Reverberation Chamber Research Articles

Characterising core body temperature response of free-moving C57BL/6 mice to 1.95 GHz whole-body radiofrequency-electromagnetic fields.

The present study investigated the core body temperature (CBT) response of free-moving adult male and female C57BL/6 mice, during and following a 2-h exposure to 1.95 GHz RF-EMF within custom-built reverberation chambers, using temperature capsules implanted within the intraperitoneal cavity and data continuously logged and transmitted via radiotelemetry postexposure. Comparing RF-EMF exposures (WBA-SAR of 1.25, 2.5, 3.75, and 5 W/kg) to the sham-exposed condition, we identified a peak in CBT within the first 16 min of RF-EMF exposure (+0.15, +0.31, +0.24, +0.37°C at 1.25, 2.5, 3.75, and 5 W/kg respectively; statistically significant at WBA-SAR ≥ 2.5 W/kg only), which largely dissipated for the remainder of the exposure period. Immediately before the end of exposure, only the CBT of the 5 W/kg condition was statistically differentiable from sham. Based on our findings, it is apparent that mice are able to effectively compensate for the increased thermal load at RF-EMF strengths up to 5 W/kg. In addition, the elevated CBT at the end of the exposure period in the 5 W/kg condition was statistically significantly reduced compared to the sham condition immediately after RF-EMF exposure ceased. This would indicate that measures of CBT following the end of an RF-EMF exposure period may not reflect the actual change in the CBT of mice caused by RF-EMF exposure in mice.

Sound absorption coefficient in the audible and ultrasonic frequency range measured in a reverberation chamber with an omnidirectional parametric loudspeaker

An omnidirectional parametric loudspeaker (OPL) is a sound source that contains hundreds of ultrasonic transducers set on the surface of a sphere. Each transducer emits an ultrasonic collimated beam consisting of an audible exponential sweep sine (ESS) modulated in amplitude. This results in hundreds of ultrasonic sound waves emitted in all directions, but also audible sound waves generated by the parametric acoustic array (PAA) phenomenon, by which a modulated signal returns to the audible range thanks to nonlinear propagation in the air. This work examines how the OPL performs in assessing room acoustics, specifically in determining material sound absorption coefficients within reverberation chambers. The ISO 354 standard requires measuring reverberation times with the chamber empty and with the material sample. These are measured using ESS, which has proven effective for the OPL performance. This signal concentrates all the sound power into a single frequency, which improves ultrasonic power levels and thus the signal-to-noise ratio of the audible sound field. The absorption coefficient is next evaluated following the standard, and the results are compared with those obtained using a standard dodecahedral loudspeaker. Finally, the absorption coefficient in the ultrasonic range is examined to evaluate the OPL performance in this frequency range.

Selection of the averaging time for the interrupted noise method of measuring reverberation time

The author wrote the requirements for the selection of the averaging time for the interrupted noise method of measuring reverberation time in the international standard ISO 354:1985 "Acoustics-Measurement of sound absorption in a reverberation room". When this standard was revised to become ISO 354:2003, the evaluation range used to measure the reverberation time was changed from 30 dB to 20 dB. Because the working group which revised the standard did not know the reasons for the averaging time requirements, they incorrectly changed them to supposedly take account of the change to the evaluation range. The purpose of this paper is to give the reasons for the averaging time requirements so that they are corrected and not incorrectly changed again. It is necessary that the averaging time is not so large that information is discarded and so that, in the case of exponential averaging devices, the measured decay rate is not biased by the decay rate of the exponential averaging device. On the other hand, it is desirable that the averaging time be as large as possible so that the ensemble standard deviation of the measured reverberation times is as small as possible.

Towards estimating acoustic surface impedances at low frequencies in a reverberation chamber

To accurately simulate interior acoustical fields, reliable descriptions of the physical properties of the acoustical space are required. While geometry, propagation medium properties, and source characteristics are, in general, easily accessible, the absorption characteristics of the bounding surfaces of a space can be difficult to obtain. For commonly used materials, databases of absorption coefficients could be used. However, absorption coefficient databases are of limited use at low frequencies, especially below 100 Hz. Furthermore, measurement-based estimations tend to be more reliable. Thus, there is incentive to perform measurement-based estimations of the absorption characteristics of a room's bounding surfaces at low frequencies. This work considers the estimation of sound absorption, given in terms of locally-reacting surface impedance, at modal frequencies. This is made feasible by solving an inverse eigenvalue problem. The spatially-uniform, frequency-dependent impedance of the bounding surfaces of a reverberation chamber is estimated. The method is validated by comparing measured transfer functions to simulated transfer functions, which are generated using the estimated impedance. It is shown that the proposed method can be used to estimate locally-reacting surface impedance from measured room impulse responses at low frequencies.

