Sound Absorption Measurements Research Articles

Measurement of sound absorption coefficient in a reverberant room using probability density function of damping constant

The measurement method of the sound absorption coefficient in a reverberation room is standardized in ISO 354. However, the measurement accuracy often deteriorates at low frequencies. This paper proposes a method that improves the measurement accuracy of the sound absorption coefficient at low frequencies. It calculates the sound absorption coefficient using reverberation time (RT) that is derived from the distribution of a damping constant for a sinusoidal input. The measured values by the proposed method were compared with those by the ISO 354 method. As a result, the proposed method reduces the spatial variability of RT and gives a better agreement with the statistical absorption coefficient that is calculated by a transfer matrix model at low frequencies.

Loudspeaker configuration in reverberation rooms for sound absorption measurement using room mode determination

For Sabine absorption coefficient measurements in reverberation rooms, it is preferable to excite as many room modes as possible. But the number of excited room modes is significantly affected by many factors, among which one dominating factor is the loudspeaker location particularly at frequencies below the Schroeder frequency. In turn, this could significantly influence the statistical absorption coefficient obtained according to ISO 354. Therefore, this study aims to investigate the impact of loudspeaker configurations on the excitation of room modes. Frequency response functions are measured using a broad band steady state noise signal using four sound sources at various locations, and the numbers of excited room modes in the one-third octave bands are quantified and compared against those by Green's function simulations. A procedure for determining favorable loudspeaker positions based on the excited room modes is proposed, which can be a useful input to the working group of the ISO 354 standard.

An optimization method for reverberation room design

The ISO 354:2003 standard relating to sound absorption measurements is currently under revision to improve the reproducibility of the procedure it describes. Round robin tests conducted across various reverberation rooms indeed revealed significant disparities between sound absorption measurements of the same sample. One of the reasons is that, at low frequencies, the sound field in a single laboratory cannot be considered fully diffuse. However, the average sound field across different laboratories may be considered diffuse if the interaction between the finite sample and the diffuse field is duly accounted for and the direct field close to the absorber is disregarded. In this work, a method is developed for optimizing reverberation room design such that measured absorption values are as close as possible to ensemble average diffuse values. The reverberation room is modelled using the finite element method and standardized measurements of an absorptive sample are simulated. The distance between resulting absorption coefficients and diffuse target values is minimized in an optimization procedure having the geometrical characteristics of the model as input parameters. The results are anticipated to participate to the revised ISO 354 as guidelines for the construction of new reverberation rooms or the improvement of existing ones.

Trial applications at gymnasiums of in-situ sound absorption measurement method by ensemble averaging technique

The authors have been published a series of papers on a measurement method for sound absorption characteristics of materials using ensemble averaging technique, i.e., EA method. The papers' results included measurement mechanisms, measurement uncertainty, and so on. Herein, to examine adaptability, especially in in-situ conditions, the EA method is applied to measure absorption characteristics of materials installed in two gymnasiums. A glass-wool panel with the dimension of 0.5 m by 0.5 m by 0.05 m and with the density of 32 kg m^-3 was brought around and measured to check the measurement consistency. Several measurements were conducted during badminton plays were undergoing. Measured sound absorption coefficients revealed that most results agree well with those measured in reverberation rooms. Certain improvement is necessary for the specimen brought to the in-situ measurement to keep the consistency. The inconsistency is considered to originate from unstable conditions between the specimen and floor.

Calculation of statistical absorption coefficient using ensemble averaged surface normal impedance of material

To predict and control the indoor sound field, it is important to comprehend sound absorption characteristics of building materials. In the previous studies, the authors have proposed an in-situ sound absorption measurement method of materials using ensemble averaging technique, namely EA method. The method yields a simple and efficient in-situ measurement of surface normal impedance of materials at random-incidence. In this paper, the authors calculate the statistical absorption coefficient using the surface normal impedance of material by the EA method to obtain random incidence absorption coefficient. At first, the procedure of calculating the statistical absorption coefficient from the normal impedance by EA method is described. Next, the sound absorption characteristics for five kinds of materials are measured by the EA method and the reverberation room method. Finally, the statistical absorption coefficients are calculated from results obtained by the EA method and are compared with absorption coefficients by the reverberation room method.

Molecular relaxation dynamics and self-association of dexamethasone sodium phosphate solutions.

