Reverberation Chamber Method Research Articles

Shielding Effectiveness Measurement of an Electrically Large Enclosure Using One Antenna

A new method is proposed to measure the shielding effectiveness (SE) of an electrically large enclosure. Traditionally, the SE measurement requires at least two (in an anechoic chamber) or three antennas (in a reverberation chamber). In this paper, only one antenna is needed in the whole measurement procedure. There is no requirement to use a reference antenna. It is interesting to note that if the aperture of the enclosure under test (EUT) can be covered (by using a conductive sheet or aluminum foil), the SE can be measured quickly by comparing the Q factor or the decay time of the covered and uncovered EUT. Measurements have been conducted to evaluate and verify the proposed method. The results are compared with those obtained using the nested reverberation chamber method; limitations of the proposed method are also discussed and identified. It is found that when the EUT is well shielded, the uncertainty of the proposed method could be relatively large. Nevertheless, the proposed method does offer a new simple and efficient approach to measure the SE of an electrically large enclosure.

A shielding effectiveness prediction method for coupled reverberant cavities validated on a real object

In the last years, different works on reverberant microwave propagation and high frequency coupling inside oversized structures have been published in order to make progress in the investigation of wireless systems and electromagnetic threats. In this paper, a new focus to the shielding effectiveness (SE) prediction using the reverberation microwave propagation formalism has been applied to a real complex object attaining very good agreement with the measured data obtained from nested reverberation chamber method. The used formalism is the combination of the simplified propagation model with the updated and more accurate formulation for cavity Q-factor and energy dissipation mechanisms. A very wide frequency range, from 500 MHz to 40 GHz, has been analyzed, which allows us to draw conclusions about how each parameter affects the final SE result at different frequency bands. Furthermore, a tuning study has been performed taking into account the tolerance in the most important parameters, such as the cavity size, the Q-factor, the main dissipation energy mechanisms, and the aperture shapes.

Investigation of the Inlet and Outlet Location on the Soundproofing Ventilation Unit

This paper deals with A-type and B-type rectangular cuboid SVU (soundproofing ventilation unit) contain an inlet and outlet in opposite surfaces. The difference between them is whether they are located in small or big cross section area. Following the theoretical calculation, the shape and location of the inlet and the outlet are determined by an investigation of the distribution of higher order mode waves formed inside the SVU. Finally, experimental results of A- type and B-type based on the reverberation chamber method are shown to be in reasonable agreement with our theoretical predictions. The results of theoretical calculation and experiments conducted lead to the conclusion that A-type is suitable for soundproof ventilation unit.

Sound power levels of the Caxirola and different types of caxixis

In 2014, Brazil will host the FIFA World Cup and Brazilian Musician Carlinhos Brown created the caxirola as the official music instrument, adapting an old African instrument—the caxixi. In both instruments, the sound is generated by hard particles impacting on the walls of a closed basket. While the caxirola is made of environmental-friendly polymer, the caxixi is handcrafted of natural components. At a previous ASA meeting [Pozzer and Paul, J. Acoust. Soc. Am. 134(5), 4187 (2013)], we presented both instruments and sound pressure levels measured at users ears. Being handmade makes the caxixi to be highly variable in size and proportions, in contrast to the caxirola which is an industry product. We now present sound power level (SWL) measurements made by the hemi-anechoic room method and the reverberation chamber method. The SWL measured for the caxirola was 79 to 86 dB (80 to 85 dBA) for transversal and longitudinal use, respectively. The SWL of four different caixixis measured ranged between 69 and 80 dB (70 to 79 dBA). Octave band SWLs were found to be higher for the caxirola compared to all four caxixi in the 500 Hz to 4 kHz bands.

Improvement of the measurement of the sound absorption using the reverberation chamber method

The random incidence absorption coefficient is measured in a reverberation room according to ISO 354 or ASTM C423-09a. It is known that the inter laboratory accuracy under Reproducibility conditions of these results is still not very well. It is generally assumed that the limited diffusion properties of reverberation rooms, especially with a strongly sound absorbing sample, are the main reason for the bad reproducibility values for the sound absorption between laboratories. Reverberation rooms should be made much more diffuse to reduce the interlaboratory differences. However there are practical limitations in quantifying and improving the diffuse field conditions. The measured sound absorption still seems to be the most sensitive descriptor of the diffuse field conditions. A way to further reduce the inter-laboratory differences is the use of a reference absorber to qualify a room and to calibrate the results of a sound absorption measurement. In the presentation an overview will be given of the research performed and some suggestions for the new version of ISO 354 will be given.

