Sound Absorption Coefficient Of Materials Research Articles

Sound absorption uncertainty in small reverberation chambers

Manufacturers in the mobility industry typically measure and specify the sound absorption coefficient of materials using a small reverberation chamber according to SAE J2883. These chambers have a 6 to 25 cubic meter volume, use a 1.0 meter x 1.2 meter sample size and report sound absorption in the one-third octave bands from about 400 Hz to 10,000 Hz. This is in contrast to a full-size chamber built to satisfy the requirements of ASTM C423 or ISO 354, which have about a 200 cubic meter volume, use a 2.4 x 2.7 or 3.2 x 3.2 square meter sample size and report sound absorption in one-third octave bands from 100 to 5,000 Hz. Although the room and sample size dimensions scale, many of the factors that contribute to uncertainty in the measurement do not scale. These include relative humidity, temperature, and other physical measurements and if the same instrumentation is used, microphone calibration and measurement uncertainty in the higher frequency range. This paper examines the potential measurement uncertainty in these chambers over the 400 to 10,000 Hz frequency range and compares this to past uncertainty analyses of full-size chambers over a similar frequency range.

Development of an empirical model for the prediction of the sound absorption coefficient for thin and low-density fibrous materials

Currently, FEA software such as ABAQUS uses empirical models to predict the sound absorption coefficient of poroelastic materials. However, based on a recent review of the literature it was found that the current sound absorption empirical models are inadequate for accurate prediction of thin (t < 20 mm), low-density materials (ρB < 50 kg/m3). Therefore, the predictions of the sound pressure levels in vehicle cabins, using such software, will be inaccurate since the trim materials are thin and have a low density. Thus, this research aimed to develop an empirical model that can accurately predict the sound absorption coefficient of these materials. Hence, polypropylene fibres consisting of four different diameters were manufactured and converted into nonwovens. Thereafter, airflow resistivity and impedance tube experimental testing were performed on the specimens. Subsequently, statistical analysis of the data was performed using SAS software. SAS was used to identify which independent variables should be included in the models to be developed. The empirical models were developed using the regression analysis toolbox in Microsoft Excel. Once the models were developed, various checks were performed to validate the assumptions of linear regression. The software NumXL was used to perform Cook’s distance tests. Thereafter, the models were validated against the validation dataset, where it was found that the developed exponential model performed best. Finally, the exponential model was compared to existing models using two data sets i.e. an internal dataset, and an external dataset derived from the literature. The developed model outperformed all the historic models on both datasets.

The feasibility of building an impedance tube to measure sound absorption coefficients of materials

This presentation focuses on the design and build of an impedance tube to measure sound absorption coefficients of materials. A practical approach was taken to develop an impedance tube from readily available ‘off the shelf’ materials, with the ultimate goal of determining feasibility and accuracy of such a build. The design theory from multiple sources will be discussed, as well as a detailed breakdown of the dimensions and materials used for the apparatus. Measurement methodologies, testing set-up and data processing techniques are also presented to contextualize the results. Finally, comparisons of the tube results are made to existing known sound absorption coefficient data of materials to ultimately assess the accuracy of the low-cost impedance tube and any deficiencies. Methods to improve the design of the tube are also explored, as well as future design improvements.

Practical applications at field environments of sound absorption measurement method using the ensemble averaging technique

To construct appropriate boundary conditions for wave-based room acoustics simulations, the authors have proposed a measurement method for sound absorption characteristics, surface impedance, and sound absorption coefficient of materials using the ensemble averaging technique. We have also shown the robustness as well as the reproducibility of the results obtained by the method at various environments. The method consists of two kinds according to sensor types employed so far: i.e., two-microphone and PU-sensor. Generally speaking, the method using a PU-sensor is more advantageous than one using a two-microphone because of its geometrical straightforwardness. However, at various field environments where environmental noise and/or wind blowing exist, the method using a two-microphone is expected to give more favorable results. In this paper, several applicational measurements of the method conducted at field environments using the two sensor types are summarized to show the plausibility of the method.

