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09.03 Microperforated materials

Optimization of space-constrained micro-perforated absorbers by causality criteria
Time: 11:20 am

Author: Teresa Bravo

Abstract ID: 1402

The problem of space-constrained absorbers in the low frequency range constitutes an area of continuous research. Micro-perforated panels are advantageous because they can be tuned by a proper selection of their constitutive physical parameters including the diameter of the perforations and their separation distance, their thickness and the length of the backing cavity. However, such optimal selection is not straightforward, especially when considering multi-layer partitions. Current optimization algorithms are based on the maximization of the total absorption coefficient averaged over a frequency band, that requires a compromise between the bandwidth and the thickness of the control device. In this work, the problem is analysed on the basis of a causality criterion. This principle is generalized from its formulation in the field of electromagnetism to obtain a relation that correlates the thickness-to-bandwidth performance of a micro-perforated absorber to its total absorption coefficient. Using this relation, an optimization procedure is presented for the sequential selection of the optimal physical parameters for single-layer partitions. An excellent agreement has been found between the optimal values obtained by the causality criterion and those achieved by critical coupling conditions.

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Enhanced modal matching method for perforated and micro-perforated partitions
Time: 11:40 am

Author: Teresa Bravo

Abstract ID: 1404

Perforated multilayer partitions are widely used in many problems for the aerospace and automotive industries and air-conditioning systems. Combinations of macro and micro-perforated perforations across the constitutive partition layers can provide different physical mechanisms for diverse control strategies depending on the problem requirements. However, there is a lack of unified numerical or analytical description able to provide accurate results over a broad frequency range for a wide range of diameter perforations ranging from supra-millimetric to sub-millimetric apertures. Furthermore, most of them do not account for the beneficial effects on the partition dissipation of the in-hole non-planar modes, albeit evanescent. In this work, an enhanced modal matching (EMM) method is presented that accounts for in-hole high-order modes as well as visco-thermal boundary layer effects inside the holes and over the outer surfaces surrounding the holes. The analytical results have been compared against effective models, numerical models and impedance tube measurements. They show good agreement for single and multi-layer partitions within the corresponding bandwidths of validity. Parametric studies have concluded that the panel thickness-to-hole diameter ratio is a key factor that plays a crucial role on the prominence of the in-hole radial modes and outer visco-thermal effects in the dissipation properties.

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Sound absorption of a finite micro-perforated panel backed a fibrous porous material
Time: 6:20 am

Author: Ho Yong Kim

Abstract ID: 2470

Back by a rigid cavity filled with a layer of porous layer, the sound absorption performance of a micro-perforated panel (MPP) can be enhanced in comparison with other resonance based sound absorbers. In this paper, a theoretical model of a finite flexible MPP back by a rigid air cavity filled with a fibrous porous material is developed to predict normal sound absorption coefficients. Displacements of MPP and sound pressure field in fibrous porous material and acoustic cavity are expressed using a series of modal functions, and the sound absorption coefficients of MPP system are obtained. Additionally, comparison of energy dissipation by MPP and fibrous material is performed to identify effects of a fibrous material on the sound absorption of a MPP. As expected, at anti-resonance frequency of an MPP, the fibrous material provide an alternative energy dissipation mechanism.

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Customized sound mitigation with micro-perforated panels
Time: 11:00 am

Author: Sebastian Floss

Abstract ID: 2489

Micro-perforated panels in combination with an air-tight back volume constitute the micro-perforated absorber (MPA), an alternative means in the category of sound absorbing (meta)materials. In contrast to the conventional porous and fibrous materials, the MPA does not mitigate sound in broad frequency range, but rather has to be customized to a specific noise frequency range. In this contribution, we demonstrate the simulation framework based a genetically fitted equivalent fluid model. Application examples show the advantages and disadvantages of using MPAs in room acoustic scenarios as well as with a background mean flow in ducts. The investigations found that the MPA’s effectiveness strongly depends on the sound field characteristic. The MPP’s surface roughness and back volume composition in-part significantly influence the efficiency of an adjacent turbo-machine.

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Experimental study of smart sound absorber using multimode electromechanical coupling control in the low-frequency range
Time: 6:40 am

Author: Xiang Liu

Abstract ID: 2529

To construct a smart sound absorber in the low-frequency range with a wide control band, a piezoelectric ceramic (PZT) shunted with multiple resonance circuit is attached onto a micro-perforated panel (MPP) to perform as a smart sound absorber. The absorption can be controlled by the shunt circuit parameters conveniently. This smart micro-perforated panel (MPP) is investigated experimentally to explore the feasibility and design procedure in practical use. Based on the coupling among the acoustical, electrical, and mechanical fields, the proposed broadband sound absorber can achieve good acoustic performance on subwavelength scales. The electrical response of the shunt circuit is tested with a Network Analyzer. The acoustic performance of the smart sound absorber is measured in an impedance tube with the two-microphone transfer function method. The experimental results validate that the shunt circuit can resonate with the PZT patch at multiple frequencies, and hence improve the sound absorption of the smart absorber at these frequencies.

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Broadband sound absorbers of multilayered micro-slit panels using Bayesian probabilistic inference
Time: 12:00 pm

Author: Michael Hoeft

Abstract ID: 2621

Micro-perforated panel absorbers can typically achieve either visual transparency or broadband absorption, but not both. This paper assesses the potential of Multilayer Micro-Slit panels to maintain both of these characteristics simultaneously. Micro-slit panels are similar to micro-perforated panels, and can similarly achieve high absorption coefficients without fibrous backing materials. The arrangement of slits are better suited to visual transparency than perforated holes because it provides more unobstructed panel per perforated area. However, these types of absorbers are limited to a narrow frequency bandwidth of effective absorption. By combining several panels into a multilayer assembly, broadband absorption becomes possible. The inherent complexity stemming from optimizing the parameters for multiple layers to meet a given design criteria necessitates the use of the Bayesian framework. This probabilistic method rapidly hones in on the best parameters of each individual layer so that the overall composite meets the design goal. Furthermore, Bayesian inference implemented cyclically alongside panel fabrication and testing allows for corrections of fabrication tolerances while assessing visual transparency.

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Proposal of Acoustic Liners Combined with Fine-Perforated-Film
Time: 6:00 am

Author: Yo Murata

Abstract ID: 3116

This paper deals with a resonant type liner panel with a special surface structure. A typical resonant type liner panel generally consists of a perforated face plate, cells, and a back rigid plate. One of the technical challenges of the acoustic liners applied to the future ultra-high bypass ratio engines is to increase the sound absorption efficiency under grazing conditions because the nacelle, covering of the engine, tends to reduce its length and the lined area. It is known that the sound absorption of the conventional liners tends to deteriorate as grazing flow increases. The authors introduced a special thin acoustically transparent film over the face plate of the acoustic liner. The film, a fine perforated film (FPF), is expected to prevent the interaction of the grazing flow with the opening of the liner face plate. An experimental result with a flow duct rig in JAXA confirmed that the proposed combination of the acoustic liner and the FPF improved the absorption in acoustic energy under grazing conditions, compared with the sole acoustic liner and simple treatment of the FPF.

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