Detailed concentration-dependent measurements of sound absorption and velocity have been performed in dexamethasone sodium phosphate (DSP) aqueous solutions in the MHz frequency range. A single well-resolved relaxation process dominates the experimental acoustic spectra following a Debye-type distribution function. The analysis of the temperature-dependent ultrasonic relaxation data also revealed analogous effect with concentration on the relaxation spectra. All acoustic parameters were estimated by means of a fitting procedure. The behavior of the relaxation frequency and amplitude with concentration allowed us to assign the observed process to self-association mechanism. Combining the ultrasonic and electric conductivity data, the self-association scheme has been established. The thermodynamic constants and the rate of the aggregation due to hydrophobic interactions have been estimated in view of the Eyring’s theory. The concentration dependence of relaxation amplitude and characteristic frequency revealed that the presence of additional relaxation processes in the spectra related to additional mechanisms, such as conformational changes and proton-transfer reaction is excluded and the self-association process considered here was found to dominate in this frequency range. The results have been discussed in view of the fair ability of DSP for hydrophobic interactions and aggregate formation in aqueous environment.Graphic abstract

Deterministic and statistical methods for the characterisation of poroelastic media from multi-observation sound absorption measurements

This paper proposes a framework for the estimation of the transport and elastic properties of open-cell poroelastic media based on sound absorption measurements. The sought properties are the Biot-Johnson-Champoux-Allard model parameters, namely five transport parameters, two elastic properties and the mass density, as well as the sample thickness. The methodology relies on a multi-observation approach, consisting in combining multiple independent measurements into a single dataset, with the aim of over-determining the problem. In the present work, a poroelastic sample is placed in an impedance tube and tested in two loading conditions, namely in a rigid-backing configuration and coupled to a resonant expansion chamber. Given the non-monotonic nature of the experimental data, an incremental parameter estimation procedure is used in order to guide the model parameters towards the global solution without terminating at local minima. A statistical inversion approach is also discussed, providing refined point estimates, uncertainty ranges and parameter correlations. The methodology is applied to the characterisation of a sample of melamine foam and provides estimates of all nine parameters with compact uncertainty ranges. It is shown that the model parameters are retrieved with a lower uncertainty in the multi-observation case, as compared with a single-observation case. The method proposed here does not require prior knowledge of the thickness or any of the properties of the sample, and can be carried out with a standard two-microphone impedance tube.

Modeling acoustic metamaterials based on reused buttons using data fitting with neural network.

Metamaterials are designed by arranging artificial structural elements according to periodic geometries to obtain advantageous and unusual properties when they are hit by waves. Initially designed to interact with electromagnetic waves, their use naturally extended to sound waves, proving to be particularly useful for the construction of containment and soundproofing systems in buildings. In this work, a new metamaterial has been developed with the use of a polyvinyl chloride membrane on which buttons have been glued. Two types of buttons were used, with different weights, placing them on the membrane according to a radial geometry. Each sample of metamaterial was subjected to sound absorption coefficient measurements using the impedance tube. Measurements were made using the samples by setting three configurations, creating a cavity with different thicknesses. The results of the measurements were subsequently used as input for training a simulation model based on artificial neural networks. The model showed an excellent generalization capacity, returning estimates of the acoustic absorption coefficient of the metamaterial very similar to the measured value. Subsequently, the model was used to perform a sensitivity analysis to evaluate the contribution of the various input variables on the returned output.

Extension of frequency range of the eight-microphone method in normal-incidence sound absorption coefficient measurement by eliminating influence of the first radial mode

Extension of frequency range of the eight-microphone method in normal-incidence sound absorption coefficient measurement by eliminating influence of the first radial mode

A versatile acoustic gas sensing method via extracting intrinsic molecular internal specific heat

Gas-coupled acoustic techniques show advantageous capability in analyzing gas mixtures. It is still a challenge to identify the intrinsic characteristics of gas compositions from the acoustic measurements. In this work, we propose an acoustic method to extract the intrinsic internal specific heat of gas composition from molecular relaxation for gas sensing. The internal specific heats of multi-relaxation process in a gas mixture are fast synthesized using measurements of sound speed and absorption at several fixed frequencies. The relative change of internal specific heat due to variation of environmental temperature is employed to identify gas species, and the ratio of the internal specific heat of mixture to that of corresponding pure gas determines the composition concentration. The robustness of the proposed gas sensing method is validated with multi-component mixtures. The acoustic sensing method provides an effective route to identify the intrinsic characteristics of gas compositions in unknown complex gas mixtures.

