Search Results (37)

To address the technical bottleneck of balancing the wideband noise (100–800 Hz) and aerodynamic performance of multi-blade centrifugal fans in air conditioning indoor units, this paper proposes a “frequency-domain target-driven” parametric design method for micro-perforated silencers. This method began with the precise identification of the 1/3-octave band noise spectrum to determine two noise peak clusters—200–400 Hz and 700–800 Hz—as the target suppression frequency bands. Based on Ma Dayou’s micro-perforated plate theory, a reverse mapping chain of “target frequency range → resonance frequency → structural parameters (hole diameter, perforation rate, rear cavity depth)” was established to enable the quantitative calculation of the silencer’s geometric parameters. Addressing engineering constraints related to manufacturing precision, clogging prevention, and structural stiffness for the theoretically optimal aperture size (0.24 mm), the aperture was adjusted to 1.0 mm through iterative recalculation of the relationship between plate thickness and perforation rate, while maintaining the perforated plate constant k ≈ 1.40 to ensure that the theoretical sound absorption frequency band remained unchanged. Test results show that under semi-anechoic chamber conditions (background noise ≤ 10 dB(A)), the micro-perforated silencer maintains airflow and power without attenuation across all airflow rates; noise peaks in the 200–400 Hz and 700–800 Hz frequency bands are significantly suppressed, and the full-band spectrum tends toward flatness. The average total noise level was reduced by 1 dB(A), and the average peak sound pressure level was reduced by 4 dB(A); the reduction in peak levels was four times that of the total reduction, revealing that the noise reduction mechanism of this method is “frequency-selective resonant absorption” rather than “uniform attenuation across the entire frequency band.” This study provides a quantifiable and reproducible design process for micro-perforated silencers, offering methodological support and engineering references for the development of low-noise compact fan systems in household appliances. Full article

The valorization of agricultural residues and post-consumer plastics is critical for achieving a circular economy. This study presents a sustainable pathway to fabricate eco-friendly acoustic panels by melt-blending recycled high-density polyethylene (rHDPE) with 10–50 wt% rice straw waste-derived nanofillers. Multi-stage chemical refinement (10% NaOH mercerization and H₂O₂ bleaching) before ball milling isolated nanofibrils under 50 nm. XRD analysis showed a crystallinity index increase from 37.0% (untreated) to 67.2% (bleached), confirming amorphous phase removal. FTIR and SEM verified successful delignification and excellent interfacial wetting. Consequently, the 50 wt% bleached cellulose composite exhibited the highest reinforcement, increasing flexural strength by 126% and flexural modulus by 132.6% over neat rHDPE. The hydrophilic framework enhanced environmental biodegradability, yielding a 15.18% maximum weight loss after a 90-day soil burial test, providing a viable end-of-life alternative to persistent synthetics. To optimize acoustic utility, a 1.76% geometric micro-perforation ratio was engineered into the panels. Backed by a 6 cm air cavity, the 50 wt% untreated composite achieved an outstanding peak sound absorption coefficient of 0.98 at a low frequency of 400 Hz. These findings establish these high-filler bio-nanocomposites as high-performance, low-carbon alternatives for noise control in construction and automotive infrastructure. Full article

The use of sound absorption materials has traditionally been restricted to medium-to-high frequencies due to their limitations at low frequencies, where the large wavelength of sound waves imposes rather bulky solutions. However, recent materials and designs allow for the absorption of sound waves with more practical sizes and weights, reviving interest in this frequency range. Some of these low-frequency absorbers, also named acoustic metamaterials or sub-wavelength sound absorbers, based on microperforated panels, are reviewed in this article. These include multilayer and multicavity microperforated panels, hybrid passive–active absorbers, multiple Helmholtz resonators, and microperforated panels with labyrinthine cavities or sonic black holes. Full article

Noise in long highway tunnels and underground interchanges poses a significant environmental concern, affecting both drivers and nearby residents. This research develops an acoustic finite element model of a long tunnel in Leuven Measurement Systems (LMS) Virtual Lab to characterize the tunnel noise field, and the effectiveness of different noise mitigation measures was also evaluated and optimized accordingly. The model is validated against in situ monitoring data, with deviations controlled within 3 dB(A) and strong agreement confirmed by the Kappa consistency test. Both simulations and measurements show that sound pressure levels (SPLs) are generally highest near the tunnel center and lower toward the portal, exhibiting periodic fluctuations rather than a monotonic decrease. The dominant noise energy is concentrated between 125 Hz and 500 Hz. SPLs at 1.8 m above the road surface are noticeably higher than at 1.2 m and 1.5 m, indicating greater noise exposure for drivers of large vehicles compared with smaller vehicles. Noise reduction performance is further assessed for different lining materials and pavement types. Installing sound-absorbing panels in the tunnel midsection provides effective attenuation, with expanded perlite panels, single-layer metal micro-perforated panels, and FC quiet perforated panels (FC-PP) performing best, while porous asphalt shows superior noise reduction compared with conventional dense-graded asphalt pavements. Full article