Sustainable multiple resonator sound absorbers made from fruit stones and air gap

This article investigates the sound absorption coefficient of materials manufactured from natural wastes. Fruit stones from some crops are one of the most available natural wastes in the Mediterranean Region. Recycled and vegetable products are becoming an interesting alternative to traditional materials to be used as sound-absorbing panels.Fruit stones can be profitable for a number of applications, such as biomass to produce energy. This research work intends to demonstrate that one of their applications can be ecological sound absorbers in building acoustics.Different four fruit stone samples, with different air gap volume percentages, display similar behaviour to multiple Helmholtz resonators (MHRs). By adding a 40 mm-thick rockwool layer, the sound absorption coefficients are compared for each sample.The experimental results allow establishing some analogies between MHRs and the new absorbing materials according to thickness, fruit type and the air gap volume. These fruit stones have been demonstrated as a good choice from acoustic and sustainable points of view.

Characterization of acoustic materials at arbitrary incidence angle using sound field synthesis

Standardized methods for measuring sound absorption such as the impedance tube and reverberation chamber methods are limited to normal or diffuse incidence, respectively. Two research axes have been generally followed in the literature to develop alternative techniques, the first one focusing on the measurement part, that is from the two-microphone technique to the use of microphone arrays or pressure-velocity sensors. The second axis focuses on the excitation part with for instance the use of sound field synthesis techniques. Since acoustic impedance and sound absorption coefficient of materials are classically defined under normal and oblique plane wave excitation, synthesizing an “ideal” plane wave using a loudspeaker array would allow measuring these acoustics quantities using a simple microphone pair. In this article, the effect of the different parameters of a loudspeaker array on acoustic plane waves reproduction on a material’s surface is first numerically studied. Then, numerical and experimental results for the estimation of both impedance and absorption coefficients are reported. These results show that sound field synthesis allows to characterize a material for arbitrary incidence angles over a wide frequency range, thus offering an alternative method to standard techniques and an improvement over existing works.

On the development of a virtual test bench to assess sound absorption of materials in an alpha chamber

A description is given of a virtual test-bench (VTB) designed by engineering center of Peter the Great St. Petersburg Polytechnic University to calculate a sound absorption coefficient of various materials. Developed VTB differs from known programs in that it allows a sound absorption coefficient of various materials to be determined with minimum involvement of an engineer. This VTB differs from other programs also by using infinite elements jointly with finite elements, which increases adequacy of the discrete model being used, and also the configuration of the boundary of an alpha chamber being used. The known programs use various phenomenological mathematical models of porous materials such as Johnson-Champoux-Allard (JCA) model. The VTB is based on fundamental mathematical models and statistical energy analysis (SEA) that allow describing adequately the established or transitional processes of sound absorption and reflection by a porous material the properties of which are not homogenized. The value of this VTB, which is created on the basis of a VA One software complex supplemented by a set of files of boundary conditions, files of solvers’ settings, secondary finite element (FE) models, is that VTB allows standardized calculations to be performed to determine the sound absorption coefficient of a material with minimum involvement of an engineer and the obtained result to be submitted for detailed processing. Developed virtual test-bench enables determination of a sound absorption coefficient for various materials within the entire finite frequency range. The result of the calculation is displayed as a graph of dependency of the material sound absorption coefficient on frequency.

