Micro-perforated Panel Absorber Research Articles

Effects of the backing cavity on the acoustic absorption of a microperforated panel absorber

The attractive features of the microperforated panels (MPPs) make them an alternative to conventional fibrous materials for acoustic absorption in environment where high hygiene standards are required. To evaluate the absorption effect, a model for the MPP with a backing cavity, i.e. MPP absorber (MPPA), has to be developed. Usually, a MPPA model is developed based on the one-dimensional assumption such that only the backing cavity modes associated with the depth contribute to the resonance absorption. For the real case, however, a MPPA is of finite-size and the modes associated with the lateral dimensions of the backing cavity will contribute to the coupling as well. In this paper, efforts are made to understand the coupling of different types of backing cavity modes to the MPP. It is found that the participation of those backing cavity modes not only influences the acoustic property of the MPPA but is also possible to deteriorate the absorption performance.

Effect of Fabric Layer on Sound Absorption of Micro-Perforated Panel

Micro-Perforated Panel (MPP) absorber becomes an alternative to common fibrous porous absorber without owning health and environmental issues. The system is considered as the next generation of absorber in noise control. However, such a system suffers from narrow absorption bandwidth so that its application becomes limited. Theoretically, the frequency range with effective absorption is determined by the surface acoustic impedance controlled by the perforation ratio, hole diameter and cavity depth. Hence, by keeping those parameters to be constant, an intervention to the change of impedance is expected to be useful in widening the absorption bandwidth. In this research, textile materials from woven fabrics namely cotton fabric, plain fabric and satin fabric were used to cover the surface of Micro Perforated Panel (MPP) absorber either at the front or at the back surface. The sound absorption coefficients were measured for the normal incidence using an impedance tube. From the measured results, it is found that the presence of the fabric on MPP surface can improve the MPP absorber in terms of amplitude as well as the frequency bandwidth of absorption. Moreover, the placement of the woven fabric at downstream area is more beneficial than at the upstream area.

Structure design of low-frequency broadband sound-absorbing volute for a multi-blade centrifugal fan

A novel low-frequency broadband sound-absorbing structure is proposed for the noise reduction of an existing multi-blade centrifugal fan in range hoods. Incorporating the advantages of both sound absorption materials and micro-perforated panel absorbers in series and in parallel with unequal cavity depths, the compound absorber is designed to suppress and absorb the radiated noise mainly in the wide- and low-frequency range below 2 kHz. Parametric studies for influences of structural parameters on sound absorption coefficient and absorption bandwidth are performed with numerical method, respectively for sound absorption materials and micro-perforated panel absorbers in series and parallel arrangements. Due to the non-uniform directivity of far-field sound radiation, the optimal installation position of the compound sound-absorbing structure at the volute, the helical surface or the two lateral side planes, is investigated as well. Taking the actual structures of the centrifugal fan, special working environment of the range hood, processing cost and difficulty into consideration, a practical low-frequency broadband compound absorber with an elaborate selection of structural parameters is designed and applied to the appropriate positions. Results of the acoustic test in the anechoic chamber according to the Chinese national standard indicate that the A-weighted sound power level on the monitoring spherical surface meets the acoustic target of lower than 64.0 dB(A) under restrictive conditions of large flow rate, high pressure and small size.

Experimental study of a compact piezoelectric micro-perforated panel absorber with adjustable acoustic property.

This work aims to investigate the acoustic characteristics of a piezoelectric micro-perforated panel (MPP) absorber, which is made of a perforated polyvinylidene fluoride (PVDF) film with a backed airgap of 2 cm, as a combination of an active component and passive absorber. In addition to its inherent passive dissipation, as the PVDF-MPP was driven with proper voltages and oscillation frequencies, sound absorption coefficients of the absorber adjacent to the driving frequencies were significantly increased. Compared with mostly previous reported hybrid passive-active absorbers, this one is more compact, and its acoustic property is adjustable, it may provide an approach to achieve intelligent noise control.