Underwater acoustic coatings are widely used to suppress low-frequency noise radiation and sonar reflection in underwater vehicles. In this study, an underwater acoustic coating model consisting of viscoelastic rubber layers and micro-perforated panel (MPP) structures is investigated, with particular emphasis on the low-frequency sound absorption mechanism and predictive modeling. Based on an improved transfer function method, a novel Micro-Perforated Panel Acoustic Coating Layer (MPPACL) model is developed to describe the coupled acoustic behavior of multilayer coatings under underwater conditions. The low-frequency sound absorption performance is primarily governed by the viscoelastic characteristics of the rubber layer, including material damping and complex modulus, while the incorporation of the MPP further enhances absorption through resonance effects. To efficiently explore the relationship between structural parameters and acoustic response, an ensemble learning-based deep neural network (ELDNN) is constructed using analytically generated data, enabling both forward prediction of sound absorption performance and inverse prediction of structural design parameters. The results show that the frequency prediction accuracy of the IDNN model is 3.7 times that of the DNN model. Furthermore, the proposed MPPACL model has achieved a significantly enhanced sound absorption effect within the frequency range of 50 to 2000 hertz. This effect has also been further verified through underwater experiments. The proposed framework provides an efficient and reliable approach for the design and optimization of underwater acoustic coatings. Full article

Unlike classical multi-layered micro-perforated panels (MPPs), which rely on sub-millimeter orifices for sound dissipation, we propose a multi-layered porous Helmholtz resonators absorber. It consists of alternately layered perforated porous material panels and perforated rigid panels with millimeter- to centimeter-scale orifices, primarily relying on porous materials for sound energy dissipation. Theoretically, perforated porous material panels are modeled as homogeneous fluid layers using double porosity theory, and the total surface impedance is derived through bottom-to-top impedance translation. A double-layered prototype was tested to validate the theoretical and numerical models, achieving near-perfect absorption peaks at 262 Hz and 774 Hz, with a subwavelength total thickness of 11 cm and a broadband absorption above an absorption coefficient of 0.7 from 202 Hz to 1076 Hz. Simulations of sound pressure, particle velocity, power dissipation, and sound intensity flow confirm that Helmholtz resonances in each layer enhance sound entry into resistive porous materials, causing absorption peaks. Parameter studies show this absorber maintains high absorption peaks across wide ranges of orifice diameters and panel thicknesses. Finally, an optimized triple-layer porous Helmholtz resonators absorber achieves an ultra-broadband absorption above a coefficient of 0.95 from 280 Hz to 1349 Hz with only 16.5 mm thickness. Compared with conventional MPPs, this design features significantly larger orifices that are easier to fabricate and less susceptible to blockage in harsh environments, offering an alternative solution for low-frequency and broadband sound absorption. Full article

This study presents a predictive model for estimating the sound absorption coefficient of perforated and non-perforated wooden panels, based on experimental data. Measurements were conducted on four wood species: fir wood (Abies alba), pine wood (Pinus sylvestris), pedunculate oak (Quercus robur), and sessile oak (Quercus petraea) in three panel thicknesses (11 mm, 18 mm and 25 mm), with perforation ratios of 0%, 10%, and 20%. The normal-incidence absorption coefficient was measured using the impedance tube method in accordance with ISO 10534-2. Measurements were performed in a 100 mm impedance tube, selected to match the specimen dimensions; therefore, the analysis is limited to the valid plane-wave frequency range of this tube, between 250 and 1600 Hz. Previous studies have shown that both panel thickness and perforation ratio significantly influence mid- and high-frequency absorption. Our results confirm that increased panel thickness and perforation enhance absorption, consistent with findings reported for micro-perforated and porous wood panels. Based on the measured values, we developed first-order regression functions linking the absorption coefficient to material density, thickness, and perforation percentage. The resulting equations allow reverse estimation of one or more physical parameters to meet target acoustic performance requirements. This data-driven approach provides a practical tool for designing wooden absorbers with predictable behavior and complements existing analytical models for acoustic optimization. Full article