Comparison of Static Wave Ratio and transfer Function Method in Determining the Sound Absorption Coefficient of Materials

Comparison of Static Wave Ratio and transfer Function Method in Determining the Sound Absorption Coefficient of Materials

The effect of changes in atmospheric conditions on the measured sound absorption coefficients of materials for scale model tests

The absorption of scale model materials is generally measured in a scaled reverberation chamber, with frequency scaled according to the scale factor. Moreover, the acoustic absorption of the chamber needs to be reduced, especially the absorption of air. It depends not only on the volume of the chamber, but also on the air parameters: pressure, temperature and relative humidity. Most frequently, in order to reduce the air absorption, the air inside the model is dried or replaced with dry nitrogen, or a digital air absorption compensation is used. Air-drying methods are time consuming and require specialised equipment; digital compensation may in turn result in errors, hence the question arises whether scaling the air absorption is really necessary to achieve good accuracy of the sound absorption coefficient measurements. In order to answer this question, the authors firstly ran a theoretical analysis of the issue based on ISO 9613-1 formulas describing air acoustic absorption. Based on this analysis, the influence of particular air parameters on the total air absorption in the model reverberation chamber, Aair (m2), was examined, together with the possibilities of scaling down the Aair values. This indicated, that the Aair may be scaled by manipulating temperature or relative air humidity. For the needs of the study, the relative air humidity was chosen, as it was used by the predecessors and may be easily manipulated. In the next step, the experimental studies of sound absorption coefficient measurements were conducted in a 1:8 scale reverberation chamber for representative specimens at different relative air humidity values (3–45%). The gathered data was then statistically analysed. The results showed that the influence of relative air humidity on the accuracy of sound absorption coefficient measurements in model tests is negligible and therefore such measurements can be performed in the ambient air humidity of the test room. As a consequence, a conclusion was drawn that the Aair value does not need to be scaled. Moreover, an attempt was made to identify the cause of the inaccuracy of the measurements – the analysis indicated that the values of the intensity attenuation coefficient m calculated according to ISO 9613-1, at model frequencies are not sufficiently accurate.

The influence of the distribution of sound absorbing materials on the estimation of reverberation time in rooms

Reverberation time in rooms depends on many factors, e.g. cubature, surface of envelopes, sound absorption coefficient of materials used for the construction of the envelopes, geometry of rooms or the distribution of sound absorbing materials. The arrangement of sound absorbing materials in rooms has an impact on the dispersion of acoustic field, yet theoretical calculation models do not take into account this impact. According to these models, regardless of the arrangement of sound absorbing materials, the reverberation time in a room will remain unchanged. The present paper investigates the above problem by means of computer simulations. For the needs of the simulation, three rooms with different dimensions were adopted, i.e. type 'p' - a cuboidal room with a square base, type 'd' - a cuboidal room (with one side of the 'p' room lengthened), type 'w' - a cuboidal room (with the height of the room lengthened 'p'). During the simulation, the way of acoustic field dispersion was being changed and its influence on the reverberation time in the rooms was being determined. The authors investigated two situations. The first one involved a non-dampened room, in which the sound absorbing material was being arranged differently. The second one involved a welldampened room, and the dispersion of sound field was analyzed depending on the location of the reflecting material.

Study of Sound Absorption Properties on Rigid Polyurethane Foams using FEA

Objective: The range of raw material used for the manufacture of Polyurethane using Polyol has grown enormously during the past fifty years. A wide range of products is now available, which allows the researchers to produce, collectively known as Polyurethane Foams as Porous Material. This paper examines the sound absorption properties of porous material from the view point of the manufacturer by the experimental and analysis way. Method/Analysis: In this experimental study, finite element method is well established in estimating acoustic transmission loss or sound absorption coefficient of a porous material. Impedance tube is used to measure the acoustic impedance of a sound absorbing porous material, with acoustic source at one end and an acoustic porous material. That is polyurethane foam at other end. There are two common methods used to measure the impedance value of the porous material. The first method involves a moveable microphone that transverses along the impedance tube and the second method is named as “transfer function” or “two-microphone”, the two microphone method is followed in this section. In this study, a 3D impedance tube is modelled using finite element software and the results will be compared with experimental data. Finding: The effort for this study determines a straight forward route to the production of Rigid Polyurethane foams through a direct reaction process. This system has potentially wide relevance as polyurethane foams. Here, an initial study has shown that using different way to calculate the sound absorption coefficient of a material, either by experimental way or by analysis using FEA software ansys. Application/ Improvement: Besides much other application, rigid polyurethane foam is used as acoustic layer in automobile industries, thermal insulation and as energy absorber medium in front bumper of auto motives. Hence by comparing the sound absorption coefficient value of the foam by the both experimental and analysis method we can improve the understanding level and efficiency of the rigid polyurethane foam.

