Influence of Pore Size on the Acoustic Absorption Properties of Open-Cell AlSi Porous Cylinders.

Absorption of sound by acoustic materials is a critical issue for acoustical engineering. Applications included sound isolation, improving acoustical quality of room and buildingdesign. Knowing the acoustic properties of materials is important for designing such applications. The normal incidence sound absorption coefficient and other acoustic propertiesof materials can be found out by experiment with an impedance tube.The traveling microphone method is reliable for determining the sound absorption coefficient, however, this method is time consuming and also limited with single frequency operations. The two-microphone method is approximately 40 times faster than the traveling microphone method, but this technique requires more accurate instrumentation setup and experimentation. The setup and experiment procedures of the two-microphone method had discussed in this paper. Amplitude and phase mismatch between microphones can be a source of error. The microphone spacing was also found to be critical, accuracy of results decreased with large microphone spacing at high frequencies. An optimum 50mm microphone separation is suggested for multi-frequency signal range from 125Hz to 2000Hz.The experiment results from the two-microphone method had been compared with results from the traveling microphone method. Results were closed to each other within an interval of +/-0.10. For precision measurements, it is still recommended using the traveling microphone method. However, the two-microphone showed a very high potential can be as accuracy as the traveling microphone method. Suggestions had been given for improving the experiment accuracy.

Noise, vibration, and harshness (NVH) of a vehicle are important factors for vehicle users and essential for successful commercialization of HEVs that can provide both substantially improved ride-comfort and reduced energy-consumption. The noise and vibration behavior in hybrid vehicle is significantly different from the conventional vehicle. In conventional vehicles, certain noise phenomena are masked by the engine noise. However, when the combustion engine is turned off in hybrid vehicle, these noise components can become dominant and annoying. While driving, the low frequency interior noise below 2000 Hz causes the main component that irritates the auditory acoustic sense. In order to remove this low frequency noise, passive noise control methods were used using sound absorbing materials or isolating the noise source. However, it is found that the sound absorbing materials are effective only for high-frequency noise compared to low frequency noise. The present work deals with the sound absorption properties of open-porous Polylactic acid (PLA) material structures that were produced using 3D printing technology. The material's ability to damp sound was evaluated based on the normal incidence sound absorption coefficient and the noise reduction coefficient, which were experimentally measured by the transfer function method using an acoustic impedance tube. The present investigation showed that the 3D printed openpore structure are efficient to reduce the noise by absorbing sound.

Noise pollution is a negative factor that affects our environment. It is, therefore, necessary to take appropriate measures to minimize it. This article deals with the sound absorption properties of open-porous Acrylonitrile Butadiene Styrene (ABS) material structures that were produced using 3D printing technology. The material’s ability to damp sound was evaluated based on the normal incidence sound absorption coefficient and the noise reduction coefficient, which were experimentally measured by the transfer function method using an acoustic impedance tube. The different factors that affect the sound absorption behavior of the studied ABS specimens are presented in this work. In this study, it was discovered that the sound absorption properties of the tested ABS samples are significantly influenced by many factors, namely by the type of 3D-printed, open-porous material structure, the excitation frequency, the sample thickness, and the air gap size behind the sound-absorbing materials inside the acoustic impedance tube.

Wheat straw, which is a by-product of wheat production and has a tubular structure, is typically used for animal feed and compost. This study estimated the sound absorption coefficient of wheat straw based on cross-sectional computed tomography (CT) images. After image processing, the surface area of the wheat straw skeletal outline and the volume of the void area were determined. The propagation constant and characteristic impedance of the void area were obtained by approximating the clearance between two parallel planes representing the void area walls. Each CT image was represented as a transfer matrix to calculate the sound pressure and particle velocity, and the transfer matrix method was used to derive the normal incidence sound absorption coefficients. The measured tortuosity was considered when calculating the normal incidence sound absorption coefficient. The CT images were corrected to reflect the lack of sound absorption by the porous part of the thick-walled portion by considering it as a solid structure. The theoretical sound absorption coefficients calculated from the corrected images were in good agreement with the measured sound absorption coefficients.

Acoustic absorption modeling of single and multiple coiled-up resonators

Porous materials are widely used in acoustic absorption applications, including building acoustics and noise control, among others. Accurate characterization of the acoustic behavior of actual materials is essential for understanding sound absorption and predicting performance. However, current methods predominanlty depend on experimental techniques, which are resource-intensive and time-consuming. These approaches often fail to facilitate the identification of optimal solutions or explain why certain materials outperform others. This study addresses these limitations by validating a numerical approach for predicting the sound absorption properties of porous materials. High-resolution 3-D geometries are obtained using X-ray micro-computed tomography, and simulations using GeoDICT predict key parameters which are applied to the Johnson–Champoux–Allard model to estimate acoustic absorption. Numerical predictions are validated using two experimental approaches: a direct method measuring normal incidence sound absorption coefficients with an impedance tube, and an indirect method determining the materials’ acoustic properties, incorporated into the JCA model for predicting the absorption coefficient. The results show strong agreement between numerical simulations and experimental measurements, confirming the reliability of the numerical approach. This validated methodology holds promise for characterizing virtual porous materials that have yet to be fabricated, thereby enabling numerical optimization of porous structures.