EMC Management in HEV/EV Applications

EMC Management in HEV/EV Applications

Mean-free-paths in concert and chamber music halls and the correct method for calibrating dodecahedral sound sources

The Eyring/Sabine equations assume that in a large irregular room a sound wave travels in straight lines from one surface to another, that the surfaces have an average sound absorption coefficient αav, and that the mean-free-path between reflections is 4 V/Stot where V is the volume of the room and Stot is the total area of all of its surfaces. No account is taken of diffusivity of the surfaces. The 4 V/Stot relation was originally based on experimental determinations made by Knudsen (Architectural Acoustics, 1932, pp. 132-141). This paper sets out to test the 4 V/Stot relation experimentally for a wide variety of unoccupied concert and chamber music halls with seating capacities from 200 to 5000, using the measured sound strengths Gmid and reverberation times RT60,mid. Computer simulations of the sound fields for nine of these rooms (of varying shapes) were also made to determine the mean-free-paths by that method. The study shows that 4 V/Stot is an acceptable relation for mean-free-paths in the Sabine/Eyring equations except for halls of unusual shape. Also demonstrated is the proper method for calibrating the dodecahedral sound source used for measuring the sound strength G, i.e., the reverberation chamber method.

The overview of measuring the shielding effectiveness of the materials in complex electromagnetic environment

The wide-spreading use of innovative lightweight materials like advanced composites and metal coated plastics in several fields of applications has pushed toward an increasing interest in the characterization of the shielding performances of test materials. In the paper, we revisit the conventional methods of measuring shielding effectiveness of materials, and introduce the nested reverberation chamber methods (two reverberation chambers). The deficiencies in the existing approaches were illustrated. We advised the new approach should account for aperture, cavity size, and chamber loading effects, for which some current reverberation chamber approaches do not correctly account. This work is useful in estimating the shielding ability of the materials in complex electromagnetic environment.

Electromagnetic shielding performance of carbon foams

The electromagnetic shielding performance of carbon foams is measured using the nested reverberation chamber method in the frequency band 1–4GHz, which is the frequency range of most cellular phones and microwave ovens. Good values of shielding effectiveness are obtained even with a small thickness of carbon foam. The results are used to assess the correct electrical conductivity values for the investigated materials using a multiple reflection model to compute the material transmission coefficient. The potential applications of such materials are in the lightweight microwave shielding structures that are partially transparent to optical frequencies.

Sound insulation characteristics of multi-layer structures with a microperforated panel

Multi-layer structures have issues with sound insulation at low and mid-frequencies due to mass-air-mass resonance. The purpose of this study is to investigate improvements to the sound insulation performance of multi-layer structures using a microperforated panel (MPP), which can absorb well over a wide frequency range. Although MPPs have been investigated over the last several decades, almost all studies have been conducted in terms of sound absorption. Herein the sound transmission loss of multi-layer structures with flexible MPPs of infinite extent is theoretically investigated. The calculation is based on the wave equation and the equation of panel vibration including the effect of perforation of the panel. Additionally to consider a more realistic sound insulation performance, the effect of the directional distribution of the incident energy in a reverberation chamber is taken into account. Experiments are conducted using an acoustic tube to validate the calculated results and the reverberation chamber method to verify the actual sound insulation characteristics. Both experiments agree well with the theoretically calculated perforation effects. Consequently, MMPs are confirmed to improve the deterioration of sound insulation performance due to mass-air-mass resonance of multi-layer structures.