Broadband sound absorption has long been a concern in noise control engineering, but the inverse design of multi-parallel microperforated panels (MPPs) for broadband sound absorption remains challenging. To address this issue, we propose a deep learning model that combines a variational autoencoder (VAE) with a physics-informed neural network (PINN) to accelerate the inverse design process of a multi-parallel MPP. Following Maa’s theory, we generated a dataset of 500,000 samples to train the model. By incorporating the PINN, we added an acoustic physical constraint to the loss function, promoting model convergence and the derivation of stable, unified parameters. The efficacy of the inverse design model was validated through theoretical analysis, finite element simulations, and impedance tube experiments. The experimental results show that the average sound absorption coefficient of multi-parallel MPPs within the frequency range of 500–1200 Hz is 0.85. Our work contributes to accelerating the inverse design of multi-parallel acoustic metamaterials. Full article

This study examines the characteristic parameters required for non-circular-hole microperforated panels (MPPs) to achieve sound absorption performance comparable to that of conventional circular-hole MPPs. Through numerical analysis, the flow resistivity and perforation ratio were found to be key parameters influencing the absorption characteristics of MPPs with square and equilateral triangular holes. The results indicate that for square-hole MPPs, matching either the flow resistivity alone or both the flow resistivity and perforation ratio to those of circular-hole MPPs leads to similar sound absorption characteristics. In contrast, for equilateral triangular-hole MPPs, both the above parameters must be matched to ensure comparable performance. Furthermore, this study explores MPPs incorporating a combination of circular and non-circular holes. It was confirmed that by appropriately matching the flow resistivity and perforation ratio, such mixed-hole MPPs can achieve sound absorption characteristics similar to those of MPPs composed solely of circular holes. These findings contribute to the broader design possibilities of MPPs, providing a foundation for optimising hole geometries in practical applications where manufacturing constraints or aesthetic considerations may necessitate non-circular hole patterns. Full article

Laminar flow offers significant potential for increasing the energy efficiency of future transport aircraft. At the Cluster of Excellence SE²A—Sustainable and Energy-Efficient Aviation—the laminarization of the wing by means of hybrid laminar flow control (HLFC) is being investigated. The aim is to maintain the boundary layer as laminar for up to 80% of the chord length of the wing. This is achieved by active suction on the leading edge and the rear part of the wing. The suction panels are constructed with a thin micro-perforated skin and a supporting open-cellular core structure. The mechanical requirements for this kind of sandwich structure vary depending on its position of usage. The suction panel on the leading edge must be able to sustain bird strikes, while the suction panel on the rear part must sustain bending loads from the deformation of the wing. The objective of this study was to investigate the energy absorption properties of a triply periodic minimal surface (TPMS) structure that can be used as a bird strike-resistant core in the wing leading edge. To this end, cubic-sheet-based gyroid specimens of different polymeric materials and different geometric dimensions were manufactured using additive manufacturing processes. The specimens were then tested under quasi-static compression and dynamic crushing loading until failure. It was found that the mechanical behavior was dependent on the material, the unit cell size, the relative density, and the loading rate. In general, the weight-specific energy absorption (SEA) at 50% compaction increased with increasing relative density. Polyurethane specimens exhibited an increase in SEA with increasing loading rate, as opposed to the specimens of the other investigated polymers. A smaller unit cell size induced a more consistent energy absorption, due to the higher plateau force. Full article

Hybrid Laminar Flow Control (HLFC) is a promising technology for reducing aircraft drag and, therefore, emissions and fuel consumption. The integration of HLFC systems within the small space of the wing leading edge, together with de-icing and high lift systems, is one of the main challenges of this technology. This challenge can be tackled by using microholes along the outer skin panels to control suction without the need for an internal chamber. However, microperforations modify the mechanical properties of titanium sheets, which bring new challenges in terms of wing manufacturability. These modified properties create uncertainty that must be investigated. The present paper studies the mechanical properties of micro-drilled titanium grade 2 sheets and their modeling using the Finite Element Method (FEM). First, an experimental campaign consisting of tensile and Nakajima tests is conducted. Then, an FEM model is developed to understand the role of the anisotropy in sheet formability. The anisotropy ratios are found by combination of Design of Experiments (DoE) and the Response Surface Method (RSM); these ratios are as follows: 1.050, 1.320, and 0.975 in the directions Y, Z, and XY, respectively. Some mechanical properties are affected by the presence of microholes, especially the elongation and formability that are significantly reduced. The reduction in elongation depends on the orientation: 20% in longitudinal, 17% in diagonal, and 31% in transversal. Full article

