Development of Acoustic Metamaterial for noise mitigation in ducts: prototype, testing and optimisation

Metamaterials help designing systems of which the physical properties belong to the geometrical configuration of the structures rather than the constitutive material and recently they have become very interested in many fields of engineering. So far, they have shown to have interesting characteristics for applications in the field of applied acoustics related to building and mechanical engineering. Specifically in recent years metamaterials have been exploited for multiphysical applications combining acoustics and fluid dynamics. However, at the moment it is still not clear how multiphysical domains interact. This study investigates the possible application of AMMs for the attenuation of noise inside the ventilation ducts through a multiphysics approach. This has been possible thanks to a preliminary numerical study of the prototypes through FEM and experimental analysis. A cylindrical duct with a diameter of 100 mm was studied considering turbulences with a parametric study (Ma=0-0.05) and sound waves propagation. Finally, the two physics where combined and two metamaterials-based ducts were optimised. Results show how a moving fluid can affect the propagation of sound within the duct with or without the metamaterial attenuators (one resonator in series, one resonator in parallel and three different coiled-up resonators). At the same time, the two optimised models were tested also experimentally with an impedance tube and Insertion Loss (IL) was compared with the numerical results. Overall a good matching between experimental and numerical method was proved for the monophysical acoustic study. On the other side a significant opening ratio (48\%) proved a higher suitability for reducing pressure drops, while attenuating the ventilation noise (IL=4-10 dB on 125-1600 Hz). These preliminary results could be applied to many flow and frequency range case studies and implemented for Mechanical and Civil Engineering applications.