Sound Absorption Measurements Research Articles

Less Is More: Hollow-Truss Microlattice Metamaterials with Dual Sound Dissipation Mechanisms and Enhanced Broadband Sound Absorption.

Being a lightweight material with high design freedoms, there are increasing research interests in microlattice metamaterials as sound absorbers. However, thus far, microlattices are limited to one sound dissipation mechanism, and this inhibits their broadband absorption capabilities. Herein, as opposed to improving performances via the addition of features, a dissipation mechanism is subtractively introduced by hollowing out the struts of the microlattice. Then, a class of hollow-truss metamaterial (HTM) that is capable of harnessing dual concurrent dissipation mechanisms from its complex truss interconnectivity and its hollow interior is presented. Experimental sound absorption measurements reveal superior and/or customizable absorption properties in the HTMs as compared to their constitutive solid-trusses. An optimal HTM displays a high average broadband coefficient of 0.72 at a low thickness of 24mm. Numerically derived, a dissipation theorem based on the superimposed acoustic impedance of the critically coupled resistance and reactance of the outer-solid and inner-hollow phases, across different frequency bands, is proposed in the HTM. Complementary mechanical property studies also reveal improved compressive toughness in the HTMs. This work demonstrates the potential of hollow-trusses, where they gain the dissipation mechanism through the subtraction of the material and display excellent acoustic properties.

Deviations in sound absorption measurements in the reverberation room and impedance tube

Deviations in sound absorption measurements in the reverberation room and impedance tube

A Study on the Sound Absorption Properties of Mycelium-Based Composites Cultivated on Waste Paper-Based Substrates.

Mycelium-based composites have the potential to replace petrochemical-based materials within architectural systems and can propose biodegradable alternatives to synthetic sound absorbing materials. Sound absorbing materials help improve acoustic comfort, which in turn benefit our health and productivity. Mycelium-based composites are novel materials that result when mycelium, the vegetative root of fungi, is grown on agricultural plant-based residues. This research presents a material study that explores how substrate variants and fabrication methods affect the sound absorption properties of mycelium-based composites grown on paper-based waste substrate materials. Samples were grown using Pleurotus ostreatus fungi species on waste cardboard, paper, and newsprint substrates of varying processing techniques. Measurements of the normal-incidence sound absorption coefficient were presented and analyzed. This paper outlines two consecutive acoustic tests: the first round of experimentation gathered broad comparative data, useful for selecting materials for sound absorption purposes. The second acoustic test built on the results of the first, collecting more specific performance data and assessing material variability. The results of this study display that cardboard-based mycelium materials perform well acoustically and structurally and could successfully be used in acoustic panels.

Noise Reduction in the Cab of a Special Vehicle

Noise is highly associated with adverse effects on health, the human psyche and performance. Current special vehicles do not possess sufficient technologies to suppress the transmission of noise or vibration, which typically results in loss of control, comfort, driving safety, the performance of tasks, etc. At the same time, noise reduces attention and work efficiency while increasing fatigue, leading to hazards, dangerous situations, and missions that may not be completed. This article briefly presents selected parts of a project which included additional soundproofing of a special armoured mobile vehicle on a TATRA wheeled chassis. Basic theoretical, experimental, and practical information about this project is presented. To reduce noise, a selected damping material was used, which is a combination of recycled PUR foam and black rubber with a rough structure. The damping material was selected based on repeated experimental testing of sound absorption measurements from several damping materials. The material was chosen for suitable damping effects and corresponding technological properties (resistance to high temperatures, non-flammability, etc.). In the engine compartment and the cab of the vehicle, the damping properties were experimentally verified after retrofitting, while the noise was significantly reduced.

Estimation of Sound absorption Coefficient from the Sound Power

Sound absorption measurements were carried out in the reverberant room in accordance with ISO354. The present paper gives an experimental technique to estimate the sound absorption coefficient of a material based on the sound power in the reverberation room according to ISO 3741. This is in order to be able to calculate the sound absorption coefficient of the material needed to reduce the sound pressure levels in the room. Depending on Newton’s (iteration) method inferred equivalent sound absorption area and sound absorption coefficient will be obtained from the sound power equation Measurements were taken on four different materials to test the method's validity (wood, rubber, Gypsum, and polyurethane sponge). For boor sound-absorbing materials, acceptable results were obtained for the corresponding sound absorption area. Due to the mutual dependency on both the properties of the material and the parameters of the reverberant environment, there was an unstable discrepancy in the results of the sound absorption coefficient for high sound-absorbing materials. The relative standard deviation for sound absorption was 0.00 while the uncertainties for sound power was 0.7dB