This study investigates lightweight and efficient candidates for sound absorption to address the growing demand for sustainable and eco-friendly materials in noise attenuation. Juncus effusus (JE) is a natural fiber known for its unique three-dimensional network, providing a viable and sustainable filler for enhanced sound absorption in honeycomb panels. Microperforated-panel (MPP) honeycomb absorbers incorporating JE fillers were fabricated and designed, focusing on optimizing the absorber designs by varying JE filler densities, geometrical arrangements, and MPP parameters. At optimal filling densities, the MPP-type honeycomb structures filled with JE fibers achieved high noise reduction coefficients (NRC) of 0.5 and 0.7 at 20 mm and 50 mm thicknesses, respectively. Using an analytical model and an artificial neural network (ANN) model, the sound absorption characteristics of these absorbers were successfully predicted. This study demonstrates the potential of JE fibers in improving noise mitigation strategies across different industries, offering more sustainable and efficient solutions for construction and transportation. Full article

Noise pollution is a major threat to the health and well-being of the entire world; this issue forces researchers to find new sound absorption and insulating material. In this paper, the sound absorption coefficient and vibration damping factor of panels manufactured from Cyperus pangorei rottb and ramie fiber reinforced with epoxy resin are explored. Cyperus pangorei rottb grass fiber and ramie fiber are widely available natural fibers. Cyperus pangorei rottb grass fiber is used in mat manufacturing, whereas ramie is widely used as a fabric. Using both of these fibers, six variant panels using a vacuum resin infusion process (VRIP) were fabricated. The panels were named C, R, CR, RCR-Flat, RCR-Curved, and RCR-Perforated. All the panels were tested for the sound absorption coefficient using an impedance tube with a frequency ranging up to 6300 Hz. Modal analysis was carried out by using the impulse hammer excitation method. A micro X-ray computed tomography (CT) scan was used to study the voids present in the panels. The results were compared among the six variants. The results show that the RCR-curved panel had the highest sound-absorbing coefficient of 0.976 at a frequency range between 4500 Hz to 5000 Hz. These panels also showed better natural frequency and damping factors. The presence of internal voids in these panels enhances sound absorption properties. These panels can be used at higher frequencies. Full article

Recently, increased attention has been given to minimize the effects of noise pollution on living beings. The attenuation and manipulation of sound waves with low-frequency components are quite difficult with traditional absorbers due to inherent properties induced by large wavelengths and yet are particularly critical to modern designs. In this study, a parallel arrangement of a coiled-up space cavity and micro-perforated panel (MPP) is considered as the absorber configuration. The coiled-up space consists of a front panel with an orifice and a rigid backing panel enclosing an arch-shaped concentric channel. The entire coiled-up space length is provided with two varying cross-sections. By this arrangement, the sound path is squeezed into a reasonably small volume enabling sound absorption at low frequencies. A thin panel with numerous perforations is the main constituent of MPP. It is backed by an air cavity and terminated by a rigid backing. Here in this configuration, micro perforations are provided on the front panel of the coiled-up space, which ensures simultaneous entry of acoustic waves into the micro-perforations and coiled-up space structure. The absorption characteristics of the present configuration are studied numerically and analytically. The combined structure with parallel combination of coiled-up space and MPP resulted in the abatement of more than 70% of sound in the frequency range of 321 Hz to 853 Hz. The present absorber has only a 5.5 cm thickness, which is subwavelength$\left(\frac{\lambda }{19}\right)$ also. Full article

A challenging issue is currently the design of non-fibrous ultra-thin acoustic absorbers that are able to provide broadband performance in demanding environments. The objective of this study is to compare using simulations and measurements the broadband absorption performance of highly porous micro-capillary plates (MCPs) to that of micro-perforated panels (MPPs) under normal incidence while considering unbacked or backed configurations. MCPs are unusual materials used for sound absorption with micron-sized channels and a high perforation ratio. Impedance-based modeling and Kundt tube experiments show that MCPs with suitable channel diameters have a pure constant resistance that outperforms the acoustic efficiency of MPP absorbers. Unbacked MCPs exhibit a controllable amount of high absorption that can exceed 0.8 over more than five octaves starting from 80 Hz, thereby achieving a highly sub-wavelength absorber. MCPs still provide broadband high absorption when backed by a rigid cavity. Their bandwidth-to-thickness ratio increases toward its causal limit when the cavity depth decreases. A parallel MCP resonant absorber partly backed by closed and open cavities is proposed. Such MCP-based absorbers could serve as short anechoic terminations for the characterization of acoustic materials at low frequencies. Full article