Open-cell porous materials have been reported as a promising concept for mitigating turbulent boundary-layer trailing-edge noise. This manuscript examines the aeroacoustics of a porous trailing edge to study its noise reduction mechanisms. Numerical investigations have been carried out for a NACA 0018 aerofoil with three different types of trailing edge: a baseline solid trailing edge, a fully porous trailing edge and a blocked-porous variant in which a solid core is added at the symmetry plane. The latter prevents flow interaction between the two sides of the aerofoil. Flow-field solutions are obtained by solving the explicit, transient and compressible lattice-Boltzmann equation, while the Ffowcs-Williams and Hawkings acoustic analogy has been used to compute far-field noise. The porous material is modelled using an equivalent fluid region governed by Darcy's law, in which the properties of a Ni-Cr-Al open-cell metal foam are applied. The simulation results are validated against reference data from experiments. The regular porous trailing edge reduces noise substantially, particularly at low frequency, whereas the blocked variant retains similar noise characteristics as the solid one. By employing a beamforming technique, the dominant source is found at the trailing edge for the solid and blocked trailing edges, while for the fully porous one, the dominant source is located near the solid-porous junction. The analysis of the scattered sound suggests that the permeability of the porous trailing edge allows for acoustic scattering along the porous medium surface that promotes destructive interference, and in turn, attenuates far-field noise intensity. The spectra and spanwise coherence of surface pressure fluctuations at the trailing edge are hardly affected by the presence of the porous material, which are found to be insufficient to justify the noise reduction. The flow field inside the porous medium is also examined to explain the differences between the fully porous and blocked-porous trailing edges. While the mean velocity components are similar for both, substantial difference is found for the velocity fluctuations. The impedance of the porous medium is computed as the ratio of velocity and pressure fluctuations. Unlike the blocked variant, the impedance in the fully porous trailing edge gradually decreases along the downstream direction, which leads to the distributed noise scattering along the porous medium surface. Additionally, the scattering efficiency at the actual trailing edge location is reduced due to the smaller impedance discontinuity.

Aerodynamic effects of leading edge (LE) slats and slotted trailing edge (TE) flaps on NACA-2412 airfoil in prospect of optimization

A sound absorbing structure of multilayer porous metal materials backed with an air gap is proposed to enhance the normal incidence sound absorption performance. The new sound absorbing structures could improve the sound absorption coefficient in a low frequency range and decrease the fluctuation of the sound absorption coefficient in a high frequency range. The Johnson-Allard model is employed to study the sound absorption characteristics of the porous metal materials. And acoustic wave transmitting in the porous metal materials obeys the law of acoustic wave propagation. The sound absorption coefficient and normalized acoustic impedance of the multilayer porous metal materials with or without an air gap are analyzed theoretically, and then compared with existing experimental values. Furthermore, the influence of several physical parameters on sound absorption characteristics, such as the thickness of the air gap, the number of layers, thickness, porosity and flow resistivity of the porous metal materials, are evaluated. The results indicate that the sound absorbing structures of the multilayer porous metal materials backed with an air gap present a satisfying sound absorption characteristic over a wide frequency range.

The effect factors of acoustical properties and tire/road noise, such as cruising speed, vehicle categories, pavement surface type and pavement condition are uncertain. Many of the exiting prediction models for acoustical properties and traffic noise have still application limitations and accuracy problems. Therefore, acoustical properties and tire/road noise measured on laboratory and field test is conducted in this study. The equipments of impedance tube and sound intensity instrument are employed for measurement of sound absorption coefficient and tire/road noise, respectively. The GM (1, 2) with Fourier correction Grey Model (FGM) is introduced to predict normal incidence sound absorption coefficient and tire/road noise. The test results show the thinner porous surface has, the topper peak of sound absorption coefficient has at higher frequency. However, the thicker porous surface tend to lower frequency has more peak frequency than the thinner one. Generally, the maximum tire/road noise occurs between 800 Hz and 1200 Hz. Therefore, using the thin porous surface with high air void contents could be effectively reduction of tire/road noise. The Michelin tire compared to the Dunlop tire that has a 1.2 decibels (dB) differences at 1000 Hz frequency and 80 km/h cruising speed. The absolute mean error (AME) and regression analysis shows the GM (1, 2) with FGM is useful for making predictions of normal incidence sound absorption coefficient and tire/road noise

