To broaden its’ effective frequency range and to improve its transmission loss performance, a modified design of a Helmholtz resonator is proposed and evaluated by implementing a rigid baffle in its cavity. Comparison is then made between the proposed design and the conventional one by considering a rectangular duct with the resonator implemented in the presence of a mean grazing flow. For this, a linearized 2D Navier-Stokes model in frequency domain is developed. After validated by benchmarking with the available experimental data and our experimental measurements, the model is used to evaluate the effects of (1) the width Lp of the rigid baffle, (2) its implementation location/height Hg, (3) its implementation configurations (i.e., attached to the left sidewall or right sidewall), (4) the grazing mean flow Mu (Mach number), and (5) the neck shape on a noise damping effect. It is shown that as the rigid baffle is attached in the 2 different configurations, the resonant frequencies and the maximum transmission losses cannot be predicted by using the classical theoretical formulation ω2=c2S/VLeff, especially as the grazing Mach number Mu is greater than 0.07, i.e., Mu>0.07. In addition, there is an optimum grazing flow Mach number corresponding to the maximum transmission loss peak, as the width Lp is less than half of the cavity width Dr, i.e., Lp/Dr≤0.5. As the rigid plate width is increased to Lp/Dr=0.75, one additional transmission loss peak at approximately 400 Hz is produced. The generation of the 12 dB transmission loss peak at 400 Hz is shown to attribute to the sound and structure interaction. Finally, varying the neck shape from the conventional one to an arc one leads to the dominant resonant frequency being increased by approximately 20% and so the secondary transmission loss peak by 2-5 dB. The present work proposes and systematically studies an improved design of a Helmholtz resonator with an additional transmission loss peak at a high frequency, besides the dominant peak at a low frequency.
In this work, acoustic damping performances of double-layer in-duct perforated plates are studied at low Mach (Ma) and Helmholtz number (He) to evaluate the effects of (1) Ma, (2) the porosities (i.e., open-area ratio) σ1 and σ2 of the front and back plates, and (3) the axial distance Lc between these two plates. The orifices’ damping is characterized by sound absorption coefficient α denoting the fraction of incident sound energy being absorbed. For this, a quasi-steady acoustic model is developed first and experiments are then conducted. When Ma = 0, α is experimentally found to oscillate with He, whatever the porosities of σ1 and σ2 are set. However, when Ma is increased to and above 0.037, the power absorption troughs, i.e., local αmin of the double-layer plates with σ1,2 ≤ 9% are more separated and shallower. Furthermore, when σ1 = 9% or σ2 = 9%, the damping performances are quite different in terms of the local αmax peaks and their number. In addition, increasing Lc with respect to the downstream pipe length Ld gives rise to an increase of αmin and αmax by 10%. Finally, the double-layer plates are shown to involve a larger α than that of single-layer one over a broader He range.
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