Interlaboratory comparison of ISO 354 sound absorption coefficient measurements for four sound absorbing materials

Sound absorption coefficients are among the most important acoustic parameters of building materials. The coefficients are measured in reverberation chambers using the ISO354 measurement procedure. The complex reverberant sound field that builds up in such facilities is closely related to the concept of the diffuse sound field and is difficult to characterize experimentally. Due to the limited quantitative assessment of the sound field, there is no general agreement in the scientific community on how to improve the method of measuring sound absorption coefficients, although significant differences have been reported between the results of different laboratories. To address part of this challenge, we investigate how specific the variations between laboratory results are for a particular material. In our investigation, we evaluated four different sound absorbing materials in five independent acoustics laboratories. Four of the reverberation chambers used suspended diffusers, while the complex shape of one chamber was designed to achieve a diffuse sound field. The results show that the interlaboratory deviations are not very specific to the material tested, while general differences between the facilities can be observed. Furthermore, the meticulous geometric design of the chambers can be seen as an alternative to the use of suspended diffusers.

A study on the improvement of sound insulation performance of aluminum chassis

In recent years, the automobile industry has rapidly shifted towards electric vehicles, with an increasing number of vehicles using aluminum instead of steel for a significant portion or the entire or part of the vehicle body. Aluminum body panels reduce sound insulation performance compared to steel panels because they depend on surface density. This situation causes uncomfortable sound environment for vehicle passengers. To address the sound insulation issues associated with lightweight panels, it is necessary to research how to optimize the thickness and weight of interior and exterior car components in order to enhance sound insulation performance. In this study, the acoustic characteristics were initially confirmed by measuring indoor noise while driving a vehicle constructed from two different materials: steel and aluminum, under the same conditions as other parts. In addition, to select a sound absorbing material suitable for the characteristics of the vehicle body material, steel and aluminum panels were installed between the reverberation chamber and the anechoic chamber, respectively, and sound transmission loss was evaluated by combining the various thickness and weight. Based on these results, we decided on a proposed specification (aluminum panel with weight/thickness added) above the basic specification (steel panel with existing sound insulation parts).

Modal identification as a tool to increase the accuracy of acoustic absorption measurements in reverberation rooms

Measuring the acoustic properties of materials may be very challenging. At low frequencies, the sample may interfere with the room's modal behavior. When it occurs, the longest modal decays are influenced. Differences in reverberation time - empty Vs with-specimen conditions - are often emphasized by these modal effects, and, as a matter of fact, the absorption coefficient may be overestimated. The present work aims to increase the accuracy through data augmentation at low frequencies. Modal identification was performed on measured impulse responses of porous specimens, allowing the determination of modal-decay times within the third-octave bands of 80-160 Hz. A statistical analysis of natural modes highlights the modes whose decay exceeds the density function as outliers. This preliminary study ultimately provides some statistical considerations on the modal distribution of decay times and their relationship with the reverberation time measured through the Schroeder integral.

Consideration on the efficient calibration method for low-cost microphones

Current microphone calibration methods in the international standards are basically targeting laboratory and working standard microphones specified in the IEC 61094-2 and IEC 61094-4, respectively. These methods are promising to ensure the reliability however there are many difficulties in applying it to low-cost acoustic sensors such as MEMS microphones, which are rapidly becoming widespread, with aspects of time and cost. Moreover, since the installation conditions for these low-cost microphones are different from those of standardized measurement microphones, calibration methods that take these usage conditions into consideration are required. As potential methods to overcome the problems, the feasibility of the active coupler method applicable to surface mounted condition and the bulk calibration method in the reverberation room are investigated. Both proposed methods can be applied under actual installation conditions and enable rapid and cost-efficient calibration. In this study, methods and procedures for securing traceability are proposed, and uncertainty evaluation is conducted.

Virtual assessment of phone booths' acoustic performance in laboratory and office environments

The irrelevant speech noise is one of the significant issues affecting workers' productivity and comfort. Acoustic furniture is the most common solution addressing this problem: mobile phone booths and partially closed workstations are two examples. The acoustic performance of such devices is defined with laboratory tests according to ISO 23351-1:2020. The sound power level measured in reverberation rooms with and without the product determines the reduction of speech A-weighted sound power level (DS,A). However, predicting acoustic performance in real-world scenarios is still challenging. The present work explores the discrepancies between DS,A, and apparent speech reduction (D'S,A) through numerical models. The study assesses the performance difference between a calibrated virtual reverberation room and virtual open-plan offices maintaining the same acoustic properties, i.e., sound transmission and sound absorption, of a typical two-person phone booth. Moreover, the study investigates the fluctuation on D'S,A changing main room acoustic criteria, e.g., the reverberation time (T) and the total absorption area (A). Results help define ray-based simulations' reliability in predicting furniture ensembles' acoustic performance.

