Micro-perforated Absorbers Research Articles

Design of a single layer micro-perforated sound absorber by finite element analysis

Micro-perforated sound absorbers with sub-millimeter size holes can provide high absorption coefficients. This paper presents results of work on the development of an effective single layer micro-perforated sound absorber from the commercial composite material Parabeam ® with micro diameter holes drilled on one side. Parabeam ® is used as a structural material made from a fabric woven out of a E-glass yarn and consists of two decklayers bonded together by vertical piles in a sandwich structure with piles (thick fibers) woven into the decklayers. The paper includes, the analytical model developed for prediction of absorption coefficients, finite element solution using commercial software MSC.ACTRAN and experimental results obtained from impedance tube measurements. A simple optimization is performed based on the developed models to obtain an efficient absorber configuration. It has been anticipated that several different and interesting applications can be deduced by combining structural and sound absorption properties of this new micro-perforated absorber.

An Acoustic Window System with Optimum Ventilation and Daylighting Performance

The paper presents a series of theoretical, experimental and numerical studies on using transparent micro-perforated absorbers in a window system to allow noise attenuation whilst at the same time maintaining ventilation for comfort and daylighting. The underlying theory for micro-perforated absorbers and the application in silencers is examined. Experimental results between a semi-anechoic chamber and a reverberant chamber using a standard window mock-up are presented. Numerical simulation using finite element method is summarised, considering the effects of opening size, air gap, louvers, hood, and absorbers. It is shown that such a window system can perform better than closed single glazed windows, while still allowing for significantly more ventilation for comfort than most current systems as well as for high levels of light ingress.

A note on the prediction method of reverberation absorption coefficient of double layer micro-perforated membrane

The absorption performance of micro-perforated absorber (MPA) has been usually estimated by equivalent circuit (EC), however, it has been noted that the predicted absorption coefficient by EC does not agree completely with the experiment in some frequency range. Hence impedance transfer method (ITM) is adopted to predict the reverberation absorption coefficient of a double layer micro-perforated membrane (MPM) structure. Experimental studies show that the prediction of ITM is better than that of EC.

An acoustic window for sustainable buildings

Encouraging the use of natural ventilation is an important tendency in the green building movement, but opening windows can often cause noise problems. This research develops a window system which allows natural ventilation while reducing noise transmission. The core idea is to create a ventilation path by staggering two layers of glass and using micro-perforated absorbers (MPA) along the path created to reduce noise. The MPA are made from transparent materials so that daylighting is relatively unaffected. Starting with a brief introduction of the MPA theory and its application in ducts, the paper presents a series of numerical simulations using finite element method based software FEMLAB, and experiment results measured between a semi-anechoic chamber and a reverberation chamber. Performance in acoustics, ventilation and daylighting are all taken into account. A basic window configuration is first considered, studying the effectiveness of various window parameters. A number of strategic designs are then examined, including external hoods and louvers in the sound path. There is generally a good agreement between simulation and measurement, and the noise reduction can be as good as a single glazing, with air movement to achieve occupant comfort, rather than just for minimum air exchange. [Work supported by EPSRC.]

Sound absorption of a finite flexible micro-perforated panel backed by an air cavity

Micro-perforated absorbers have been studied for decades. In the experimental results of some previous works, an unexpected peak due to the flexible panel vibration effect was found on the absorption coefficient curve. In this paper, the acoustic absorption of a finite flexible micro-perforated panel backed by an air cavity is studied in detail. The absorption formula that is developed for the micro-perforated absorber is based on the modal analysis solution of the classical plate equation coupled with the acoustic wave equation. Another approach to derive a simpler absorption formula is also developed. The predictions from the two formulas are very close, except for those at the resonant frequencies of the higher structural modes and acoustic modes parallel to the panel surface. The theoretical results show good agreement with the measurements. It can be concluded that (1) as the panel vibration effect can dissipate more energy, the corresponding absorption peaks can widen the absorption bandwidth of a micro-perforated absorber by appropriately selecting the parameters such as panel thickness, perforation diameter, and perforation spacing, etc., such that the structural resonant frequency is higher than the absorption peak frequency caused by the perforations; (2) the comparison of the cases of different panel mode shapes does not show a significant difference in the absorption performance; and (3) the structural damping effect can improve the absorption performance at the frequencies between the structural resonant frequencies and the peak frequency of the micro-perforation effect, and decrease the peak absorption values of the structural resonances.

Feasibility of applying micro-perforated absorbers in acoustic window systems

The paper examines the feasibility of using transparent micro-perforated absorbers (MPA) in a window system to allow noise attenuation whilst at the same time maintaining high levels of comfort ventilation and daylighting. The underlying theory for micro-perforated panels and membranes and the application in silencers is presented. Experiments have been performed between a semi-anechoic and a reverberant chamber using a standard window mock-up, and the effectiveness of MPA has been demonstrated. With a constant air backing, MPA are more effective with a wider ventilation path. With an air flow of up to 2 m/s the performance of the MPA remains unchanged. Current results are based on readily available materials and relatively simple configurations, but the theoretical analysis suggests possibilities for increasing the noise reduction and widening the frequency range by using more strategically designed materials and configurations.