Reverberation Chamber Method Research Articles

A finite element study of absorption coefficient measurement at low frequencies

The frequency-dependent absorption coefficients of an acoustically absorbent material sample can be measured using the reverberation chamber method, defined by the ISO standard 354:2003. The lower frequency bound for the measurement defined in the standard is 100 Hz, since measurement data below this frequency may be unreliable. However, the ability to measure absorption coefficients at low frequencies is becoming increasingly more important, to facilitate the enhanced control of low frequency sound fields. This paper presents a finite element method study of the reverberation chamber measurement method at frequencies below 100 Hz, and the predicted performance of the ISO standard method is reported. Additionally, a recently proposed method that makes use of finite element-based eigenvalue analyses to estimate the locally reacting complex-valued impedance of a sample at modal frequencies is applied to the problem. It is shown that the proposed method can be used to estimate frequency-dependent absorption coefficients at modal frequencies below 100 Hz.

Experimental study on sound absorption performance of microporous aluminum square tube space absorber

The aluminum square tube is one of the ceiling materials that have been produced in recent years. It has become the most popular product in the decoration market because its lines are bright and clean with distinct layers, reflecting the simple modern style, and it is easy to install and dismantle. However, the aluminum square tube used in the market is mostly a single aluminum panel structure, mainly used for decoration, without sound absorption and noise reduction. In order to improve the sound absorption properties of the aluminum square tube, a microporous aluminum square tube space absorber is proposed. The reverberation chamber method is used to investigate the sound absorption properties of the microporous aluminum square tube. The results show that the sound absorption performance of the aluminum square tube can be greatly improved by the formation of micropores on it. The sound absorption performance of the aluminum square tube perforated on three sides is better than that of the aluminum square tube perforated on only two sides. In the case of perforation on only two sides, with the increase in the height of the perforated surface, the peak value of sound absorption is gradually increased. In addition, the sound absorption peak gradually shifts to the low-frequency direction as the distance between the perforated surfaces increases. The reasonable design of structural parameters of microporous aluminum square tube can make it have beautiful decorative effect and good sound absorbing effect.

Experimental investigation of the acoustic emission of an electrically driven propeller in thrust and recuperation mode in a wind tunnel

This paper presents the results of an experimental performance study of an electrically driven propeller in thrust and power recuperation mode in a subsonic wind tunnel. Using the reverberation chamber method, acoustic measurements were performed to evaluate them as a reference in relation to the performance data obtained. The three approaches to recuperation were investigated: Fixed pitch propeller at speed reduction (case 1), controllable pitch propeller with negative angle of attack (case 2), and inverted, backward running propeller (case 3). Results show that in-flight recuperation can be optimized in terms of low noise emissions and generating shaft power from the flow efficiently by choosing the respective operating point. For the latter, case 3 shows the greatest potential for a two bladed propeller, while case 2 has the lowest noise emissions and might reach higher values in recuperated power by increasing the propeller blade number.

Measurement of the diffuse field sound absorption using a sound field synthesis method

A method for measuring the diffuse field sound absorption coefficient of a material using sound field synthesis is proposed. A planar loudspeaker array is first used to generate acoustic plane waves with variable incidence angle on the surface of a material under test. Using a two-microphone probe positioned closely to the sample’s surface, the angle-dependent sound absorption coefficients are then estimated. Finally, the diffuse field absorption coefficient is computed following Paris formula. Numerical simulations are used to evaluate the respective effects of the maximum incidence angle value and the number of individual incidence angles that are required for a robust calculation of the diffuse sound field absorption. Measurements are conducted on three different materials and compared with either simulation results obtained using the Johnson-Champoux-Allard theory, or with measurement results obtained using the standard reverberation chamber method. For all considered materials and over a wide frequency range, the proposed method leads to results that are in better agreement with theoretical predictions than those obtained using standardized methods.

