Micro-perforated Plate Research Articles

Enhanced low- to mid-frequency sound absorption using parallel-arranged perforated plates with extended tubes and porous material

Porous sound absorptive material (PSAM) and micro-perforated plate (MPP) limited by space are weak in absorbing low- to mid- frequency noise. In order to improve sound absorption in 100–1600Hz, a compound sound absorber comprised of perforated plates with extended tubes (PPET) and a PSAM is proposed in this paper. A theoretical model is described to predict the sound absorption coefficients of two combinations: one has three parallel-arranged PPETs and a PSAM layer; the other one has two parallel-arranged PPETs, an MPP and a PSAM layer. The calculated results are then validated by the results measured in an impedance tube. The proposed combinations demonstrate superior sound absorption performance over more than three octaves in the targeted frequency range.

Non-conforming finite element method for efficiently computing multilayered microperforated plate absorbers

The concept of acoustic impedance is a very useful model approach to efficiently compute configurations for sound absorption. Thereby, the measurements are performed by an impedance tube and the obtained data is used for the first order Robin (impedance) boundary condition within the numerical simulation. However, this approach is only valid for sound incidence perpendicular to the boundary. A second approach is to resolve the volume of the absorber and use a rigid or even elastic frame model. Especially for multilayered silencers based on microperforated plates (MPPs) the volume resolving approach is beneficial. Here, a main challenge is to cope with the quite different mesh sizes needed for accurately resolving the waves in the MPPs and the surrounding air regions. To efficiently simulate such designs, we apply a Nitsche-type mortaring within the Finite Element Method to allow for non-conforming meshes and thereby directly connect the different mesh sizes in the MPPs and surrounding air regions. We will discuss in detail our absorber design, the performed measurements and numerical simulations and plan to publish the complete setup and results as a benchmark case for computational acoustics.The concept of acoustic impedance is a very useful model approach to efficiently compute configurations for sound absorption. Thereby, the measurements are performed by an impedance tube and the obtained data is used for the first order Robin (impedance) boundary condition within the numerical simulation. However, this approach is only valid for sound incidence perpendicular to the boundary. A second approach is to resolve the volume of the absorber and use a rigid or even elastic frame model. Especially for multilayered silencers based on microperforated plates (MPPs) the volume resolving approach is beneficial. Here, a main challenge is to cope with the quite different mesh sizes needed for accurately resolving the waves in the MPPs and the surrounding air regions. To efficiently simulate such designs, we apply a Nitsche-type mortaring within the Finite Element Method to allow for non-conforming meshes and thereby directly connect the different mesh sizes in the MPPs and surrounding air regions. We will...

Investigation of the impedance of a micro-perforated plate with two-sided grazing flow

Acoustic impedance for perforates is mainly quantified by semi or totally empirical formulas, which depend on the experimental set-up used and are specific for each type of tested sample. In this work, the acoustic impedance of a micro-perforated plate (MPP) with two-sided low Mach grazing flow is educed by a modified semi-analytical inverse technique. The inputs to this technique are complex acoustic pressure measured at eight positions at the wall of the duct, upstream and downstream of the MPP section. Several measurements on the two sided grazing flow rig are conducted and trends in the impedance shift are observed. These trends serve to quantify the change in the resistance and reactance of a MPP when it is subjected to two sided grazing flow. An empirical formula is also proposed which is based on these trends. This formula helps in approximating the behavior of MPP’s in applications such as perforated cooling fans, perforated guide vanes, and turbine nozzles. It was observed that with increased flow velocity on either side of the duct the resistance tends to increase while the reactance drops.

A parametric study of flexible micro-perforated panels with a patch-impedance numerical model

The absorption characteristics of a single flexible micro-perforated panel with a back cavity are compared for different combinations of physical parameters. Provided that the panel is made of a rigid plate, the acoustic properties of a micro-perforated panel are determined by the plate thickness, the distribution and diameter of the perforations, the porosity, the edge profile and back cavity depth. Nevertheless, some applications can involve a material choice or plate thickness values invalidating the rigidity assumption. In such cases, additional absorption peaks are observed in the absorption frequency spectrum, which cannot be explained by the classical micro-perforated plate theory. The vibro-acoustic coupling of the flexible plate and the acoustic medium has to be taken into account. This is done by a thin shell model employing a patch-impedance approach, modeling each perforation separately. Using this model, absorption coefficient values are calculated for various combinations of plate thickness, perforation diameter and perforation distribution. The results of this study provide a better understanding of the influence of the design parameters of flexible micro-perforated panels on the sound absorption. The method is numerically efficient and can be used for optimizing the acoustic absorption of such panels.

