Rainbow-trapping absorbers: Broadband, perfect and asymmetric sound absorption by subwavelength panels for transmission problems

Noé Jiménez,Vincent Pagneux,Vicent Romero-García,Jean-Philippe Groby

doi:10.1038/s41598-017-13706-4

Noé Jiménez, Vincent Pagneux + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-13706-4

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Abstract

Perfect, broadband and asymmetric sound absorption is theoretically, numerically and experimentally reported by using subwavelength thickness panels in a transmission problem. The panels are composed of a periodic array of varying crosssection waveguides, each of them being loaded by Helmholtz resonators (HRs) with graded dimensions. The low cut-off frequency of the absorption band is fixed by the resonance frequency of the deepest HR, that reduces drastically the transmission. The preceding HR is designed with a slightly higher resonance frequency with a geometry that allows the impedance matching to the surrounding medium. Therefore, reflection vanishes and the structure is critically coupled. This results in perfect sound absorption at a single frequency. We report perfect absorption at 300 Hz for a structure whose thickness is 40 times smaller than the wavelength. Moreover, this process is repeated by adding HRs to the waveguide, each of them with a higher resonance frequency than the preceding one. Using this frequency cascade effect, we report quasi-perfect sound absorption over almost two frequency octaves ranging from 300 to 1000 Hz for a panel composed of 9 resonators with a total thickness of 11 cm, i.e., 10 times smaller than the wavelength at 300 Hz.

Highlights

Perfect, broadband and asymmetric sound absorption is theoretically, numerically and experimentally reported by using subwavelength thickness panels in a transmission problem
In ref.[20] a similar approach was presented using detuned Helmholtz resonators (HRs) in a rigidly-backed waveguide. An extension of these ideas has been used to produce multiple slow waves inside a rigidly-backed graded structure of porous material to improve the broadband behaviour[33], but critical coupling conditions were not fulfilled at most resonances and perfect absorption was only observed at a single frequency
For frequencies below f2, propagation is allowed in the main waveguide and the effect of the second HR may be visible inducing a decrease of the reflection coefficient

Summary

Introduction

Broadband and asymmetric sound absorption is theoretically, numerically and experimentally reported by using subwavelength thickness panels in a transmission problem. In ref.[15] the authors used a graded set of QWRs in a slightly-subwavelength thickness structure to obtain quasi-perfect and broadband absorption This last configuration can fulfil the critical coupling conditions at more than one frequency, exhibiting perfect and broadband absorption[18]. An extension of these ideas has been used to produce multiple slow waves inside a rigidly-backed graded structure of porous material to improve the broadband behaviour[33], but critical coupling conditions were not fulfilled at most resonances and perfect absorption was only observed at a single frequency

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Conclusion