A Comparative Study of Error Vector Magnitude Measurement in a Reverberation Chamber

This letter shows a comparative analysis between the conventional and reverberation chamber (RC) measurements of error vector magnitude (EVM). The conventional methods, including direct link (DL) and anechoic chamber (AC), have been used to measure EVM for several years. However, the over-the-air testing of devices under test (DUT) is emphasized, and the EVM measurement in RC is a difficult question. For mass production testing, instead of using two chambers (RC and AC), it is expected to integrate EVM testing into the existing RC measurement system to give a pass/fail conclusion quickly. Hence, we measured EVM in 2 GHz (2412 MHz) and 5 GHz (5180 MHz) using RC, AC, and DL methods. From the measurement results, we find that the measured EVM is repeatable, stable, and insensitive to the location of DUT. Besides, the measured EVMs obtained from all scenarios deteriorate with the decrease of received power. Comparing the results from RC with that from the conventional methods, it could be concluded that with the reduction of multipath effect, the measured EVMs approach to the ideal values measured in AC. The differences among the methods could be used to calibrate the measured EVMs and give a pass/fail conclusion efficiently for mass production.

Дослідження акустичних характеристик дослідних зразків звукопоглинаючих конструкцій для газотурбінних двигунів

It is shown that the problem of aviation noise affecting the population near airports is still relevant. Pollution from aircraft noise is not a less influential factor than air or water pollution. The international community is paying close attention to tackling aviation noise by introducing requirements and noise limits for aircraft under development or in operation. Among the sources of aircraft noise, one of the main sources that should be singled out - the noise generated by gas turbine engines. Among the sources of noise of gas turbine engines should be noted to highlight the noise of the fan. To reduce the noise level of engines, various methods are used, among which should be noted the use of acoustic liners placed in the nacelles of the engine. To meet modern international requirements, acoustic liners must reduce the noise level in a wide frequency range. It is shown that there are several methods for determining the acoustic characteristics of acoustic liners: the standing wave method, the transfer function method, the reverberation chamber method, and the use of a flow-through acoustic channel. Each of these methods can be used in the stages of development of acoustic liners for gas turbine engines. It is shown that the standing wave method should be used at the initial stage of research, as it allows to determine the most promising acoustic liners for further research. This paper presents the results of research for six prototypes of acoustic liners. The measurement results of the prototypes were compared with the results of the measured design used to reduce the noise of the D-18T engine. Two prototypes should be distinguished among the prototypes. In the designs of the first group, noise reduction in various frequencies was achieved by the effect of viscous friction due to the holes in the filler cells as in porous fiber materials. In the designs of the second group, the expansion of the absorption frequency range was achieved due to the placement of cells with different resonant frequencies. Studies have shown that it is possible to create acoustic liners that absorb noise in a wide frequency range, so a prototype has a sound absorption coefficient of more than 0.8 in the frequency range from 1600 Hz to 3500 Hz.

Influences of Various Parameters on Sound Absorption Properties of Vetiver Grass Fiber-based Developed Composite Material

Recently, the trend of natural fibers based eco-friendly materials utilization in the field of acoustic application has been growing rapidly in place of traditional synthetic materials to counter environmental issues, due to their cheap processing cost and other several advantages. In this work, sound absorption and thermal insulation properties of the vetiver grass (Chrysopogon zizanioides) based material have been investigated. Efforts have been carried out in developing porous vetiver grass fibers based composite material. The developed samples tested for the sound absorption coefficient (alpha) over a wide range of frequencies (100 Hz – 4000 Hz) by the reverberation chamber method as per IS 8225-1987/ISO 354- 2003/ASTM 423-909-19 and noise reduction coefficient (NRC) also been calculated. The experimental outcome proved that vetiver grass-based manufactured material has comparable sound absorption to already developed traditional synthetic materials. Further, two different bulk densities, thickness and fibre lengths-based samples were developed and their sound absorption acoustic performances were evaluated to see the influences of these parameters on sound absorption characteristics. Apart from these, some required physical properties as per application requirements were also discussed. However, in this study major focus has been done on acoustical properties.

