Locally resonant metamaterial curved double wall to improve sound insulation at the ring frequency and mass-spring-mass resonance

A locally resonant metamaterial curved double wall is proposed and studied. The aim is to improve the sound insulation by introducing a metamaterial design targeting a narrow fre-quency band region associated with characteristic frequencies of curved double walls, thus enabling an overall improvement of sound insulation properties in a broader frequency range. This metamaterial is realized by introducing periodically distributed resonators to a curved double wall. The sound transmission loss properties of such curved double walls are first investigated by using the concept of 'apparent impedance', which expresses the properties of the whole structure in terms of the impedances of the constituting panels and air cavity. The apparent impedance approach is validated against Finite Element models. It is shown that, instead of a dip in the sound transmission loss around the ring fre-quency of a single curved panel, the curved double wall may exhibit a broad 'valley' with low sound transmission loss, whose bandwidth is determined by the spacing between the two characteristic frequencies of the structure (associated with the ring frequency and mass-spring-mass resonance of the curved double wall). The curved double wall is then specifically designed by adjusting the two characteristic frequencies to be close to each other in order to narrow the region associated with a low transmission loss. This enables, subsequently, to improve the transmission loss in this region by effectively inserting tuned local resonators. The design principles are discussed, and applications of double walls consisting either of the same curved panels or different curved panels are both included. (c) 2020 The Authors. Published by Elsevier Ltd.