Investigation and correction for the calibration error of using two-microphone impedance tube method on low absorption materials

The two-microphone impedance tube method has been widely used for determining the sound absorption coefficients of materials. In this method, a calibration procedure is performed to correct the amplitude and phase mismatch of the two microphones, through repeated measurement of a specimen with the two channels interchanged. Both ASTM E1050-12 and ISO 10534-2 suggest the use of a highly absorptive material as the calibration specimen and the calibration, once complete, is valid for all successive measurements. When following the Standards, however, test materials of low absorption capability exhibited a pattern of peaks and dips in the absorption coefficient curve at certain frequencies, which are obviously not a part of the material’s own behavior. This paper will discuss the cause of these peaks and dips and also suggest a simple solution to the issue. Using the new method, one is able to obtain smooth absorption coefficient curves for low absorption test materials just as expected.

Method of Determining the Sound Absorbing Coefficient of Materials within the Frequency Range of 5 000–50 000 Hz in a Test Chamber of a Volume of about 2 m3

Abstract Sound absorption coefficient is a commonly used parameter to characterize the acoustic properties of sound absorbing materials. It is defined within the frequency range of 100-5 000 Hz. In the industrial conditions, many appliances radiating acoustic energy of the frequency range of above 5000 Hz are used and at the same time it is known that a noise within the frequency range of 5 000-50 000 Hz can have a harmful effect on people,hence there is a need to define the coefficient in this frequency range. The article presents a proposal for a method of measurement of the sound absorption coefficient of materials in the frequency range from 5 000 Hz to 50 000 Hz. This method is a modification of the reverberation method with the use of interrupted noise.

Experimental Determination of Sound Absorption Coefficients of Four Types of Malaysian Wood

Currently, one of the important topics in acoustic science is noise control. It is important to control the noise in order to minimize extraneous noise in rooms, buildings, and our environment. Noise control can be achieved by reducing the intensity of sound to the level that is not harmful to human ear. There are four basic principles employed to reduce noise which is absorption, isolation, vibration isolation, and vibration damping. In fact, the most recognized technique to reduce noise is sound absorption on the materials itself. Sound absorption on material such as wood and porous material have been developed and studied by few researchers. Materials that reduce the sound intensity as the sound wave passes through it by the phenomenon of absorption are called sound absorptive materials. There are lot of methods can be used on determining the sound absorption coefficient of materials. In this paper, a preliminary work has been carried out experimentally to determine the sound absorption coefficient of four types of Malaysian wood. They are Tapang (Koompassia excels), Pulai (Alstonai angustiloba), Selunsor merah (Tristianiopsis beccariana) and Jelutong (Dyera polyphylla). The test was performed using the ASTM E1050-98/ISO 10534-2 (American Society for Testing and Material) standards for the sound absorption coefficient testing. This method is known as impedance tube method (Two-Microphone Method). The absorption coefficient depends on the frequencies. In this study the values of the frequencies used was in the range from 350 Hz to 1000 Hz.

Sound Absorption Measurement in a Circular Tube Using the Echo-Pulse Method

An alternative method for measuring normal-incidence sound absorption coefficients of materials or structures was examined, that is based on the echo-pulse method in a circular tube. The so-called Butterworth impulse was generated in a standing wave tube with a flat frequency response over a wide frequency range. The inverse filtering principle was adopted to compensate for the loudspeaker frequency response at higher and lower frequencies. Two types of acoustical material and structure were employed: glassfibre material and tube exit. Through comparison with the traditional, well-established transfer function method, it can be seen that the new method can be as accurate as the established method. The main errors of this new method result from the accuracy of the measurement of the amplitude of the reflection coefficient of the sample.