Determining and Comparison of Sound Absorption Coefficients using Small Reverberation Chamber and Test Tube Methods

Sound absorbing materials have been widely used to decrease hazardous noise in indoor and outdoor environments. In the present study, we designed and constructed an experimental laboratory-scale chamber to measure the sound absorption coefficients of porous materials in comparison with the measurements of the test tube method. The main reason was to design and construct a small chamber to enable testing of acoustic material samples in small dimensions allowing easy and rapid testing of acoustic materials. The acoustic chamber method was based on the formation of reverberation field of the acoustic waves across testing chamber locations, but differences in sound pressure throughout the chamber may result in measurement errors. Therefore, the chamber was constructed with a volume of 2.85 m3, wall reflectors, and a rotating sound source was designed to ensure a diffusive field. The tests were conducted with samples of 12.4m2 installed on interior surfaces of the chamber. Sound absorption coefficients of acoustic polyethylene and polyurethane absorbents were measured across the central frequencies of the octave band. Sound absorption coefficients under reverberant random incidence and normal incidence were related to the sound frequency. The chamber method predicted higher sound absorption coefficients compared to the coefficients obtained by the tube test method for all tested porous materials. Based on the results of the proposed small chamber, it can be concluded that sound absorption coefficients measurement of samples in an environment was more similar to real situations.

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.

Consistent Shielding Effectiveness Measurements for Small Enclosures in Reverberation and Semianechoic Chambers

In this paper, a correction method is proposed for removing the antenna effects when measuring the shielding effectiveness (SE) using the IEEE 299.1 standard. For a small shielding enclosure with a maximum dimension of less than 2 m, the IEEE 299.1 standard provides reverberation chamber (RC) and semi-anechoic chamber (SAC) based methods for SE measurements. However, these two methods are inconsistent each other. Even for the same enclosure, the measured SE has a different value depending on which measurement method is used. Such an inconsistency results in an incompatibility between the two approaches, and thus the IEEE 299.1 standard uses the maximum dimension of an enclosure to determine which method should be applied for measuring the SE. The inconsistency is caused by the measurement antennas used inside and outside of an enclosure. Procedures to remove the antenna effects are also proposed herein. After the removal of the antenna effects, consistency is established between the SEs measured at the RC and SAC. The SE is measured for three enclosures having an SE of 30 to 100 dB at 1 GHz, and the antenna effects are removed from the measured SE to investigate the consistency. The consistency is investigated at up to 16 GHz for a 100-dB enclosure. Removal of the antenna effects and the resulting consistency make it possible to measure the SE using either the RC or SAC method irrespective of the maximum dimension of the enclosure to be measured. In addition, an overestimation is prevented in the SE measurement, improving the measurement accuracy.

An Energy Model for the Calculation of Room Acoustic Parameters in Rectangular Rooms with Absorbent Ceilings

The most common acoustical treatment of public rooms, such as schools, offices, and healthcare premises, is a suspended absorbent ceiling. The non-uniform distribution of the absorbent material, as well as the influence of sound-scattering objects such as furniture or other interior equipment, has to be taken into account when calculating room acoustic parameters. This requires additional information than what is already inherent in the statistical absorption coefficients and equivalent absorption areas provided by the reverberation chamber method ISO 354. Furthermore, the classical diffuse field assumption cannot be expected to be valid in these types of rooms. The non-isotropic sound field has to be considered. In this paper, a statistical energy analysis (SEA) model is derived. The sound field is subdivided into a grazing and non-grazing part where the grazing part refers to waves propagating almost parallel to the suspended ceiling. For estimation of all the inherent parameters in the model, the surface impedance of the suspended ceiling has to be known. A method for estimating the scattering and absorbing effects of furniture and objects is suggested in this paper. The room acoustical parameters reverberation time T20, speech clarity C50, and sound strength G were calculated with the model and compared with calculations according to the classical diffuse field model. Comparison with measurements were performed for a classroom configuration. With regard to all cases, the new model agrees better with measurements than the classical one.