The normal incidence sound absorption coefficient of single-layered porous materials predicted using some prediction models is well known. The published acoustic behaviors prediction models, such as Biot model, Zwikker and Kosten model, Delany and Bazley model, and Champoux and Allard model, can give acceptable prediction results by only taking specific flow resistivity and material thickness as independent variables to estimate the normal incidence sound absorption coefficient. However, the existing literature fails to provide proper knowledge regarding the acoustic characteristics of dual-layered porous nonwoven absorbers. So, the aim of this paper was to propose a theoretical acoustic model for dual-layered porous nonwoven absorber and to verify the proposed model experimentally. In theory aspect, the study focused on the extension algorithm of the Zwikker and Kosten model for dual-layered nonwoven absorber. The theoretical analysis of the impact of thickness and porosity of outer and inner layer on sound absorption coefficient was detailed using numerical simulation method. In experiment aspect, we particularly designed 20 dual-layered nonwoven absorbers with four types of meltblown polypropylene nonwoven materials and five types of hydroentangled E-glass fiber nonwoven materials firstly. Secondly, the calculated sound absorption coefficients using the proposed model were compared with the measured ones of the 20 dual-layered nonwoven absorbers at 250, 500, 1000, and 2000 Hz. Experimental results indicate that the measured and the calculated data have very similar trend with the change of thickness, porosity, and the sound frequency, apart from the obvious difference between them at low frequency.

This study investigated polyurethane foam, a porous sound-absorbing material. Samples with and without membranes sealing the framework openings were imaged using nano-computed tomography (nano-CT), and the internal voids were modeled as slit-shaped gaps between two parallel planes. The normal incidence sound absorption coefficient was estimated using the transfer matrix method, incorporating the measured tortuosity using the ultrasonic method, and the theoretical results were compared with the experimental results. Experimentally, the normal incidence sound absorption coefficient was determined using a two-microphone impedance tube. Image processing techniques were applied to extract the skeletal cross-section and reduce residual noise from the CT images. Stable sound absorption coefficients were obtained for the original and membrane-removed foam samples through image processing of high-resolution CT scans, even with variations in binarization threshold. By accounting for tortuosity and adjusting the correction factor for the skeletal surface area, the theoretical estimates for the foam samples closely matched the experimental values.

In order to broaden the sound absorption bandwidth of single-leaf microperforated panel(MPP),The structure of MPP with partitioned cavity is introduced and its sound absorption performance is studied by impedance tube experiment. Results show that the MPP with partitioned cavity has two sound absorption peaks. Meanwhile, the sound absorption bandwidth is broaden and the sound absorption performance of the low and middle frequency is improved. Introduction With the development of economic construction, urban noise is more and more serious; the noise becomes an issue of popular concern. Reduce noise and create a beautiful acoustic environment is an important element to improve the quality of life of the people. A microperforated panel (MPP) absorber has become widely known as the most attractive alternative for the next generation sound absorbing material [1]. The MPP is first proposed by Maa, who has established its theoretical basis and design principle [2-4]. The MPP absorber is a thin panel or membrane less than 1mm thick with perforation of less than 1% perforation ratio with air-back cavity and a rigid backing [5]. The fundamental absorbing mechanism of the MPP absorber, which is typically backed by an air cavity and a rigid wall, is Helmholtz-resonance absorption [6]. This type of absorption is mainly due to frictional loss in the air flow of the apertures [6]. With the rapid development of processing technologies and computational methods, micro-perforated panel sound absorption theory has also been further development [7-9]. But usually the single-leaf MPP sound absorbing structure is generally only one resonance absorption peak. Although, the absorption coefficient of absorption peak is able to control above 0.9, but the sound absorption bandwidth is usually limited to about two octaves [2-4]. In order to broaden the absorption frequency range, Maa has proposed a double-leaf MPP backed by a rigid-back wall with an air-cavity [10]. Recently, Asdrubali and Pispola have studied this type of absorber for its application to noise barriers [11]. This absorber is intended to produce two resonators so that a broader absorption frequency range can be obtained. The acoustical properties of a structure composed of two parallel MPP with an air-cavity between them and no rigid backing is studied numerically by Sakagami [12]. Although lots of researches have been done on broaden the absorption frequency range of MPP, it appears that no relevant reports have been given for the MPP with partitioned cavity. The main purpose of this paper will focus on the acoustic performance studies of MPP absorbers with partitioned cavity. Structure of this paper will be arranged as follows: In Section 2, sample of the MPP with partitioned cavity will be shown. In Section 3, the sound absorption performance of MPP absorber without a rigid backing will be studied. Finally, the conclusions will be given in section 4. Sample construction MPP absorbers with partitioned cavity are constructed by a MPP and a structure of partitioned cavity. MPP used is aluminum panel 100mm in diameter. Thick, porosity and aperture for the MPP are 0.8mm, 1% and 0.8mm, respectively. Figure 1 shows the structure of MPP. The structure of International Industrial Informatics and Computer Engineering Conference (IIICEC 2015) © 2015. The authors Published by Atlantis Press 345 partitioned cavity is added to MPP by attaching. Figure 2 shows the structure of MPP with partitioned cavity. The deep of partitioned cavity is 30mm. Figure 1. microperforated panel Figure 2. microperforated panel with partitioned cavity Impedance tube experiments We study the property of MPP with partitioned cavity by impedance tube experiments. Figure 3 is the test system of impedance tube. Figure 4 compare the normal incidence sound absorption coefficient of the single-leaf MPP and MPP with partitioned cavity. The deeps of backing cavity are 50mm in impedance tube experiments. Through figure 4, we find that the single-leaf MPP has one sound absorption peaks at 756 Hz. However, the MPP with partitioned cavity has two main sound absorption peaks at 536Hz and 1336Hz respectively. In addition, there also exists another peak 0.579 at 936 Hz on the structure of the MPP with partitioned cavity. The reason may be the vibration of partitioned plate. Meanwhile, the maximum sound absorption peak of MPP with partitioned cavity is 0.988 is higher than 0.960 of the single-leaf MPP’s. The sound absorption coefficient of the MPP with partitioned cavity over 0.4 in the frequency range 256-1600Hz, which is much wider than 396-1416 Hz of the single-leaf MPP. If the partitioned cavities are well designed, it will improve the sound absorption performance of the partitioned cavity. Figure 3. The test system of impedance tube