Results of the 10-Year Arizona Quiet Pavement Pilot Program

In April 2003, the Arizona Department of Transportation (ADOT) initiated a Quiet Pavement Pilot Program in cooperation with the Federal Highway Administration (FHWA). Under this program, many freeway segments in the Phoenix metropolitan area with Portland Cement Concrete pavement surfaces received 25.4-mm thick asphalt rubber friction course overlays to reduce highway-related traffic noise. This pilot program represented the first time that pavement type was allowed as a noise mitigation strategy on federally funded projects. As part of this program, ADOT developed a 10-year research program with FHWA to evaluate the long-term acoustic performance of this noise mitigation approach. The final measurements were completed in 2015. The program was done with three types of measurements: Type 1 examined tire/pavement noise at the source at 115 milepost locations; Type 2 examined noise in residential neighborhoods near the freeways; and Type 3 evaluated noise using direct measures of traffic noise adjacent to the freeways. Sound absorption measurements were also made at specific sites at various times throughout the project. Type 1 measurements documented an average initial reduction of 8.7 dBA and an average increase of 0.5 dB/year. The Type 2 measurements revealed an average initial reduction of 5.2 dBA, whereas the Type 3 measurements showed an average initial reduction of 9.1 dBA and an average increase of 0.5 dB/year. The measured reductions were also compared with ADOT and FHWA noise abatement criteria.

Optimization and Characterization of Polyurethane Electro-Spun Nano-Membranes Used for the Surfaces of Sound Absorbent Multi-Layer Sheets

With more and more attention paid to the quality of life, noise pollution has been ranked the third in environmental pollution. In this study a symmetrical multi-layer sheet was designed for a sound absorbent material. Needleless electro-spinning nano-membranes of polyurethane (PU) were used as the surfaces of the sound absorbent multi-layer sheets. The effects of the PU concentration, solvent ratio, membrane thickness and solvent additives on the surface morphology of the nano-membranes and the sound absorption coefficient of the sound absorbent sheets were carefully investigated by scanning electron microscopy and sound absorption measurements. The results showed that the optimal spinning solution was 10 wt.% PU and the solvents ratio of N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) at 1:1. Under this condition, the average sound absorption coefficient of the multi-layer sheets increased from 0.29 for the 0 mm nano-membranes to 0.57 for the 1.2 mm nano-membranes (spinning time 240 minutes). For the nano-membranes in the range of 0.2–1.2 mm, it was noted that the average sound absorption factor of the multi-layer sheets increased with the increasing of the nano-membranes’ thickness and the absorption frequency for which the highest sound absorption coefficient occurred moved to lower frequencies with the increasing of the membranes’ thickness. By adding sodium dodecyl sulfonate (0.5 wt.%) and sodium chloride (1.0 wt.%) into the optimal solution, a nano-membrane with a nano-cobweb structure of 100% coverage rate was formed. Under the spinning condition of 10 wt.% PU, 1.0 wt.% NaCl, 0.5 wt.% SDS, 1:1 DMF/THF and 240 minutes, a nano-cobweb multi-layer material with an average sound absorption factor of 0.63 was obtained. Although not a large increase from the sound absorption coefficient for the nano-membrane multi-layer sheet material of 0.57, the results showed that the multi-layer sheet with nano-cobweb layer had a great improvement of sound absorption in the range of 500–2000 Hz relative to the nano-membranes. Therefore, a good sound absorption sheet with multi-layer structures was obtained successfully.

Implementation of Sound Absorption Measurement Based on ISO and ASTM Standards in BSN

Implementation of Sound Absorption Measurement Based on ISO and ASTM Standards in BSN

The design of an impedance tube and testing of sound absorption coefficient of selected materials

The paper describes a design, building and testing a low-cost impedance tube for measuring the acoustic parameters of small testing samples. The measurement of sound absorption coefficient is based on transfer function and the two microphone method in accordance with ISO 10534-2 and ASTM E 1050 standards. With small configuration updates and after installation of additional microphones on measuring apparatus testing of complex acoustic material properties can be performed. The impedance tube has been used also as a teaching tool.

Sound absorption properties of rigid polyurethane foam composites with rubber-wood sawdust as a natural filler

Rigid closed-cell polyurethane (PU) foam is well-known for its ability to withstand external force. On the other hand, due to the closed-cell nature, its ability to absorb sound is doubtful since hollowed materials are supposed be better in sound absorption. However, the composites fabrication with non-reactive fillers was thought to improve sound absorption ability since it might prohibit the formation of closed cell in the process. In this research, rigid polyurethane foam was prepared from polyether polyol and polymeric methylene di-isocyanate (p-MDI) formulated with water as a blowing agent. Natural rubber-wood sawdust was used as a filler. The percentages of rubber-wood sawdust in the polyurethane foam composites were varied by 0, 1, 3, 5 and 7%. The measurement of sound absorption coefficient has been done using impedance tube technique. The results showed that the rigid closed-cell polyurethane foam with 0% filler exhibits poor acoustic absorption. Additionally, sound absorption coefficient results showed that a sample with higher amount of rubber-wood sawdust allowed the sound wave to be absorbed at wider frequency range. This could be explained that the rubber-wood sawdust filler played an important role in sound absorption property.