Test and Response Analysis of Shielding Performance of The Cable Under HEMP Irradiation

The shielding efficiency of coaxial cable is characterized by its anti-interference ability in electromagnetic environment. The testing methods include three-axis method, GTEM chamber method, line injection method, power absorption clamp method, reverberation chamber method and so on. In practical application, it is found that the shielding performance of the cable under high altitude electromagnetic pulse (HEMP) does not accord with the shielding efficiency measured by the cable. While a typical multi-layer shielded cable is selected as the test object, a test method is designed and tested under electromagnetic simulator, and then the test results are verified and extended by CST simulation. Finally, the reason of this phenomenon is analysed theoretically, which has important reference significance for the prediction and testing of shielding efficiency of cable in HEMP environment.

Radio frequency susceptibility test of Beidou airborne receiver

The radio frequency susceptibility testing method of RTCA/DO-160G section 20 has been widely utilized to verify the reliability of airborne equipments in civil aviation. However, there are only a few of theory analysis and practical test of Beidou airborne equipment based on the procedure of radio frequency susceptibility test originated from RTCA/DO-160G, the research of radio frequency susceptibility test stemmed from RTCA/DO-160G is aiming at provide theoretical reference and practical bases for civil Beidou airborne receiver. Firstly, a theoretical model of calibration for conducted susceptibility and radiated susceptibility is derived and validated its effectiveness, the derived theoretical model indicate that choosing a suitable injection probe that meets the requirements of standard is much more important than the choose of a high power amplifier, a method for estimating the input power of antenna is also obtained under far field zone. Secondly, an equivalence model based on the susceptibility probability is formulated between the anechoic chamber and reverberation chamber method of radiated susceptibility test, the theory and experimental results show that the model can predict the difference between these two test methods well. Finally, a susceptibility evaluation model is constructed in terms of the needed test signal. The practical minimum radiated susceptibility threshold for continuous wave and pulse modulated signal are 5 V/m and 40 V/m, which indicates the receiver shows a better endurance to pulse interference than continuous wave.

The Application of Transfer Matrix Method in Acoustic Performance Analysis of Composite Materials and Acoustic Protection Design of Ship

This research introduced the transfer matrix method to calculate the acoustic performance of composite materials, which solving the problem of traditional methods existing in calculating acoustic performance of complex composite structure. Based on the impedance tube method and the reverberation chamber method, the experiment of measuring sound absorption and insulation parameters of typical composite materials was conducted. And then, the validity of theoretical calculation of the transfer matrix method was verified. In addition, under the application of the statistical energy method, the model simulation was carried out by taking a ship as an example, and the transfer matrix method in the noise protection design of the ship was discussed.

Carbon foam electromagnetic mm-wave absorption in reverberation chamber

The electromagnetic (EM) absorbing cross section (ACS) of carbon foams in the mm-wave frequency range 50–70 GHz is analyzed by means of a reverberation chamber (RC) set-up. Carbon foams are lightweight cellular materials which are believed to provide benefits in EM interference/compatibility (EMI/EMC) issues, due to their significant thermo-mechanical and electrical properties. With respect to usually employed microwave test set-up, the range of frequency investigated allows to analyze the details of the foam bulk structure using shorter wavelengths. Moreover, the RC method allows to study the EM interaction with materials by taking into account not a single wave propagation mode (like in transmission line methods) but rather an homogeneous and statistically random EM field propagation. A full analysis of the material behavior can be thus carried out, by reproducing conditions similar to what really happens within operational environments. The foam EM properties are related to specific structural parameters, such as apparent density, open porosity, cell size/shape and 3-D arrangement. Significant ACS values are appreciated in low density foams; further, a functional dependence between EM mm-wave absorption capability and foam inner morphology is revealed, thus promoting the employed microwave set-up as effective method for supporting the structural analysis of porous materials.