Reducible Experimental Deviations between ISO 354 and ASTM C- 423

The main standards used to determine the sound absorption coefficient (α) of materials in the reverberation room are ISO 354 and ASTM C- 423. Really there is a difference between the two standards. The materials with the highest sound absorption determined by ASTM C423 standards (αTM) increases more than that determined by ISO 354 standards (αIS) and decreases as the ability of the material decrease to absorb sound, as in the case of wood. The number of source locations has no effect on the sound absorption of materials. To reduce the difference between ISO (αIS) and ASTM (αTM) methods for sound absorption measurements, a new idea of a sample area was proposed with 9.36m2 and only two locations for sample placement in the reverberation room. The sound absorption coefficient (αNA) of (9.36m2 ) approximates the findings of the sound absorption coefficient in highly absorbent materials with ISO standards method, and vice versa for low absorption materials, when the sound absorption coefficient results converge with ASTM standards. For all situations of source locations (2, 3, 4), the sound absorption average (SAA) established by ASTM standards is greater than that determined by ISO standards for high sound-absorbing materials (rockwool (k), polyurethane sponge (s)), and vice versa for weakly absorbing materials (wood (w)). In the reverberation room, the relative standard deviation ranged from 0.01 to 0.04.

A New Methodology Based on Cell-Wall Hole Analysis for the Structure-Acoustic Absorption Correlation on Polyurethane Foams.

Polyurethane foams with a hybrid structure between closed cell and open cell were fabricated and fully characterized. Sound absorption measurements were carried out in order to assess their acoustic performance at different frequency ranges. The cellular structure of these systems was studied in detail by defining some novel structural parameters that characterize the cell wall openings such as the average surface of holes (Sh), the number of holes (h), and the area percentage thereof (%HCW). Therefore, these parameters allow to analyze quantitatively the effect of different structural factors on the acoustic absorption performance. It has been found that the parameters under study have a remarkable influence on the normalized acoustic absorption coefficient at different frequency ranges. In particular, it has been demonstrated that increasing the surface of the holes and the percentage of holes in the cell walls allows increasing the acoustic absorption of these types of foams, a promising statement for developing highly efficient acoustic insulators. Additionally, we could determine that a suitable minimum value of hole surface to reach the highest sound dissipation for these samples exists.

Finite Element Multi-Physics Analysis and Experimental Testing for Hollow Brick Solutions with Lightweight and Eco-Sustainable Cement Mix

Combining eco-sustainability and technological efficiency is one of the “hot” topics in the current construction and architectural sectors. In this work, recycled tire rubber aggregates and acoustically effective fractal cavities were combined in the design, modeling, and experimental characterization of lightweight concrete hollow bricks. After analyzing the structural and acoustic behavior of the brick models by finite element analysis as a function of the type of constituent concrete material (reference and rubberized cement mixes) and hollow inner geometry (circular- and fractal-shaped hollow designs), compressive tests and sound-absorption measurements were experimentally performed to evaluate the real performance of the developed prototypes. Compared to the traditional circular hollow pattern, fractal cavities improve the mechanical strength of the brick, its structural efficiency (strength-to-weight ratio), and the medium–high frequency noise damping. The use of ground waste tire rubber as a total concrete aggregate represents an eco-friendlier solution than the ordinary cementitious mix design, providing, at the same time, enhanced lightweight properties, mechanical ductility, and better sound attenuation. The near-compliance of rubber-concrete blocks with standard requirements and the value-added properties have demonstrated a good potential for incorporating waste rubber as aggregate for non-structural applications.

On the 50th anniversary of ASTM E33, a history of ASTM International standards on building and environmental acoustics

This year marks the 50th anniversary of ASTM International Committee E33 on Building and Environmental Acoustics which combined all ASTM activity in these areas into one committee, though work on ASTM standards for measurement of sound absorption and transmission had started prior to 1950. This presentation will discuss the history of the committee and ASTM International standards in acoustics including the leadership that has made it possible.

Modeling of reverberation chambers for sound absorption measurements using a diffusion equation

The sound field in reverberation chambers used for measuring sound absorption coefficients is usually non-diffuse, leading to inaccuracies and disagreements in the results. In this paper, a diffusion equation model is applied to simulating reverberation chambers in order to obtain reverberant energy distributions and sound energy flows of the chamber under investigation in a more efficient way than wave-based simulation models. The computational efficiency of the diffusion equation lies in the fact that the meshing condition of the simulation domain is dictated by the room's mean-free path length (MFPL). Further investigation shows one-twelfth of MFPL as the meshing condition is considered practically sufficient for obtaining random incidence absorption coefficient of standardized sample sizes. With the computational efficiency given by a meshing of up to twelfth MFPL, an inversion calculation of random incident absorption coefficients of highly absorptive materials is possible with a mixed boundary condition [Jing and Xiang, JASA 123, pp. 145–153 (2008)]. Based on analysis of both chamber-based measurements and simulated data, ways to scrutinize measurement results of absorptive materials are investigated.