Investigation of the acoustic properties of corrugated micro-perforated panel backed by a rigid wall

Extensive studies have been carried out on the acoustic performance of a micro-perforated panel (MPP) absorber which typically consists of a flat MPP fitted in front of a rigid backing wall. The present study concerns its acoustic properties when the conventionally flat perforated panel is replaced with a corrugated one that finds many applications in architecture. A three-dimensional finite element model is used to simulate the acoustic performance of a such corrugated MPP absorber at normal incidence, oblique incidence and diffuse field. Results show that the absorption performance of corrugated MPPA can be very different from conventional flat MPPA when the acoustic wave length is smaller than the corrugation depth. At low frequencies, the corrugated MPP performs similarly as a flat one of the same bulk perforation ratio. The absorption levels of the corrugated MPPA are considerably enhanced at the dips of absorption curves, which is favorable for reverberation control or broadband random noise reduction in large spaces and buildings. Further modal analysis shows that multiple modes of the corrugated configuration are excited at the peak and dip frequencies, and the acoustic response by the non-resonating modes give rise to the performance improvement. Absorption coefficients are also measured experimentally using an impedance tube. The measured normal incidence absorption coefficients compare well with the numerical predictions, which validates the numerical model and, in part, the numerical findings.

Coefficients of End Correction Depending on Geometric Properties of Micro-perforated Panel

The purpose of this paper is to calculate an accurate sound absorption coefficient for a micro-perforated panel absorber (MPPA). When the sound absorption coefficient is calculated, there is a difference between the measurement and the calculation. In the calculation, a resistive end correction coefficient (β) of 2 or 4 and a reactive end correction coefficient (δ) of 0.85 are generally applied. However, using an impedance tube test, we confirm that end correction coefficients change depending on the specification of the micro-perforated panel. The resistive end correction coefficient is in the range of 2–4 and increases when the hole diameter and the porosity decrease and the panel thickness increases. The reactive end correction coefficient is less than 0.85 and increases when the hole diameter and the panel thickness increase and the porosity decreases. Based on our experimental results, we derive new coefficient formulas expressed by the hole diameter (d), the hole spacing (b), the porosity (p) and the panel thickness (t). To verify the proposed formulas, the sound absorption coefficient calculation is compared with the measurement, which confirms that the absorption coefficient is affected by the ratio between the air mass inside the hole and the added air mass outside the hole. The relation between the two masses can be confirmed by the ratio of the panel thickness (t) to the hole diameter (d). In a single-layer MPPA, when t/d is greater than or equal to about 2, the absorption peak level of calculations, no matter what β is applied, is similar to the measurement. On the other hand, when t/d is less than about 2, the calculation is greatly affected by β. The sound absorption coefficients for both single- and double-layer MPPAs are compared according to the proposed formulas and coefficients of other researchers. The calculation of the proposed formulas among calculations is in good agreement with the measurement.

Wideband sound absorption of a double-layer microperforated panel with inhomogeneous perforation

Micro-perforated panel (MPP) absorber is increasingly gaining popularity as an alternative sound absorber in buildings compared to the well-known synthetic porous materials. A single MPP has a typical feature of a Helmholtz resonator with a high amplitude of absorption but a narrow absorption frequency bandwidth. To improve the bandwidth, a single MPP can be cascaded with another single MPP to form a double-layer MPP. This paper proposes the introduction of inhomogeneous perforation in the double-layer MPP system (DL-iMPP) to enhance the absorption bandwidth of a double-layer MPP. Mathematical models are proposed using the equivalent electrical circuit model and are validated with experiments with good agreement. It is revealed that the DL-iMPP produces a wider half-absorption bandwidth compared to the conventional homogeneous double-layer MPP. The absorption bandwidth to higher frequencies can be effectively controlled by reducing the air cavity between the iMPPs and to the lower frequencies by increasing the back cavity depth at the second layer.

Natural Ventilation Performance Simulation and Analysis of New Type Sound Insulation Window

To evaluate the ventilation performance of new type sound insulation window, a window model with streamline-structure micro-perforated panel absorber (MPA) was simulated based on computational fluid dynamics (CFD). Finite element model of the insulation window was built to analyze the flow field in muffler channel, velocity in vent and pressure loss of the channel. To contrast with the simulation model, an insulation window artifact was produced and tested in ventilation quantity detection. The results show that the dominant pressure loss is local hydraulic loss that comes from MPA structure. The total pressure loss increased by 4.3 Pa under 0.5 m/s inlet wind velocity. The difference of pressure loss between simulation and actual test is 6.3%. CFD simulation is an effective and low-cost method to evaluate the ventilation performance with visual results. The results provide valuable reference for optimal design of insulation window.