Incorporating measurement standards in an advanced acoustics laboratory course

In an advanced acoustics laboratory course at Brigham Young University, students are introduced to ANSI measurement standards in the context of sound power. They are introduced to the anatomy of a typical acoustics standard and then plan and carry out sound power measurements of an electric leaf blower using both reverberation chamber and sound intensity methods. The students are required to write a technical memorandum describing (a) the blower’s radiated sound power levels over an appropriate frequency range, as obtained with the two methods; (b) setup documentation and deviations from the standards’ recommended practices; and (c) how any deviations might have contributed to discrepancies between the sound power levels obtained with the two methods. In this talk, a description of the experience from the faculty and student perspectives is given, along with plans for future improvements.

Rapid impedance tube determination of fabric transparency

Acousticians are continually being asked to verify fabric transparency for applications with absorptive and diffusive surfaces, as well as in sound reinforcement. Standard reverberation chamber methods can be used, but require large fabric and fiberglass samples. A quick and simple impedance tube method has been developed requiring only a 160 mm x 160 mm sample. Two measurements are made. One with an anechoic wedge termination and another with a rigid termination in which the fabric is applied to a 50 mm 6pcf fiberglass substrate. Four microphones are placed at a quarter and three quarters of the square tube’s width and height and the signal is summed. This unique microphone placement minimizes the 1st, 2nd, and 3rd harmonics resulting in a fourfold increase in the upper frequency limit. Impulse response measurements are made at three distances from the sample to calculate the reflection factor, impedance and normal incidence absorption coefficient from 63 Hz to 4000 Hz in one measurement. The small fabric sample is large enough to minimize non-homogeneous effects. A study of fabrics with a wide range of transparencies reveals how both acoustically transparent and backed fabrics can be used as sonic equalizers depending on the application.

Absorption cross section of building materials at mm wavelength in a reverberation chamber

The reverberation chamber (RC) method is used to estimate the average absorption cross section of building materials at mm wave frequencies. Analysed samples include concrete, travertine and bricks of different types. The investigation is carried out in the frequency range between 50 GHz and 68 GHz, which is of interest in the next generation of mobile telecommunication system. A cylindrical cavity is transformed into a RC through the use of a mechanical model stirrer. The chamber field is statistically homogeneous and depolarized; therefore it can be used to probe the average response of the sample under test. In particular, through a differential measure of the average quality factor (average insertion loss) it is possible estimate the fraction of power absorbed by the sample under test. Several cube-shape samples have been characterized and compared. Obtained results show that analysed samples have remarkably different levels of the electromagnetic wave absorption, depending on both material density and chemical composition. The absorption of pure water is used as a baseline to determine the dynamic range of the measurement.

Comparison of two internationally recognized methods for determining the sound absorption coefficient

With today’s growing focus on noise control issues and the emergence of sound quality as an important aspect of product design, acoustic material testing is becoming increasingly relevant to engineers, designers and manufacturers from a broad range of industries. Acoustic material testing is the process by which acoustic characteristics of materials are determined in terms of absorption, reflection, impedance, admittance and transmission loss. Many different methods can be used to determine the acoustic properties of materials. There are standing wave method, two-microphone free field and multimicrophone techniques, reverberation chamber method, in-situ methods and many other alternative methods based on measurement of internal properties of porous materials. The point is that some methods are preferred over others because the environment in which they are measured is closer to real practice. The article deals with the comparison of the results of the measurement in the reverberation chamber and in the interferometer.

A Modified Two-Antenna Method to Measure the Radiation Efficiency of Antennas in a Reverberation Chamber

A modified two-antenna method is proposed to measure the radiation efficiency of antennas in a reverberation chamber. By combining the conventional reference antenna method and the one-antenna method, and introducing a virtual antenna, the proposed method does not need a reference antenna with known efficiency and can be applied to highly lossy antennas, which could be problematic for the existing methods. Measurements are conducted to verify the effectiveness of the proposed method. The proposed method can be regarded as a generalized form of the two-antenna method and is compared to the existing reverberation chamber methods. The advantages and limitations of the proposed approach are also discussed.