# Multi-Layered Porous Helmholtz Resonators for Low-Frequency and Broadband Sound Absorption

## TL;DR

This paper introduces a new sound absorber using layered porous materials and large orifices to achieve efficient low-frequency and broadband sound absorption.

## Contribution

A novel multi-layered porous Helmholtz resonators absorber with large orifices and high absorption performance is proposed.

## Key findings

- A double-layered prototype achieved near-perfect absorption peaks at 262 Hz and 774 Hz with 11 cm thickness.
- A triple-layer design achieved ultra-broadband absorption above 0.95 from 280 Hz to 1349 Hz with only 16.5 mm thickness.
- The design uses larger orifices than traditional MPPs, making it easier to fabricate and more resistant to blockage.

## Abstract

Unlike classical multi-layered micro-perforated panels (MPPs), which rely on sub-millimeter orifices for sound dissipation, we propose a multi-layered porous Helmholtz resonators absorber. It consists of alternately layered perforated porous material panels and perforated rigid panels with millimeter- to centimeter-scale orifices, primarily relying on porous materials for sound energy dissipation. Theoretically, perforated porous material panels are modeled as homogeneous fluid layers using double porosity theory, and the total surface impedance is derived through bottom-to-top impedance translation. A double-layered prototype was tested to validate the theoretical and numerical models, achieving near-perfect absorption peaks at 262 Hz and 774 Hz, with a subwavelength total thickness of 11 cm and a broadband absorption above an absorption coefficient of 0.7 from 202 Hz to 1076 Hz. Simulations of sound pressure, particle velocity, power dissipation, and sound intensity flow confirm that Helmholtz resonances in each layer enhance sound entry into resistive porous materials, causing absorption peaks. Parameter studies show this absorber maintains high absorption peaks across wide ranges of orifice diameters and panel thicknesses. Finally, an optimized triple-layer porous Helmholtz resonators absorber achieves an ultra-broadband absorption above a coefficient of 0.95 from 280 Hz to 1349 Hz with only 16.5 mm thickness. Compared with conventional MPPs, this design features significantly larger orifices that are easier to fabricate and less susceptible to blockage in harsh environments, offering an alternative solution for low-frequency and broadband sound absorption.