Broadband measurement of normal-incidence sound absorption coefficient with eight-microphone method

Broadband measurement of normal-incidence sound absorption coefficient with eight-microphone method

FE analysis of urethane foam using X-ray CT data

Polyurethane foam for sound absorption is used to reduce noise in automobiles, houses, home appliances, etc. The Biot-Allard model is generally used to calculate the sound absorption characteristics of urethane foam, which is used as a sound absorbing and insulating material for automobiles. Biot parameters used in calculations are measured with a dedicated measuring instrument, but it is difficult to connect them to actual material development. In order to clarify the sound absorption mechanism, 3D data of the urethane skeleton was obtained using an X-ray CT device. The FE model of the skeleton and surrounding air was created from the 3D data of the skeleton, and vibroacoustic analysis was performed. We report the results of comparison between the results of normal incidence sound absorption coefficient measurement using acoustic tubes and the calculation results of the FE model, and the results of various parameter studies.

Measuring and modelling the effect of base zone on sound absorption of persian rug

The aim of this study is to investigate the role of base zone on rug sound absorption and to predict the acoustic behavior of typical Iranian rugs using a mathematical model. For this purpose, four rug samples (1.5 cm thickness) were produced with different base structures, so that the base warp yarns are divided into two equal groups and arranged from each other at a different adjustable distance (0.0, 0.1, 0.2 and 0.3 cm). Sound absorption measurements were carried out using an impedance tube. Then, in order to focus on the base zone and to separate the piles contribution from the global rug sound absorption, the pile yarns were shaved from all samples and the non-piled samples were measured again. The five-parameter model of Johnson-Champoux-Allard (JCA model) was fitted to experimental data, assuming the rug is a two-layer (pile + base zone) porous absorber. The agreement between the model and the experiments is satisfactory. This optimized model can be used to predict the sound absorption coefficient of Iranian rugs. The results indicate that the rug base plays an important role in the overall rug sound absorption and that the air gap between the two groups of warp yarn improves the rug sound absorption.

Fabrication of cellulose-based aerogel for thermal and acoustic insulation applications

A method to prepare aerogels from rice straw and polyvinyl alcohol in the presence of fiberglass and glutaraldehyde is herein reported. The morphology, pore structure and physical properties of the aerogels were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), thermal conductivity, water contact angle (WCA) measurements, oil adsorption and sound absorption measurements. The obtained aerogels should be considered as a promising material for upcoming applications, since it has high porosity (up to 93.72%), low density (0.083-0.127 g/cm3), super low thermal conductivity (0.032-0.048 W/mK), high Young modulus (0.201-1.207 MPa), high sound absorption (absorption coefficient of 0.87) and a good oil adsorption capacity (4.8 g/g).

Acoustic absorption modeling of single and multiple coiled-up resonators

This work proposes an improved design procedure of sound absorption metamaterials consisting of acoustic coiled-up quarter-wave resonators that fit into a limited space, in order to broad the range of possible applications even in the low-frequency regime. Different types of models were used to understand the propagation of sound in long, narrow, and square-section tubes, including the visco-thermal loss within the structures for accurate modeling. An analytical model, which includes the definition of the different loss contributions with equivalent fluid models, has been developed: the Johnson-Champoux-Allard model for inlet losses and the “Narrow Region” model for visco-thermal loss along the tubes. A geometric parameterization procedure, for maximizing acoustic absorption at certain target frequencies, was carried out. To validate the theoretical acoustic absorption predictions, the 3D printing technique was used to fabricate samples in a plastic material (PLA) for normal incidence sound absorption coefficient measurements. Moreover, systems containing several quarter-wave resonators of different dimensions coupled in parallel or in series were designed, with the aim of broadening the absorption band; in this case, for practical reasons, only the medium-high frequency regimes were experimentally investigated.

On the Determination of Acoustic Properties of Membrane Type Structural Skin Elements by Means of Surface Displacements

The article focuses on the determination of the acoustic properties (sound transmission loss, sound absorption and transmission coefficient under acoustic plane wave excitation) of membrane-type of specimens by means of a combination of incident plane wave sound pressure and membrane surface displacement information, measuring the sound pressure with a microphone and the membrane displacement by means of a laser Doppler vibrometer. An overview of known measurement methods and the theoretical background of the proposed so-called mobility-based method (MM) is presented. The proposed method was compared with the conventional methods for sound transmission loss and absorption measurement in the impedance tube, both numerically and experimentally. Finite element model (FEM) simulation results of two single layer membrane samples of different shape configurations were compared, amongst which six different variations of the backing wall termination. Four different approaches to determine the sound transmission loss and two methods to determine sound absorption properties of the membranes were compared. Subsequently, the proposed method was tested in a laboratory environment. The proposed MM method can be possibly used to measure the vibro-acoustic properties of building parts in situ.