2P-21 A method for measuring diffuse-field sound absorption coefficients of materials using parametric loudspeaker(Poster Session)

2P-21 A method for measuring diffuse-field sound absorption coefficients of materials using parametric loudspeaker(Poster Session)

Field testing of sound absorption coefficients in a classroom

Formal procedures for determining the sound absorption coefficients of materials installed in the field do not exist. However, the U.S. Air Force requested such tests to prove that the sound-absorbing material used in classrooms at Beale AFB in Marysville, CA, met the specified NRC of 0.80. They permitted the use of two layers of 0.5-in. fiberboard or 1-in.-thick fiberglass panels to meet the specified NRC rating. Post-construction tests showed reverberation times longer than expected. Unrealistic sound-absorption coefficients for room finish materials had to be used with the Sabine equation to achieve agreement between the measured and predicted reverberation time. By employing the Fitzroy equation and generally published absorption coefficients for ceiling tile, carpet, and fiberboard, the model provided excellent agreement with the measured reverberation times. The NRC of the fiberboard was computed to be 0.35, agreeing with published data. Since this did not meet project specifications, the Fitzroy model was used to learn the type and quantity of material needed to meet design goals. Follow-up tests showed good agreement between the predicted and measured reverberation times with material added, and project specifications were met. Results are also compared with the requirements of ANSI 12.60.

Sound absorption properties of thermally bonded nonwovens based on composing fibers and production parameters

AbstractThis article reviews the noise absorption capacity of thermally bonded nonwovens in the range of audible frequencies (125–2500 Hz). First, we focus on the effects of the properties of the fibers, which constitute nonwovens, on the sound absorption properties, and then we consider the web orientation angle of nonwovens. We also investigate a composite model of the sound absorption properties of nonwovens, including the surface roughness and panel vibration. We have used an impedance tube interferometer, which provides the normal incidence sound absorption coefficient of materials, for the determination of the noise absorption properties of nonwovens produced under different conditions. The noise absorption capacity of nonwovens depends primarily on the thickness and surface characteristics of specimens, but the effects of the fiber contents are only marginal. Interestingly, when there is a panel in front of nonwovens, the noise absorption capacity increases significantly at low and medium frequencies (250–1000 Hz). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2295–2302, 2004

Effects of room shape and diffusing treatment on the measurement of sound absorption coefficient in a reverberation room

In this paper, the measurement of sound absorption coefficient of acoustical materials in a reverberation room is investigated by numerical analysis based on the ray-tracing method. As a result, it has been confirmed that the sound absorption coefficient of the same specimen can much differ by the differences of room shape and by the effect of the sound diffusers. In order to evaluate the degree of sound diffusivity in a reverberation room, the angular dependence of the sound power incident to the room boundary and to the surface of the specimen have been calculated. From the result of the numerical analysis, the relationship between the measurement of sound absorption coefficient of materials and the diffusivity of the sound field in a reverberation room has been examined.

Measuring acoustic impedance: The results of a worldwide round robin

ASTM Committee E-33 on Environmental Acoustics has published two different test methods for determining the normal incidence acoustic impedance and sound absorption coefficients of materials using an impedance tube. Method C-384 involves the use of the traditional standing-wave apparatus using pure tones. Method E-1050 involves the use of a two-microphone technique that provides improved measurement speed and increased detail in the performance versus frequency spectrum. Committee E-33 organized a round robin test program in order to generate ASTM required precision and bias statements for both test methods. Although not required by ASTM, the program also provided an excellent opportunity to compare the results obtained by the two measurement methods. Twenty-two laboratories expressed initial interest and fourteen laboratories from five countries throughout the world eventually participated in the program utilizing either one or both of the test methods. This paper discusses the organization of the program, the test results that were reported, and the conclusions that were drawn from a statistical analysis of the reported test results.