Sound absorption of face-centered cubic sandwich structure with micro-perforations

This paper presents a novel ultra-lightweight micro-perforated sandwich structure with face-centered cubic (FCC) core as a sound absorber, which encompasses excellent mechanical features as well as acoustic properties. The structure, which consists of air cavities and small perforations on both the top panel and the corrugated plates, is considered as a combination of multi-layered micro-perforated panel absorbers (MPPAs). A theoretical model that makes use of an equivalent electrical circuit (EEC) approach is established. Validation of the model with experimental results and numerical simulations shows good agreement. The dependence of the acoustic absorption on features of the perforations and of the FCC structure is discussed. Based on the observations, an optimized structure is proposed by using simulated annealing method to yield good acoustic absorption property in the low frequency range.

Nonlinear sound absorption of ultralight hybrid-cored sandwich panels

Abstract A combined experimental, numerical and analytical approach is employed to investigate the nonlinear effect of incident sound pressure level (SPL) on the sound absorption performance of a novel ultralight sandwich panel with perforated honeycomb-corrugation hybrid (PHCH) core. Built upon the motion, continuity and heat conduction equations for compressible viscous fluids, the numerical model fully considers the nonlinear effects caused by incident sound wave with high SPL. The analytical model is constructed by using an approximation solution to the characteristic impedance of micro-perforated panel (MPP) absorbers with consideration of the compressibility of the fluid inside the perforation. The validity of both the numerical and analytical models is checked against experimental measurement results. The effects of facesheet thickness, corrugation thickness and core height on sound absorption are systematically explored. The proposed PHCH sandwich construction is ultralight. It has small total thickness and possess simultaneous load-bearing, energy absorption and sound absorption properties, showing great potential in multi-functional applications. This work provides a nonlinear analytical model and a nonlinear numerical method for the sound absorption of the ultralight hybrid-cored sandwich structures, which demonstrates their superior performance against sound with high SPL.

Implementation experiment of a honeycomb-backed MPP sound absorber in a meeting room

This paper presents the absorption performance of honeycomb-backed microperforated panel (MPP) absorbers, which are needed to improve the acoustics of an existing 91 m3 small meeting room where the reverberation time is over two seconds from 250 Hz to 2 kHz, for comfortable speech communication. In Japan, MPPs are difficult to use combustible materials, particularly, panels with holes for interior walls owing to fire regulations. Therefore, we implemented the MPP absorber as an additional attachment that can be hung on walls and ceilings. First, the absorption characteristics of the MPP absorber were designed to reduce reverberation times at mid-frequency using an electroacoustical equivalent theory. Then, a wave-based finite element method simulation was used to determine the absorber placement. Absorption coefficients of the honeycomb-backed MPP absorber were also measured in an irregularly shaped reverberation chamber. Finally, the effect of the installed MPP absorbers was checked by acoustic parameter measurements. As a result, we observed that the reverberation time was reduced under 1.5 s in all frequencies, the equivalent absorption area doubled, the early decay time values were also reduced from 2.7 s to 1.0 s at 1 kHz, the clarity (C50,mid) increased by more than 5 dB, and the speech transmission index (STI) value increased from 0.55 to 0.67 by one rank up.

Sensitivity analysis of micro-perforated panel absorber models at high sound pressure levels

Sensitivity analysis is performed using a nonlinear acoustic impedance model of micro-perforated panel absorbers constituted by a micro-perforated panel bonded on a honeycomb structure in order to assess the impacts of the input parameters on the outputs of interest namely the normalized surface impedance and the sound absorption coefficient. The theoretical results predicted by the nonlinear impedance model show good agreement with experimental measurements performed at high sound pressure level using an impedance tube. The inputs of the model are the plate thickness, the orifice diameter, the perforation ratio, the cavity depth and the sound pressure level. For higher pressure levels excitations, it is demonstrated that the perforation ratio is the key parameter which affects significantly the resistance and the sound absorption of the absorber at the resonance while the impacts of the thickness and orifice diameter of the panel are negligible. In addition, the study confirms that the acoustic reactance is dominated by the cavity depth especially at low frequencies and the sound pressure level influences strongly the acoustic behaviour of micro-perforated panel absorbers. Therefore, the design of a micro-perforated panel absorber requires the knowledge of the sound pressure level and the perforation ratio control quality must be done carefully in order to get the target performances of the absorber.