Improving low-frequency sound absorption of micro-perforated panel absorbers by using mechanical impedance plate combined with Helmholtz resonators

The traditional Micro-perforated plate (MPP) is a kind of clean and non-polluting absorption structure in the middle and high frequency and has been widely used in the field of noise control. However, the sound absorption performance is dissatisfied at low frequencies when the air-cavity depth is restricted. In this paper, a mechanical impedance plate (MIP) is introduced into the traditional MPP structure and a Helmholtz resonator is attached to the MIP. Mechanical impedance plate (MIP) provides a good absorption at low frequency by using mechanism of mechanical resonance and the acoustic energy is dissipated in the form of heat with viscoelastic material. Helmholtz resonator can fill in the defect of the poor absorption effect between the Micro-perforated plate (MPP) and the mechanical impedance plate (MIP). The acoustic impedance of the proposed sound absorber is investigated by using acoustic electric analogy method and impedance transfer method. The influence of the tube’s length of Helmholtz resonator and the number of Helmholtz resonator on the sound absorption is studied. The corresponding results are in agreement with the theoretical calculation and prove that the composite structure has the characteristics of improving the low frequency sound absorption property.

Theoretical analysis of the sound absorption characteristics of periodically stiffened micro-perforated plates

The vibro-acoustic responses and sound absorption characteristics of two kinds of periodically stiffened micro-perforated plates are analyzed theoretically. The connected periodical structures of the stiffened plates can be ribs or block-like structures. Based on fundamental acoustic formulas of the micro-perforated plate of Maa and Takahashi, semi-analytical models of the vibrating stiffened plates are developed in this paper. Approaches like the space harmonicmethod, Fourier transforms and finite elementmethod (FEM) are adopted to investigate both kinds of the stiffened plates. In the present work, the vibro-acoustic responses of micro-perforated stiffened plates in the wavenumber space are expressed as functions of plate displacement amplitudes. After approximate numerical solutions of the amplitudes, the vibration equations and sound absorption coefficients of the two kinds of stiffened plates in the physical space are then derived by employing the Fourier inverse transform. In numerical examples, the effects of some physical parameters, such as the perforation ratio, incident angles and periodical distances etc., on the sound absorption performance are examined. The proposed approaches are also validated by comparing the present results with solutions of Takahashi and previous studies of stiffened plates. Numerical results indicate that the flexural vibration of the plate has a significant effect on the sound absorption coefficient in the water but has little influence in the air.

Sound absorption of a micro-perforated plate backed by a porous material under high sound excitation: measurement and prediction

The sound absorption coefficient of perforated facings backed by porous materials 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 pressure levels ranging from 90 dB to 150 dB at the surface of the perforated facing. To corroborate the validity of the presented method, two considerations are particularly depicted in the experimental results: first, the case where the perforated facing and the porous material are both directly backed by a rigid wall and the case where there is an air cavity between the porous material and the rigid wall. Good agreement is observed between the simulation and the experimental results.

Study of micro-perforated tube mufflers with adjustable transmission loss

Micro-perforated tube muffler has the advantages of both the reactive and dissipative mufflers. The paper proposed that the change of micro-perforated tube length could change the transmission loss behaviors of the micro-perforated tube mufflers. The acoustic impedance of micro-perforated plate was derived in this paper, which was used in the finite element method to calculate the transmission loss of mufflers with various micro-perforated tube lengths. Then, a relation formula of the maximum muffling frequency and the micro-perforated tube length was fitted. This kind of adjustable mufflers could be used efficiently to control the noise of the rotating machines, whose main noises are always proportional to the rotating speed.

Hybrid noise control in a duct using a light micro-perforated plate

A plate silencer consists of an expansion chamber with two side-branch cavities covered by light but extremely stiff plates. It works effectively with a wide stopband from low-to-medium frequencies only if the plate is extremely stiff, to ensure a strong reflection of acoustic wave to the upstream in the duct. However, a plate with a slightly weak bending stiffness will result in non-uniform transmission loss (TL) spectra with narrowed stopband. In this study, a hybrid silencer is proposed by introducing micro-perforations into the plate to elicit the sound absorption in order to compensate for the deficiency in the passband caused by the insufficient sound reflection in a certain frequency range due to weaker plate stiffness. A theoretical model, capable of dealing with the strong coupling between the vibrating micro-perforated plate and sound fields inside the cavity and the duct, is developed. Through proper balancing between the sound absorption and reflection, the proposed hybrid silencer provides a more flattened and uniform TL and a widened stopband by more than 20% while relaxing the harsh requirement on the bending stiffness of the plate. Theoretical predictions are validated by experimental data, with phenomenon explained through numerical analyses.

