Metadiffusers: Deep-subwavelength sound diffusers

Noé Jiménez,Vicent Romero-García,Trevor J Cox,Jean-Philippe Groby

doi:10.1038/s41598-017-05710-5

Abstract

We present deep-subwavelength diffusing surfaces based on acoustic metamaterials, namely metadiffusers. These sound diffusers are rigidly backed slotted panels, with each slit being loaded by an array of Helmholtz resonators. Strong dispersion is produced in the slits and slow sound conditions are induced. Thus, the effective thickness of the panel is lengthened introducing its quarter wavelength resonance in the deep-subwavelength regime. By tuning the geometry of the metamaterial, the reflection coefficient of the panel can be tailored to obtain either a custom reflection phase, moderate or even perfect absorption. Using these concepts, we present ultra-thin diffusers where the geometry of the metadiffuser has been tuned to obtain surfaces with spatially dependent reflection coefficients having uniform magnitude Fourier transforms. Various designs are presented where, quadratic residue, primitive root and ternary sequence diffusers are mimicked by metadiffusers whose thickness are 1/46 to 1/20 times the design wavelength, i.e., between about a twentieth and a tenth of the thickness of traditional designs. Finally, a broadband metadiffuser panel of 3 cm thick was designed using optimization methods for frequencies ranging from 250 Hz to 2 kHz.

Highlights

We present deep-subwavelength diffusing surfaces based on acoustic metamaterials, namely metadiffusers
The transfer matrix method (TMM) and finite element method (FEM) using the effective parameters were accurately validated experimentally to model the thermoviscous losses of metamaterials using a set of N = 13 by M = 1 HRs31 and N = 3 by M = 10 HRs32
Metadiffusers, a novel design of locally reacting surfaces with tailored acoustic scattering was presented. These new structures are based on metamaterials comprising a slotted panel, with the slits loaded by a set of Helmholtz resonators

Summary

Introduction

We present deep-subwavelength diffusing surfaces based on acoustic metamaterials, namely metadiffusers These sound diffusers are rigidly backed slotted panels, with each slit being loaded by an array of Helmholtz resonators. By tuning the geometry of the metamaterial, the reflection coefficient of the panel can be tailored to obtain either a custom reflection phase, moderate or even perfect absorption Using these concepts, we present ultra-thin diffusers where the geometry of the metadiffuser has been tuned to obtain surfaces with spatially dependent reflection coefficients having uniform magnitude Fourier transforms. One approach is to set the spatially-dependent reflection coefficient according to a number sequence that presents a uniform magnitude Fourier spectrum at the design frequency In this case, a periodic array of the panel presents grating lobes with the same pressure magnitude in the far field at the design frequency

Methods

Results

Conclusion