A Study on Micro-Perforated Panel Absorber with Multi-Layered and Parallel-Arranged Structure to Enhance Sound Absorption Performance

Micro-perforated panel absorber (MPPA) can be manufactured using various materials and can be applied to various fields. There are restrictions on applying MPPA because of the narrow sound absorption bandwidth. To solve this problem, the design parameters are controlled or the MPPA is constructed in parallel or multilayer structures. However, It is not easy to make holes of diameter of 0.1 mm or less in a general method. If there are constraint conditions, MPPA may not be designed for the target frequency band. We propose MPPA combined with multi-layer structure and parallel arrangement to effectively enhance the sound absorption performance. A calculation method for the absorption coefficient of the proposed MPPA is derived and the validity of calculation results is confirmed through the experiments. The sound absorption characteristics according to change of design parameters and structural change are analyzed and the following results are obtained. First, MPP with one kind of hole and various cavity depths is more effective in enhancing the sound absorption performance than MPP with various types of holes and one type of cavity depth. Second, in double-layer MPPA, it is confirmed again that the absorption performance is improved when the hole diameter and porosity of the MPP from the source are higher than those of the MPP from a rigid wall. Third, when MPPA with multi-layer structure and parallel arrangement is designed, both MPP properties and cavity depths should be considered to achieve good performance of MPPA. Based on the results, we design the prototype MPPA. Result of reverberation room test shows that prototype has a good sound absorption performance. We will improve the prototype MPPA and conduct a field test to reduce railway noise such as squeal noise.

Broadband design of multilayer micro-slit panel absorbers for improved transparency using Bayesian inference

Multilayer Micro-Slit panels (MSP) are assessed for their potential as broadband absorbers that simultaneously maintain visual transparency. The late Dah-You Maa introduced micro-Slit panel absorbers as a continuation of his previously developed Micro-Perforated panel (MPP) absorbers. Like MPPs, Micro-Slit panels allow high absorption coefficients to be achieved without using fibrous materials but are limited to a narrow frequency bandwidth. Broadband absorption can be achieved by combining panels into a multilayer absorber. The complexity of determining the optimum set of parameters required to fulfill a design scheme necessitates implementation of the Bayesian framework. This probabilistic method automatically determines the most concise number of layers required and gives parameters for each layer of the resulting composite. Various slit patterns and algorithms introduce a family of solutions, which satisfy the design scheme, for optimization of visual transparency. The Micro-Slit samples can be fabricated and tested to validate acoustic performance and assess visual properties.

Sound absorbers with a micro-perforated panel backed by an array of parallel-arranged sub-cavities at different depths

This paper presents a type of wide band micro-perforated panel (MPP) absorber with an array of parallel arranged sub-cavities at different depths based on the quadratic residue sequence (QRS). An analytical prediction model is proposed for evaluating the normal incidence sound absorption coefficients of this type of MPP absorber in its design stage. Next, simulations with a finite element procedure and experimental measurements were conducted to provide cross-validations of the proposed analytical model and to facilitate the discussions on the sound absorption performance of this type of MPP absorber. The results show that the analytical model provides simple yet accurate prediction on the sound absorption performance of this MPP absorber; the absorption bandwidth of this type of MPP absorber can be significantly enhanced through the fine design on the sub-cavity depth sequence. It is also shown that the normal incidence absorption coefficient of this type of MPP absorber can remain beyond 0.5 in the frequency range of 450–3500 Hz, with the maximum value of 0.98.