Preliminary study of a reverberation chamber method for multiple-source testing using intermodulation

The illumination of a system under test by multiple sources can lead to intermodulation (IM) effects that give rise to threat signals at the IM frequencies. The result of this is that the electromagnetic (EM) compatibility and signal integrity of that system may be compromised by two or more signals that are, individually, out of the frequency band of concern. As the EM environment continues to be increasingly polluted and electromagnetic compatibility (EMC) tests rely, predominantly, on a single-source illumination, this is an issue that is ripe for further investigation. The statistical nature of the reverberation chamber and its behaviour in isolating the test environment from the ambient (outside) environment indicates that this is a facility worth considering for multiple source (IM) testing. This study discusses the need for such an expansion in test methods, driven by an increasing interest in functional safety. It presents the results of a preliminary, simulation based, study of the reverberation chamber used with multiple sources. The purpose of this study is to demonstrate that the reverberation chamber can provide a facility where worst-case multiple-source EM interference effects can be tested. It concludes that the proposed approach has the potential to be viable and suggests experimental studies to confirm the behaviour and also to confirm that this approach can be used as an indirect method of reducing the lowest working frequency of the reverberation chamber.

Accurate Analysis of Reverberation Field Penetration Into an Equipment-Level Enclosure

The paper focuses on the reverberation chamber method for the shielding properties evaluation of equipment-level enclosures. The enclosure under test is numerically modeled by an in-house finite-difference time-domain code that is able to predict the field inside the enclosure and the voltage captured by the probe placed inside it. The chamber fields have been modeled by applying the plane-wave superposition. The code is validated by measurements in our reverberation chamber. Subsequently, the effect of probe positioning and length on the induced voltage is analyzed. Finally, the enclosure shielding effectiveness is evaluated by applying two different definitions, and a statistical analysis is carried out, thus allowing an estimation of the measurement uncertainty.

A correction of random incidence absorption coefficients for the angular distribution of acoustic energy under measurement conditions

Most acoustic measurements are based on an assumption of ideal conditions. One such ideal condition is a diffuse and reverberant field. In practice, a perfectly diffuse sound field cannot be achieved in a reverberation chamber. Uneven incident energy density under measurement conditions can cause discrepancies between the measured value and the theoretical random incidence absorption coefficient. Therefore the angular distribution of the incident acoustic energy onto an absorber sample should be taken into account. The angular distribution of the incident energy density was simulated using the beam tracing method for various room shapes and source positions. The averaged angular distribution is found to be similar to a Gaussian distribution. As a result, an angle-weighted absorption coefficient was proposed by considering the angular energy distribution to improve the agreement between the theoretical absorption coefficient and the reverberation room measurement. The angle-weighted absorption coefficient, together with the size correction, agrees satisfactorily with the measured absorption data by the reverberation chamber method. At high frequencies and for large samples, the averaged weighting corresponds well with the measurement, whereas at low frequencies and for small panels, the relatively flat distribution agrees better.

Measuring the uniform diffusion coefficient: Synthesized aperture goniometer measurements.

There are currently two methods to measure the diffusion/scattering from acoustical surfaces: reverberation chamber measurement and in-situ measurement using a semicircular array of microphones called a goniometer. Each of these methods has advantages and drawbacks. The reverberation chamber method accounts for random incidence, but produces inconsistent results for materials of the same profile but different absorption. It does not distinguish between one-dimensional and two-dimensional surface topologies, since the sample is rotated, and is also inapplicable to high absorption materials. The goniometer measurement quantifies directivity as well as diffusion, but presents challenges related to processing power, as many microphones are required to achieve reasonable resolution. Using a procedure akin to synthesized aperture radar, a 2.5° resolution polar plot of the 180° response is achieved by scanning eight microphones rather than employing a full array of 72. Also, since the direct sound and reflected sound arrive at the receiver in close succession when measuring low incident angles, special processing must be exercised in order to resolve the relationship between the sounds. This research addresses the challenges of the goniometer measurement and attempts to use a synthesized aperture goniometer to derive accurate diffusion and directivity properties of materials.

Modeling of the Reverberation Chamber Method for Determining the Shielding Properties of a Coaxial Cable

The paper considers the reverberation chamber (RC) method for the measurement of the shielding effectiveness (SE) of coaxial cables with braided shields. In particular, the voltage at the cable termination is numerically computed and compared to that measured in an RC. The RC field is represented by a finite summation of random plane waves, and a finite-difference time-domain (FDTD) code is used to calculate the outer shield current induced by the RC field. The knowledge of the shield current distribution allows the determination of the voltage at the cable termination's internal circuit after a proper numerical averaging. It is then compared to the measured voltage averaged over stirrer rotations. The method is applied to a commercially available cable model RG58, and using the nominal value for the transfer impedance of this cable type gives results in a satisfactory agreement with the measurements. Finally, the possibility of recovering the transfer impedance from the measured SE of the RC is discussed.

