Conference_programme: 19.1 - Duct Acoustics
Lecture: Sound wave attenuation in a duct with a periodic array of ultrathin Helmholtz resonators
Author(s): Cedric Maury, Teresa Bravo
Summary:
Enhancing the broadband attenuation of low-frequency noise using compact, lightweight and fiber-free acoustic liners is still a challenging task in air conditioning systems, but also in exhaust and intake ducted flows. Locally-reacting Helmholtz resonators can be tuned and optimized to efficiently dissipate noise in a narrow mid-frequency range. A periodic combination of several resonators is known to provide a broader bandwidth of the noise attenuated at mid-frequencies, albeit at constant value of the total power attenuated over the resonance bandwidth. In this work, a theoretical study examines the efficiency of a periodic array of Ultrathin Helmholtz Resonators (UHR) to attenuate low-frequency noise components over a broad bandwidth, well below the first cut-on frequencies of the duct and of the resonators neck and cavity. Each side-branch resonator is composed of a cylindrical neck backed by a coplanar coiled air chamber that significantly increases the acoustic path length in the cavity while keeping a sub-wavelength depth of the cavity at the resonator Helmholtz resonance. A transfer matrix formulation is derived to calculate the Transmission Loss (TL) of the array of resonators, averaged over the number of resonators. The TL results tend towards the dispersion curves of the unitary transfer matrix as the number of resonators increases, revealing the emergence of stopping and passing bands that respectively inhibit and allow the propagation of sound waves in the duct. The attenuation characteristics of the array of UHRs is compared to that due to arrays of classical Helmholtz resonators in the no-flow and uniform flow cases. Of interest is to find an optimal periodic distance between the resonators, typically half-the acoustic wavelength at the Helmholtz resonance, in order to broaden the bandwidth of the first stop-band in the low-frequency range.
Corresponding author
Name: Prof Maury Cedric
e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
Country: France