Experimental evaluation and modelling of the sound absorption properties of plants for indoor acoustic applications

The Team’s approach into the Definitive Study of Acoustic Absorption Measurement involved three methods; the Acoustic Impedance Tube, the Large Reverberation Room and the Small Reverberation Chamber. The particular interest and primary focus of this study is to obtain the sound absorption coefficient, α of the various acoustic materials that were tested and compare the results between the various methods involved. There are currently two methods in measuring the energy absorption properties of acoustic materials; the impedance tube (standing wave ratio or two-microphone method) and the large reverberation room. The objective of the Small Reverberation Chamber is to explore the feasibility of performing acoustic absorption measurements in a smaller, cheaper and mobile environment with the aim of obtaining suitably close sound absorption coefficients which are comparable to those obtained from a standard large reverberation room. The large reverberation room requires space and specific construction and the high cost incurred in acquiring such a facility provides a considerable limitation. The small reverberation chamber however, is cheap in construction materials and does not require heavy and precision construction and is easily dismantled and assembled for transportation purposes to the test site for setup and experimentation. The chamber plywood planks were transported from the University of Queensland’s Mechanical Engineering workshop to the Acoustics lab for setup. The plywood planks were joined to the chamber base with 90° angle brackets. The sides and roof of the chamber were held together by self-tapping screws. Three-quarters of the roof were fixed to the sides to provide added stability and support, leaving a removable opening for access into the chamber for setting up test equipment. Three materials were tested, Soundguard Sound Sorber/PU Foam and CSR Bradford ‘Supertel’ and ‘Ultratel’ Glasswool. The test results were compared to those obtained from the large reverberation room at ACRAN, a noise control and air-conditioning company at Richlands and the acoustic impedance tube which was housed in the Acoustics lab in the Division of Mechanical Engineering. The tests were performed at one-third octave bandwidth frequencies ranging from 100Hz to 5000Hz. The reverberation time, T60 of the test materials in the small chamber was found to be substantially shorter as compared to the large reverberation room as was expected from previous studies. The Bradford Glasswool displayed a shorter reverberation time than the Sound sorber foam implying that the former had better absorption properties than the latter. The sound absorption coefficients were calculated to be reasonably similar at low frequencies of up to 250Hz in comparison to the acoustic impedance tube, the large reverberation room and the data from the material suppliers. However, the coefficients were found to be varying significantly at the medium and high frequencies. This shortcoming could be due to a number of reasons such as the intended use of the software and its application, and experimentation setup. The GNU Octave Band program adopted in this investigation was found to be limited by the smallest reverberation time measured and this affected the resulting sound absorption coefficient which was found to be significantly varying from the material supplier’s test data. It was initially intended to compare both the GNU Octave Band and Maximum Length Sequence System Analyser (MLSSA) programs. However, the PC card for the MLSSA was not functioning and the decision was made to focus on the analysis of the test results based on the GNU Octave Band program which was used for the first time in small chamber testing instead. The results from the GNU program were varying from of the MLSSA program. As such, this opens up a possibility of a further investigation in the near future to verify the validity of the MLSSA program which was adopted in a previous study and which gave reasonably satisfactory results; and to propose the use of an upgraded version of the MLSSA program in comparing the sound absorption properties of acoustic materials found in a small chamber.