Investigation on acoustical properties, thermal stabilities and water sorption abilities of finger millet straw fibers, darbha fibers and ripe bulrush fibers

Finger Millet (Eleusine coracana) is a protein-rich plant popularly known as ragi in Karnataka. The literature cites many researchers studied the nutritive-value of its grains, kernels, and it's flour. And, few researchers tried to explore the utilization of the leftover plant for xylose production, briquet making, and so forth. Darbha (Desmostachya bipinnata) has been known popularly for its medicinal importance in the traditional Hindu society since Vedic times. The research was mainly focused on drug analysis, drug authentications, anti-inflammatory, antimicrobial, and analgesic potential of Darbha. Herein, the three readily available, commonly sourced and widely explored fiber types for their medicinal properties and nutritive gain are investigated for their sound absorption efficacies. The four types include finger millet (Eleusine coracana), darbha (Desmostachya bipinnata), and ripe bulrush fibers (Typha Domingensis). All fibers were monitored for their sound absorption properties, with sample thickness of 10 mm, in accordance with ISO − 10,534 (2). The effect of sample thickness on sound absorption behaviors of two fiber types yielding best results were performed by increasing the sample thickness to 20 mm. The two fiber types yielding best results were also monitored for their thermal stabilities, surface morphological characteristics and water sorption abilities. The effect of chemical treatment on the water absorption abilities and fiber surface morphologies on sound absorption abilities were also correlated. The combinatorial results drawn from the sound absorption coefficient measurements, thermal analysis and water sorption studies supports that peroxide treated finger millet straw fiber to be the best suitable fiber for noise reduction. Also, the observed appreciable sound absorption properties of peroxide treated finger millet straw fibers support their possible interior acoustical applications.

Evaluation of Colombian Crops Fibrous Byproducts for Potential Applications in Sustainable Building Acoustics.

Local production of construction materials is a valuable tool for improving the building sector sustainability. In this sense, the use of lignocellulosic fibers from local species becomes an interesting alternative to the development of such materials. As it is thought that the properties of fiber-based materials are dependent on the fibers properties, the knowledge of such properties is fundamental to promote materials development. This study compares the physical, morphological, acoustic, and mechanical characteristics of coir (Cocos nucifera) and fique (Furcraea Agavaceae) fibers and panels. The chemical composition appears to be associated with the general behavior of the fibers and panels, regarding higher tensile strength, thermal degradation behavior, and water absorption. In most tests, fique had the upper hand, showing superior performance; however, on thermal degradation and water absorption, both materials had similar behavior. The sound absorption measurement showed that the fiber diameter affects the sound absorption at high frequencies, where fique panels showed better performance than coir panels.

An experimental and theoretical investigation of coconut coir material for sound absorption characteristics

Abstract This is comprehensive study on sound absorption performance of coconut coir fiber using experimental and theoretical approaches. Samples of coconut coir fiber with different thickness and densities were prepared to get their effect on sound absorption coefficient. Measurement of sound absorption coefficient was done by using impedance tube set up based on ISO 10534-2 and ASTM E1050-98 standards. Delany- Bazley model was used to find effect of thickness and density. In this paper the result sound absorption coefficient of theoretical and experimental study presented and compared. The result of Delany- Bazley model shows promising results compared with experimental results up to 2700–2900 Hz, further deviation in results were observed. Sound absorption coefficient of 0.84 (2900 Hz) for 35 mm thick sample with density 220 Kg/m3 was recorded. Further Sound Transmission Loss (STL) for 21 mm, 28 mm, 35 mm thickness samples with 220 Kg/m3 were measured using transfer function method of impedance tube. It was established that coconut coir fiber samples effectively dissipitate the sound energy giving fair sound absorption properties.

Effect of the excitation signal type on the absorption coefficient measurement using the impedance tube

This work evaluates the effect of the excitation signal used when measuring the absorption coefficient on an impedance tube. This paper aims to offer some guidance on the selection of the excitation signal to perform sound absorption measurements using the impedance tube. Four possible excitation signals defined in ISO 10534-2 Standard were studied: two of them, random noise and two of them sine sweep signals. Some hypotheses tests were executed to assess the homogeneity of each measurement. The signal-to-noise ratio (SNR) was also computed to verify the measurement quality. The results show that the best performing approach was accomplished using a logarithmic sweep, giving more precise sound absorption curves with an SNR of 34.15 dB. Random signals reported similar SNR (greater than 30 dB) after executing an average with 100 repetitions.

Suppression technique of sound scattering at microphones in normal-incidence sound absorption coefficient measurement using 8-microphone method

Suppression technique of sound scattering at microphones in normal-incidence sound absorption coefficient measurement using 8-microphone method