Search results for "absorption"
showing 10 items of 2701 documents
Propriétés acoustiques de matériaux poreux saturés d'air incluant une porosité partiellement ouverte
2010
International audience; Nous étudions les propriétés d'un matériau à microgéométrie complexe incluant une porosité partiellement ouverte ou de culs-de-sacs. Ce type de matériau peut être obtenu par dissolution de grains de sel (NaCl) noyés dans une matrice de métal solidifié. Cette matrice est obtenue après invasion sous pression négative et à haute température des grains de sels par le métal sous forme liquide, puis refroidissement jusqu'à solidification du métal. La comparaison des résultats expérimentaux et théoriques sur le coefficient d'absorption acoustique d'une couche de ce métal poreux montre que le modèle de Biot-Johnson-Allard ne fournit pas des résultats aussi satisfaisants que …
Holes Interaction Effects under high and medium Sound Intensities for Micro-perforated panels design
2010
International audience; Most models for predicting the acoustic response of perforated panels are based on the assumption that there are no interactions between the holes. This paper investigates the Hole Interaction Effect (HIE) and distribution effect on the sound absorption coefficient of perforated panels when submitted to medium and high sound pressure levels. It is experimentally demonstrated that though the HIE is mostly an added mass effect, the nonlinear resistance parameters depend on the separation distance between the apertures. Analysis of the data reveals the fact that even with HIE, the nonlinear reactance dependency with velocity is very low compared to the resistance-veloci…
Acoustic properties of plates with unevenly distributed macro perforations backed by woven meshes
2012
A hybrid model describing the acoustic properties of plates with macro-perforations that can be unevenly distributed on the plate surface and backed by woven or precision woven meshes with microscopic perforations is proposed. The plate perforations may be of circular or rectangular shapes. Since the perforated plate may not necessarily be considered as an equivalent fluid, its surface impedance is calculated by the Maa model [Noise Control Eng. J. 29, 77-84 (1987)], whereas the Johnson-Champoux-Allard model [J. Appl. Phys. 70, 1975-1979 (1991)] is used for the mesh. It is observed that the absorption of the carrying plate seems to depend on the hydraulic diameter of the perforations and no…
On the measurement and prediction of the sound absorption coefficient of air-cavity backed perforated plates considering the holes interaction effect…
2012
International audience; This paper deals with the measurement and prediction of air cavity backed perforated plates considering the holes interaction effect under relatively low sound excitation. A revised expression for the radiation contribution is proposed to properly account for the holes interaction effect. This expression is integrated in the characteristic impedance expression following Atalla and Sgard [J. Sound Vib. 303, 195-208 (2007)] model. Perforated plate specimens with different centre-to-centre holes distances (pitches) are built and tested using an impedance tube. The particular cases of holes interaction effect with constant low porosity and of holes interaction effect wit…
Sound absorption coefficient of a porous material covered with a low open area perforated plate under high sound excitation
2012
International audience; The sound absorption coefficient of porous materials covered with low open area perforated plate is studied under high sound intensities in the absence of mean flow. The theoretical considerations are based on the equivalent fluid following the Johnson-Champoux-Allard approach and the use of the transfer matrix method. To take into account the high sound levels effects, the air flow resistivity of each layer is modified following the Forchheimer law. Two specimens of perforated plate are built and tested when backed by a polymeric foam and a fibrous material. A specific impedance tube setup is developed for the measurement of the surface acoustic impedance for sound …
Excitons in few-layer hexagonal boron nitride: Davydov splitting and surface localization
2018
Hexagonal boron nitride (hBN) has been attracting great attention because of its strong excitonic effects. Taking into account few-layer systems, we investigate theoretically the effects of the number of layers on quasiparticle energies, absorption spectra, and excitonic states, placing particular focus on the Davydov splitting of the lowest bound excitons. We describe how the inter-layer interaction as well as the variation in electronic screening as a function of layer number $N$ affects the electronic and optical properties. Using both \textit{ab initio} simulations and a tight-binding model for an effective Hamiltonian describing the excitons, we characterize in detail the symmetry of t…
NHC-Stabilized Au10 Nanoclusters and Their Conversion to Au25 Nanoclusters
2022
Herein, we describe the synthesis of a toroidal Au10 cluster stabilized by N-heterocyclic carbene and halide ligands via reduction of the corresponding NHC−Au−X complexes (X = Cl, Br, I). The significant effect of the halide ligands on the formation, stability, and further conversions of these clusters is presented. While solutions of the chloride derivatives of Au10 show no change even upon heating, the bromide derivative readily undergoes conversion to form a biicosahedral Au25 cluster at room temperature. For the iodide derivative, the formation of a significant amount of Au25 was observed even upon the reduction of NHC−Au−I. The isolated bromide derivative of the Au25 cluster displays a…
BETWEEN SUCCESS AND UNSUCCESS IN ABSORBING EU FUNDS - POLAND VS. ROMANIA
2015
Component of the cohesion policy promoted by the European Commission, European grant financing represents an inexpensive and important resource for the social and economic recovery of the member states, offering the prospect for the budgetary sustainability, encouraging public and private investments and thereby reducing the development gaps both at national and international level. In the present study, through the comparative analysis between Romania and Poland in the field of EU funds absorption, we intend to identify the weaknesses and the strengths of each of the two member states, helping to formulate a set of recommendations that our country should follow in order to attract funds re…
A critical size for emergence of nonbulk electronic and geometric structures in dodecanethiolate-protected Au clusters.
2014
We report on how the transition from the bulk structure to the cluster-specific structure occurs in n-dodecanethiolate-protected gold clusters, Au(n)(SC12)m. To elucidate this transition, we isolated a series of Au(n)(SC12)m in the n range from 38 to ∼520, containing five newly identified or newly isolated clusters, Au104(SC12)45, Au(∼226)(SC12)(∼76), Au(∼253)(SC12)(∼90), Au(∼356)(SC12)(∼112), and Au(∼520)(SC12)(∼130), using reverse-phase high-performance liquid chromatography. Low-temperature optical absorption spectroscopy, powder X-ray diffractometry, and density functional theory (DFT) calculations revealed that the Au cores of Au144(SC12)60 and smaller clusters have molecular-like elec…
Oxysterols – how much do we know about food occurrence, dietary intake and absorption?
2021
Oxysterols are formed in foods during processing and storage, having an impact on the nutritional profile besides being associated to the development of diverse chronic and degenerative diseases. This review provides an overview on recent information and knowledge gaps on oxysterol formation and absorption from foods. Their contents should be monitored based on dietary habits, in order to have more realistic data available and to expand the currently limited daily intake data. Unraveling the mechanism of oxysterol absorption, together with a more thorough understanding of their metabolism, is needed; in this respect, bioavailability studies using in vitro and in vivo methodologies could con…