Radiated emissions and immunity of microstrip transmission lines: theory and reverberation chamber measurements

The increasing complexity of electronic systems has introduced an increased potential for electromagnetic interference (EMI) between electronic systems. We analyze the radiation from a microstrip transmission line and calculate the total radiated power by numerical integration. Reverberation chamber methods for measuring radiated emissions and immunity are reviewed and applied to three microstrip configurations. Measurements from 200 to 2000 MHz are compared with theory, and excellent agreement is obtained for two configurations that minimize feed cable and finite ground plane effects. Emissions measurements are found to be more accurate than immunity measurements because the impedance mismatch of the receiving antenna cancels when the ratio of the microstrip and reference radiated power measurements is taken. The use of two different receiving antenna locations for emissions measurements illustrates good field uniformity within the chamber.

Measuring auditorium seat absorption

There is a need for a more accurate laboratory measurement method to predict auditorium seat absorption. The traditional method tends to overpredict the absorption of the exposed front and sides of seating blocks. An alternative reverberation chamber method was studied that involves the use of barriers to obtain realistic measurements of front and side absorption. This method was validated by comparing measurements of seats made in a reverberation chamber with in situ absorption data for the same seats, calculated from reverberation time measurements in ten auditoria with and without the seats present. The accuracy of the alternative method was satisfactory in all cases, although a severe lack of diffusion in two of the halls hindered the validation process. It was found that using a frequency-constant edge correction strip to account for side and front absorption could lead to significant errors in auditorium absorption prediction.

Sound power measurements in reverberation chambers

A great deal of effort was devoted in recent years to improve the accuracy of the reverberation chamber method for sound power measurements of small sources. The paper summarizes the results achieved. The accuracy problems exist in two areas. The actual radiation of sound power as related to the power radiated into the free field. This ratio is a function of source position and source size as well as the radiation pattern of the source. Quite recently the statistics of the power radiation for a monopole source has been found (R. Waterhouse). Another area affecting the accuracy of measurements is the determination of the time-space average of pressure squared. For this purpose, various means, primarily the rotating diffusers, have been employed. The recent results on the effect of rotating diffusers on the statistical properties of the sound field is presented. Finally, suggestions for future development are given.

System for the measurement of acoustic energy density

In the reverberation chamber method of measuring the sound power radiated from an acoustic noise source, pressure microphones are used in arrays, traversed over fixed paths or used singly with a rotating diffuser. The purpose is to accurately measure the potential energy density in the sound field produced by the noise source. In general, accuracy is limited by the variance in the measurement of the potential energy density. When the total energy density (potential and kinetic) is measured, the variance should be cut in half. A system has been designed and built that measures the total energy density in a three‐dimensional sound field. Three sets of spaced pressure microphones are placed orthogonally. For a set the signals are averaged and squared to produce the potential energy at a point and the difference signal is squared to give the square of one component of the acoustic velocity vector. By multiplexing the sum of the squares of the three components of the velocity vector the kinetic energy is produced. The three pressure squared signals are averaged to produce the potential energy, halved and added to half the kinetic energy to give total energy density. Calibration is accomplished with an acoustic calibrator or pistonphone and small plane wave tube. Measurements made in one dimensional and three dimensional sound fields will be presented.

Model study of sound generation by hot jets ejecting into an augmenter tube

To obtain baseline data, first the sound power spectrum of jet exiting from a nozzle of 2.5-in. throat diameter was measured with high accuracy as a function of the pressure ratio (up to 3) and temperature (up to 3300°F) utilizing the reverberation chamber method. Subsequently, the effect of augmeter tubes and subsonic diffusers on the noise generation process was investigated utilizing a test setup consisting of two adjacent reverberation chambers (burner room and exhaust room) connected to each other by the augmenter tube. The sound power was measured simultaneously in both rooms in order to evaluate both the change in total sound power output, and the split of the sound power between the burner room and exhaust rooms as a function of nozzle augmenter entrance distance for various ratios of the augmenter nozzle diameter. The paper will report the result of the acoustical and pumping performance tests. [This work was supported in part by the United States Navy, Southern Division Naval Facilities Engineering Command.]