Use of CFD to Calculate the Dynamic Resistive End Correction for Microperforated Materials

The classical Maa theory for microperforated materials was initially formulated for constant diameter, cylindrical holes. Since then, a number of ad hoc corrections have been suggested to account for different hole shapes, in particular, rounding of the aperture. Here it is shown that the resistance and reactance of small apertures may be calculated using relatively simple CFD models in which a single hole is modeled. The fluid is assumed to be viscous but incompressible, and the geometry is assumed to be axisymmetric. It will be shown that this approach essentially reproduces the classical theory of Maa for circular, sharp-edged apertures. However, it will also be shown that the empirical correction to the resistive end correction, in particular, exhibits a clear dependence on frequency and geometrical parameters that is neglected in conventional microperforated material models

Lightweight Absorption and Barrier Systems Comprising N-Layer Microperforates

Since the concept of microperforated panels (MPPs) was introduced by Maa, there have been continuing efforts to apply MPPs, primarily as fiber-free sound absorbing materials, typically wall-mounted. The objective of the present work was to demonstrate that multi-layer MPPs can also be effective functional absorbers and lightweight barrier systems. The acoustical properties of lightweight MPPs depend on hole diameter, thickness, porosity, mass per unit area, and air cavity depth. In the case of a single layer, it is possible to find a combination of these parameters that results in good performance over one or two octaves. However, to be effective for noise control over a broader range of frequencies, it is necessary to design multi-layer MPPs. Thus here the focus was on the optimal design of multi-layer MPPs in the speech interference range, 500 to 4000 kHz. In the case of functional absorbers, the total absorption of the system was optimized, while in the case of barriers, a high transmission loss was desired, without necessarily sacrificing the absorption of the system. In the latter case, in particular, it was possible to create systems having transmission losses well in excess of the mass law over a broad range of frequencies

Computational investigation of microperforated materials: end corrections, thermal effects and fluid-structure interaction

The concept of microperforated noise control treatments was introduced by Maa 1975; in that theory, the transfer impedance of the microperforated layer was calculated based on oscillatory viscous flow within a small cylinder combined with resistive and reactive end corrections. Initially, microperforated materials were the subject of mostly academic study since practical implementations were rare owing to the cost of manufacturing the materials with acceptable accuracy. However, recently, new manufacturing procedures have dramatically lowered the cost of these materials, and perhaps as a result, there has been renewed interest in studying their properties. Since 1975, Maa’s original theory has been widely used to predict the performance of microperforated materials. However, in principal, that theory can only be used to describe cylindrical perforation, while in practice, perforations are rarely cylindrical. In addition, there have been questions about the dependence of end corrections on frequency, and on the effect of coupling between the motion of the fluid in the perforations and the solid sheet in which they are formed. Additionally, in his original paper, Maa drew a distinction between the dissipative properties of thermally conducting and adiabatic materials. The latter topic, in particular, has not been considered by any investigators since the idea was introduced. The purpose of our presentation is to introduce the numerical tools that can be used to address the open questions mentioned above, and to highlight important results obtained by using those tools

Optimal Design of Multi-Layer Microperforated Sound Absorbers

Microperforated polymer films can offer an effective solution when it is desired to design fiber-free sound absorption systems. The acoustic performance of the film is determined by hole size and shape, by the surface porosity, by the mass per unit area of the film, and by the depth of the backing air layer. Single sheets can provide good absorption over a one of two octave range, but if absorption over a broader range is desired, it is necessary to use multilayer treatments. Here the design of a multilayer sound absorption system is described, where the film is considered to have a finite mass per unit area and also to have conical perforations. It will be shown that it is possible to design compact absorbers that yield good performance over the whole speech interference range. In the course of the optimization it has been found that there is a tradeoff between cone angle and surface porosity. The design of lightweight, multilayer functional absorbers will also be described, and it will be shown, for example, that it is possible to design systems that simultaneously possess good sound absorption and barrier characteristics