Omnidirectional broadband acoustic absorber based on metamaterials

Abstract

We present the design, construction, and experimental characterization of the acoustic analogue of the so called photonic black-hole. The fabricated sample has cylindrical symmetry and consists of two parts, a shell that bends the sound towards the center and a core that dissipates its energy. The shell is made of a metamaterial that perfectly matches the acoustic impedance of air and behaves like a gradient index lens. The experimental data obtained in a multi-modal impedance chamber demonstrate that the proposed acoustic black-hole acts like an onmidirectional broadband absorber with strong absorbing efficiency.

Gradient index lenses for flexural waves based on thickness variations

“…3 shows the real part of the displacement W(r, h) for the different type of lenses listed in Table I The concentrator shown in panel (e) is identical to the photonic black hole studied by Narimanov and Kildishev 26 which was also tested for acoustic waves. 27 As expected, the elastic counterpart exhibits similar behavior. As a parameter used to prove the broadband efficiency of the method to control the refractive index with the thickness, we have used the scattering cross section (SCS) defined at the far-field as rðk; hÞ lim r!1 j ffiffi r p W sc ðr; hÞj;…”

“…26 Note that all these lenses have been defined for optical waves, however, since they are obtained using the geometrical optics approximation, we assume that the solutions are also valid for flexural waves. This assumption has been applied previously for acoustics waves, 27 where the mapping with the electromagnetic waves is well supported. Fig.…”

Transparent Gradient-Index Lens for Underwater Sound Based on Phase Advance

“…Traditional compliant materials often have larger densities and/or lower dilatation moduli than water, making them useful for applications requiring a low relative sound speed. For example, a sonic crystal lattice of rubber cylinders with a gradient filling fraction was recently used to achieve an underwater omnidirectional absorption coating [1], which requires a radially decreasing sound speed [2,3].…”

“…GRIN geometries are utilized for a broad range of metamaterial applications including scattering reduction [6,7], wave focusing [8][9][10][11][12][13][14][15][16], and bending [1][2][3]17,18]. In contrast to previous lens designs, we present a lens composed of impedance-matched, hollow-shell elements with sound speeds higher than water.…”

Thin structured rigid body for acoustic absorption

“…Furthermore, an omni-directional broadband acoustic absorber of airborne sound, the acoustic analogue of the optical black-hole, 12 has been demonstrated experimentally. 13 In this study, Climente et al utilised a graded index (GRIN) structure to guide the sound energy towards a lossy core, where 80% of the impinging acoustic energy was shown to be dissipated.…”