On the use of micro-perforates for machinery and vehicle noise control

A micro-perforated plate (MPP) is a perforated plate with holes typically in the sub-millimeter range and perforation ratio around 1%. The values are typical for applications in air at standard temperature and pressure (STP). The underlying acoustic principle is simple: To create a surface with a built in damping that effectively dissipates sound waves. To achieve this, the specific acoustic impedance of a MPP is normally tuned to be of the order of the characteristic wave impedance in the medium (400 Pa*s/m in air at STP). The traditional application for MPP's has been building acoustics, normally in the form of a so called panel absorber to create an absorption peak at a selected frequency. However, MPP's made of metal are also well suited for machinery and vehicle noise control. For instance MPP's have the potential to be used instead of porous materials in dissipative mufflers, which not only can save weight but also offer a non-fibrous alternative. Furthermore, since MPP's have a large steady flow re...

Hybrid silencer by using micro-perforated plate with side-branch cavities

A plate silencer consists of an expansion chamber with two side-branch cavities covered by light but extremely stiff plates. It works effectively with wide stopband from low-to-medium frequencies only if the plate is extremely stiff, to ensure a strong reflection of acoustic wave to the upstream in the duct. However, a plate with slightly weak bending stiffness will result in non-uniform transmission loss (TL) spectra with narrowed stopband. In this study, a hybrid silencer is proposed by introducing micro-perforations into the plate to elicit the sound absorption in order to compensate for the deficiency in the passband caused by the insufficient sound reflection in certain frequency range due to plate with weaker stiffness. A theoretical model, capable of dealing with the strong coupling between the vibrating micro-perforated plate and sound fields inside the cavity and the duct, is developed. Through a proper balancing between the sound absorption and reflection, the proposed hybrid plate silencer with moderately stiff plates is shown to outperform the typical plate silencer with very stiff plate. Whilst releasing the harsh requirement on the bending stiffness of the plate, the proposed hybrid silencer provides a more flattened and uniform TL and a widened stopband by about 30%.

A New Type of Muffler Based on Microperforated Tubes

Microperforated plate (MPP) absorbers are perforated plates with holes typically in the submillimeter range and perforation ratios around 1%. The values are typical for applications in air at standard temperature and pressure (STP). The underlying acoustic principle is simple: It is to create a surface with a built in damping, which effectively absorbs sound waves. To achieve this, the specific acoustic impedance of a MPP absorber is normally tuned to be of the order of the characteristic wave impedance in the medium (∼400 Pa s/m in air at STP). The traditional application for MPP absorbers has been building acoustics often combined with a so called panel absorber to create an absorption peak at a selected frequency. However, MPP absorbers made of metal could also be used for noise control close to or at the source for noise control in ducts. In this paper, the possibility to build dissipative silencers, e.g., for use in automotive exhaust or ventilation systems, is investigated.

Research on the Absorption Characteristic of Three-Layer Microperforate Plate of the Automotive Body

Three-layer micro perforated plate of the automotive body is researched. Acoustic impedance of the three-layer microperforated plate of the automotive body is attained according to the equivalent circuit, thus absorption coefficient is attained. Different microperforated plate with different porosity, different diameter, different thickness, different depth of cavity of the automotive body is analyzed and some conclusion is drawn to applied some theory to design these types of the micro perforated plates of the automotive body.

An electromechanical low frequency panel sound absorber

The sound absorbing properties of a thin micro-perforated plate (MPP) coated with piezoelectric material with shunt damping technology is investigated. First a theoretical model is presented to predict the sound absorption coefficients of a thin plate attached with a piezoelectric patch and electrical circuits. Then the model is extended to analyze the sound absorption for a thin plate with micro perforations and piezoelectric material. Measurements are also carried out in an impedance tube and found to be in good agreements with the theoretical model. The sound absorption of the constructions can be much improved by tuning the electrical circuits.