Improving the performance of double-expansion chamber muffler using dielectric beads; optimization using factorial design.

Noise pollution is a common health hazard worldwide which is emitted along with chemical air pollutants, simultaneously from many sources. Some studies have been conducted to control these pollutants, simultaneously with promising results being achieved. Dielectric beads have been used in air pollution control technologies, successfully and probable effectiveness of them in noise reduction can be promising in dual use of them in the exhausts emitting noise and air pollution, simultaneously. In order to investigate the effectiveness of dielectric beads in noise reduction, two types of them; namely glass and ceramic beads, were placed separately inside the connecting tube of a double-expansion chamber muffler. Then the transmission loss (TL) of muffler was examined using impedance tube. A factorial design was used to evaluate and optimize the effect of noise related parameters on TL of such a system. Results show that the presence of dielectric beads has significant effect on TL of muffler. The maximum TL was obtained as 74.76dB for muffler with ceramic beads, under the optimal condition of 5250Hz and 120dB. Measurement of TL and sound absorption coefficient (SAC) of glass and ceramic beads showed that the noise reduction in muffler with ceramic and glass beads is probably due to SAC in ceramic beads and noise reflections in glass beads, respectively. These results promise the dual use of dielectric beads in the exhausts emitting noise and air pollution simultaneously.

System error calculation and analysis of underwater sound absorption coefficient measurement experiment

Measuring the sound absorption coefficient by the impedance tube method is an important step in the research and development of the underwater sound-absorbing materials and structures. Generally, the experimental results are more persuasive than the theoretical results are. However, the system errors caused during the preparation and installation process of the sample to be tested are often ignored, which will affect the reliability of the experimental results. In order to reduce the system errors, three factors that may cause the system errors, including the slit length and width, the inclination angle and the surface finish, are proposed in this paper. First, a finite element (FE) simulation experiment environment of the underwater sound absorption coefficient measurement by the impedance tube method is established and the correctness of the FE algorithm is verified. Then the effects of the related parameters on the sound absorption coefficient of rubber are studied. When the sound absorption coefficient is within a certain acceptable error range, the restrictions of the related parameters are given. The achieved conclusions have reference value for reducing the system errors and improving the reliability of the experimental results.

Measurement and theoretical analysis of sound absorption of simple cubic and hexagonal lattice granules

A structure that possesses a continuous pore space packed with a granular material exhibits acoustic characteristics based on the principle similar to that of a porous material. Such a structure is used in low-noise pavement and ballast track. Therefore, predicting the sound absorption characteristics of a clearance generated in a granular material from the particle diameter and the physical property of gas is useful for engineering. This article deals with two packing models, namely, the simple cubic lattice model and the hexagonal lattice model, to measure the sound absorption coefficient of the structure packed with a granular material whose particle diameter is a few mm. Furthermore, a simple and accurate theoretical analysis that uses a one-dimensional transfer matrix method, which does not require numerous parameters, was performed. Comparing experimental values, when the particle diameter of the granular material was 4 mm, the percentage of the boundary layer in the clearance was moderately larger than when the diameter was 8 mm; therefore, the effect of viscosity on it was larger, and its sound absorption coefficient peak was higher. Moreover, compared with the simple cubic lattice, the hexagonal lattice had a higher percentage of boundary layer in the clearance because of its smaller volume of pore space. Then, the influence of friction due to the viscosity on it was higher, and its sound absorption coefficient peak was higher. In addition, the theoretical values obtained by the proposed analysis method typically matched the experiment values.

Numerical study of the impact of reverberation room design and test parameters on sound absorption measurements

The measurement of sound absorption in reverberation rooms following the ISO 354:2003 standard relies on Sabine's equation to derive absorption coefficients from reverberation times. This equation assumes perfect diffusivity, i.e. the sound field is composed of many statistically independent plane waves with uniformly distributed spatial phases, themselves uncorrelated to the corresponding amplitudes. In this work, both existing and fictitious reverberation rooms are numerically modelled using the finite element method. Finite porous absorbers are introduced in the rooms as equivalent fluid models. Standardized sound absorption measurement are simulated in the rooms through the determination of reverberation times. The respective effects of the sample size, sample placement, source positioning, and presence of finite panel diffusers are investigated. The resulting absorption coefficients are then confronted to the theoretical values in a perfectly diffuse sound field, that interacts with a baffled, finite-sized absorber, as obtained with a hybrid deterministic-statistical energy analysis model. The process notably underlines the strong, yet often disregarded, beneficial effect of panel diffusers at low frequencies in highly regularly-shaped rooms. Another conclusion of this work is that reverberation room design represents a crucial factor that can influence sound absorption measurements at low frequencies.