An Investigation On Neck Extensions For Single and Multi-Degree Of Freedom Acoustic Helmholtz Resonators

TL;DR

This paper investigates how neck extensions in Helmholtz resonators affect their resonance frequencies and sound absorption bandwidths through experimental and numerical methods, revealing non-linear frequency shifts and increased absorption bandwidths.

Contribution

It introduces a detailed analysis of neck extension effects in single and multi-degree Helmholtz resonators, highlighting their impact on resonance frequencies and absorption bandwidths.

Findings

Neck extensions lower resonance frequencies non-linearly.

Neck extensions increase sound absorption bandwidth.

Double degree of freedom resonators benefit from neck extensions.

Abstract

The effect of neck extensions in single and multi-degree of freedom Helmholtz resonator based acoustic liners is studied both experimentally and numerically and the resulting transmission coefficient and resonance frequencies are examined. It has been shown that a single degree of freedom liner with increasing neck extension lengths leads to the resonance frequencies being pushed to lower frequency values, however, this shift to lower frequencies is not linear with increasing length. A study on including neck extensions for the primary and/or secondary neck within a double degree of freedom liner is also presented. It is shown that both neck extension concepts lead to an increase in bandwidth of sound absorption by a double degree of freedom Helmholtz resonator.