-Actuated Acoustic Metamaterials Based on Helmholtz Resonators Reza Hedayati * and Sandhya Lakshmanan Novel Aerospace Materials group, Faculty of Aerospace Engineering, Delft University of Technology (TU Delft), Kluyverweg 1, 2629 HS Delft, The Netherlands; [email protected] * Correspondence: [email protected] or [email protected] Received: 24 February 2020; Accepted: 19 March 2020; Published: 23 March 2020 Abstract: Metamaterials are periodic structures which offer physical properties not found in nature. Particularly, acoustic metamaterials can manipulate sound and elastic waves both spatially and spectrally in unpreceded ways. Acoustic metamaterials can generate arbitrary acoustic bandgaps by scattering sound waves, which is a superior property for insulation properties. In this study, one dimension of the resonators (depth of cavity) was altered by means of a pneumatic actuation system. To this end, metamaterial slabs were additively manufactured and connected to a proportional pressure control unit. The noise reduction performance of active acoustic metamaterials in closed- and open-space configurations was measured in different control conditions. The pneumatic actuation system was used to vary the pressure behind pistons inside each cell of the metamaterial, and as a result to vary the cavity depth of each unit cell. Two pressures were considered, P = 0.05 bar, which led to higher depth of the cavities, and P = 0.15 bar, which resulted in lower depth of cavities. The results showed that by changing the pressure from P = 0.05 (high cavity depth) to P = 0.15 (low cavity depth), the acoustic bandgap can be shifted from a frequency band of 150–350 Hz to a frequency band of 300–600 Hz. The pneumatically-actuated acoustical metamaterial gave a peak attenuation of 20 dB (at 500 Hz) in the closed system and 15 dB (at 500 Hz) in the open system. A step forward would be to tune different unit cells of the metamaterial with different pressure levels (and therefore different cavity depths) in order to target a broader range of frequencies. Keywords: active noise control; acoustic metamaterial; broadband noise attenuation; pneumatic actuation; Helmholtz resonators 1. Introduction Metamaterials are periodic designer materials which offer physical properties not found in nature [1]. Particularly, acoustic metamaterials can manipulate sound and elastic waves both spatially and spectrally in unpreceded ways [2]. Such properties include super-focusing [3], super-lensing [4], active membrane structures [5,6], cloaking [7,8], phononic plates [9], fluid cavities separated by piezoelectric boundaries [10], and tunable noise attenuation based on Helmholtz resonators [11–15]. The capability of metamaterials to tune their physical behavior just based on their geometrical characteristics offers a great benefit over conventional materials for application in various high-demand industries such as aerospace, automobile, and construction. Increase in importance of customer comfort in transportation and construction sectors as well as consumer goods has been a driving force for recent advancements in acoustic metamaterials in the field of noise control. Acoustic metamaterials can generate arbitrary acoustic bandgaps by scattering sound waves, which is a superior property for insulation properties. Initially, the acoustic metamaterials were passive; their performance remained unchanged and it was in accordance to the design values [16–18]. More recently, the concept of tunable acoustic Materials 2020, 13, 1456; doi:10.3390/ma13061456 www.mdpi.com/journal/materials Materials 2020, 13, 1456 2 of 11 Materialsmetamaterials 2020, 13, x FOR has PEER emerged REVIEW in which acoustic metamaterials are able to manipulate sound2 waves of 11 in various conditions [6–8,19–21]. Active acoustic metamaterials can be responsive to different stimuli, varioussuch conditions as electric [6–8,19–21]. field [21], electromagnetic Active acoustic field meta [6,materials22], mechanical can be loadresponsive [11], and to electricitydifferent stimuli, by means of suchpiezoelectric as electric field elements [21], electromagnetic [7]. field [6,22], mechanical load [11], and electricity by means of piezoelectricAcoustic elements metamaterials [7]. are widely preferred in applications where high efficiency in noise reductionAcoustic metamaterials is expected. 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