Numerical simulation of passive–active cells with microperforated plates or porous veils

The main goal of this work consists in the numerical simulation of a multichannel passive–active noise control system based on devices involving a particular kind of cell. Each cell consists of a parallelepipedic box with all their faces rigid, except one of them, where a porous veil or a rigid micro-perforated plate (MPP) is placed. Firstly, the frequency response of a single passive cell is computed, when it is surrounded by an unbounded air domain (an anechoic room) and harmonic excitations are imposed. For the numerical solution of this three-dimensional problem, the original unbounded domain is truncated by using exact perfectly matched layers (PML) and the resulting partial differential equation (PDE) is discretized with a standard finite element method. Secondly, the passive cells are transformed into active by assuming that the opposite face to the passive one may vibrate like a piston in order to reduce noise. The corresponding multichannel active control problem is stated and analyzed in the framework of the optimal control theory. A numerical method is proposed to assess and compare different control configurations.

101 吸音微細多孔薄板音響連成振動系の挙動解明(騒音・振動の解析・改善(1),騒音・振動評価・改善技術)

101 吸音微細多孔薄板音響連成振動系の挙動解明(騒音・振動の解析・改善(1),騒音・振動評価・改善技術)

On droplet formation from capillary waves on a vibrating surface

The formation of sprays from a liquid film on a vibrating surface is used by ultrasonic atomizers for applications ranging from humidification to metal–powder manufacturing. The received opinion in the literature is that droplets are formed periodically from the apexes of an orderly pattern of standing capillary waves, with a wavelength that can be related to vibration frequency by stability analysis. It is described how this assumption may be incorrect in that, after droplet formation commences, the orderliness of the standing–wave pattern is lost due to one or more secondary instability phenomena. These phenomena, which lead to disorderliness, are investigated by using high–speed imaging techniques and a low–frequency vibrating film to model the high–frequency case, because of the difficulty of penetrating clouds of small droplets in the latter case. Different modes of droplet formation are identified and the flow patterns responsible for these modes are discussed. Physical mechanisms are proposed from which it is deduced that only a proportion of standing–wave crests can eject droplets, for a given wall–vibration period, and the identity of these ejecting waves should vary from period to period. The model thus developed demonstrates an apparently random ejection of droplets from one wave cell, even though the model itself is deterministic. The disorder of the capillary waves and the occurrence of several droplet–formation routes are sufficient to explain the range of droplet sizes that is produced by ultrasonic atomization.

PREDICTING THE ABSORPTION OF OPEN WEAVE TEXTILES AND MICRO-PERFORATED MEMBRANES BACKED BY AN AIR SPACE

Although theories for both membrane absorbers and micro-perforated plate absorbers have been well developed, it appears that no theory has been given for their combination, such as a glass-fibre textile or a micro-perforated membrane mounted over an airtight cavity. This paper presents a theoretical method for predicting the absorption of such a structure. The basic idea of the theory is to regard an open weave textile or a micro-perforated membrane as a parallel connection of the membrane and apertures. The predictions for both normal and random incidence have shown very good agreement with measurements. It has also been demonstrated that the absorption performance of such structures can be very high. Typically, with appropriate parameters the absorption coefficient of a glass-fibre textile or a micro-perforated membrane mounted at 100 mm from a rigid wall can exceed 0·4 over 3–4 octaves. With two layers of the material over the same total air space of 100 mm this range can extend to 4–5 octaves.

Comparison of Test Results for Flat-Plate, Transpired Flat-Plate, Corrugated, and Transpired Corrugated Solar Air Heaters

Five selective-black-metal-plate experimental air heaters with and without transpiration were designed, fabricated, and tested to compare their technical merits. All were 61 cm × 61 cm (2ft × 2ft), double-glazed with low-iron glass, and those with transpiration contained an additional Teflon film to form a plenum above the absorber plate. The configurations were (i) a reference collector with flow on back only, (iia) a transpired slotted plate, (iib) a transpired micro-perforated plate, (iii) a corrugated plate with flow on back, and (iv) a transpired slotted corrugated plate. The results show the magnitude of technical improvement that can be obtained through the slotting, perforating, and corrugating strategems.

Collection efficiency of aerosols by micro-perforated plates

For the first step of investigation of filtration mechanisms in membrane filters, the inertia-interception collection efficiency of a micro-perforated plate filter, such as a metallic perforated plate (MP) or a nuclepore filter (NPF), was calculated by assuming it to be an assembly of parallel pipes for various values of Reynolds number, Stokes number, porosity and interception parameter. Collection efficiency charts over the whole Re region were also prepared for convenient practical usage. The collection efficiencies of M P's and NPF's were measured for particles of polystyrene latex and stearic acid and compared with the calculated results. They agreed fairly well with each other, especially at low approach velocities, when neither particles nor filter were electrically charged.