Sound absorption performance of the acoustic absorber fabricated by compression and microperforation of the porous metal

Novel acoustic absorbers were fabricated by the compression and microperforation of the porous metal, which aimed to develop practical acoustic absorbers for the noise reduction. Sound absorbing coefficients of the five investigated acoustic absorbers were measured by the AWA6128A detector according to the standing wave method, and their trends were consistent with normal sound absorption principle of the porous metal absorber and that of the microperforated panel absorber. The results proved that with same length of the cavity, sound absorption performance could be obviously improved by the compression and microperforation. When length of the cavity was 20 mm, average sound absorbing coefficient of the compressed and microperforated porous metal panel absorber in frequency range 100–6000 Hz reached 59.69%, which was superior to that 25.70% of original porous metal absorber and that 31.49% of the microperforated spring steel panel absorber. In the constructed semi-empirical model, a fourth-order polynomial function was treated as the coupling function to express the superposition absorption effect, and its veracity and reliability was validated by two replication experiments. Micromorphology of the compressed and microperforated porous metal panel provided the intuitive explanations to the improvement of its sound absorption performance.

Design and manufacture of metal membrane microperforated panel absorbers

Metal Membrane MPP (Microperforated Panel) is a kind of promising acoustic absorber due to its steady absorption performance and the convenience in process. In order to reveal the effects of pivotal parameters on absorption coefficient, an acoustic model of MPP is derived based on classic theory. The optimized parameters are found using genetic algorithm by the aid of this model. The MPP samples made of aluminum are manufactured by laser drilling and then the absorption coefficient is measured in impedance tube. The result shows that the theoretical model can predicted the absorption behavior of metal MPP relatively accurately. And the peak absorption coefficient, peak frequency, and the half-absorption band width are about 0.9, 2000Hz and 3200Hz respectively.

Prediction of microperforated panel absorbers using the finite-difference time-domain method

Microperforated panel (MPP) sound absorbers give selective but relatively wide and high sound absorption. In addition, they are environmentally friendly, hygienic, and visually attractive. As such, MPPs are very promising alternatives for next-generation sound absorbers. The effects of MPPs have been predicted by analytical approaches as well as numerical techniques such as the finite element method, the boundary element method, and computational fluid dynamics. However, the effects have yet to be predicted by the finite-difference time-domain (FDTD) method. In this paper, the FDTD formulation is proposed as an option to predict the acoustic performance of MPPs. For the time-domain calculations, the frequency dependence of the transfer impedance for a hole in an MPP is approximated based on regression analyses, which removes the frequency dependence. Furthermore, the stability conditions for the MPP boundary are derived by considering the state transition equations. Comparisons of numerical and analytical results verify the formulation.

Theoretical model of absorption coefficient of an inhomogeneous MPP absorber with multi-cavity depths

Micro-perforated panel (MPP) absorber has been known as an alternative absorber to classical porous material given its facile installation, long durability, environmental friendliness and attractive appearance. Extensive studies of MPP absorber proposing the improvement of its absorption frequency bandwidth have been published. This study presents a MPP absorber introduced with inhomogeneous perforations and with multi-cavity depths. The MPP is divided into two sub-area, where each area has different hole diameter, perforation ratio and a separated backed cavity depth. The acoustic impedance is modelled using electrical equivalent circuit and the absorption coefficient is calculated under normal-incidence of sound. It is found that the inhomogeneous MPP can have good bandwidth of absorption by designing the sub-MPP of smaller perforation ratio with large hole diameter and the one having the larger perforation ratio with smaller hole diameter. The absorption bandwidth can be conveniently controlled by adjusting the cavity depth of each sub-MPP. The results from the experimental work show good agreement with the theoretical model.

A multilayer microperforated panel prototype for broadband sound absorption at low frequencies

Microperforated panel (MPP) absorbers are one of the most promising alternatives to porous sound absorbing materials. However, these structures cannot achieve high and broadband absorption at low frequencies. To be effective, once defined the material properties the geometrical parameters of the absorber need to be optimized to match the prescribed absorption level. This paper presents a multiple layer MPP absorber with a high sound absorption coefficient and broadband absorption at low frequencies. An electro-acoustical equivalent circuit model was used for a parametric analysis to study the relationships between the absorption mechanism and the absorbers geometrical parameters in the proposed multilayer MPP. A prototype of this absorber was machined and tested in an impedance tube test ring and the experimental acoustical properties in terms of absorption coefficient were extracted using the transfer function method. It was demonstrated that the five-layer MPP absorber was capable of guaranteeing a high absorption (constantly over 90%) in a frequency range from 400 to 2000 Hz. The results indicate that the proposed multilayer MPP absorber provides a good alternative for sound absorption applications.