A study on road noise reduction using recycled material

Recently, there has been a shift in the paradigm of the automobile industry from internal combustion engines to electric vehicles. An NVH research has also evolved rapidly from studying noise and vibration in conventional internal combustion engines to exploring methods for reducing road noise. A road noise during vehicle operation can be amplified by the resonance of the cavity inside chassis. As a countermeasure, so, filler materials have been applied inside the chassis to reduce resonance, However, fillers applied to existing cavities, such as pads and polyurethane foam, generally focus on blocking resonance inside the cavity rather than reducing vehicle road noise. In this study, a filler has been developed to fill the cavities of vehicles; Filler resources are from residual scrap gathered in manufacturing shop floor of automobile interior and exterior parts. As a result, fillers introduced in the study offer not only excellent acoustic performance but also are recyclable. The effect in terms of acoustic performance of recycled fillers was confirmed by conducting tests, sound transmission loss in reverberation and anechoic chambers. Also, actual vehicle driving tests were conducted on a proving ground.

Impact of contextual and lip-sync-related visual cues on speech intelligibility through immersive audio-visual scene recordings in a reverberant conference room

Recent hearing research has benefitted from the latest Virtual Reality systems that allowed the reproduction of immersive Audio-Visual scenarios to achieve more ecological listening tests. Indeed, efforts have been spent to identify the aspects that convey actual ecological validity, particularly investigating the effects of visual cues and self-motion on Speech Intelligibility through tests mainly based on simulated scenes. However, work must still be addressed when sceneries developed through real recordings inside reverberant environments are concerned. This study used 3rd-order ambisonics recordings and stereoscopic 360° videos inside a reverberant conference hall to create three virtual audio-visual scenes where speech intelligibility tests were performed, introducing informational noise from different angles. A 16-speaker spherical array synced with a headmounted display was used to administer the immersive tests to 50 normal-hearing subjects. Firstly, tests only composed of the auditory scenes were compared, based on the achieved scores, with tests also providing contextual and positional source-related visual cues, both with and without self-motion, for a total of four different test configurations. Then, to complete the investigation of the visual cues' impact on speech intelligibility, ten normal-hearing subjects were recruited to performaudio-visual tests incorporating lip-sync-related visual cues for the target speech.

Sound insulation of partially open windows - alternative laboratory measurement methods

Sound insulation with partially open windows has had growing attention in recent years. In Denmark, this is primarily caused by the Danish Environmental Protection Agency's guideline from 2007 "Noise from roads", which introduces noise limits with open windows (opening area of 0.35 m2) for situations with a high traffic noise level. The sound insulation of traditional windows in open position have been measured in both the laboratory and external environment, and for certain setups too different results have been observed. To investigate the reasons behind the discrepancies, the traditionally reverberant source room of laboratory measurements was modified to a semi-anechoic room to remove reflections from unwanted reflections from room surfaces and to imitate the external environment during field measurements. Four different window types were tested as part of the research projects "Optimized measurement method for sound reduction of partially open windows part 1 and 2" (MetÅV and MetÅV2) in collaboration with four Danish window manufacturers. The primary focus of the project is to explore alternative laboratory methods to the traditional diffuse field method by studying reference cases from field measurements and taking into consideration different sound source positions and window geometry. This paper describes the results so far.

Measurement uncertainty of ISO CD 23351-2 - Preliminary results

The purpose of our study was to determine the measurement uncertainty of ISO CD 23351-2 draft standard. Two sub-studies were conducted. In study 1, an accuracy experiment was conducted where 7 participants tested in the same room (in situ) two different products (full enclosure and partial enclosure). In study 2, an experiment was conducted where a single participant tested two different products in several rooms varying in room volume and reverberation time. Study 1 yielded an in-situ reproducibility standard deviation of 0.80.9 dB. Study 2 yielded a between-site standard deviation of 0.7 dB. The small standard deviations suggest that the technical content should not be essentially changed in the next versions of the standard. The results could be considered in next version of the standard ISO DIS 23351-2.

Comparative analysis of acoustic testing methods of a multi-layered material: uncovering the membrane effect

This paper presents a comprehensive investigation into the differences observed when evaluating the acoustic properties of a multi-layered material using different standardized methods. The primary focus is on the membrane effect of the upper aluminum layer, which has been shown to contribute to the overall acoustic absorption. This phenomenon, crucial for applications requiring enhanced acoustic performance at lower frequencies, is observed in tests conducted with large sample sizes but remains undetected in impedance tube measurements. Through a series of experiments, including reverberant chamber, impedance tube, and in-situ absorption measurements, this study investigates the complexities of accurately estimating sound absorption characteristics in multi-layered materials. Additionally, 2D colormaps to visualize the spatial distribution of sound absorption across different samples are examined, offering deeper insights into the uneven absorption patterns that standard testing methods may overlook. The experimental findings highlight the importance of selecting an appropriate evaluation technique for multi-layered materials, particularly in applications where acoustic performance is critical at lower frequencies.

Validation of an eigenvalue-based inverse method for estimating locally-reacting surface impedance

Accurate boundary condition descriptions are essential for creating accurate computational models for room acoustics. These descriptions are often defined in terms of absorption coefficients or surface impedances. However, obtaining complex-valued, frequency-dependent values for an acoustically absorbing material sample at low frequencies is a challenging task. In fact, the current measurement standard ISO 354:2003 does not provide a method for measuring absorption coefficients below 100 Hz. In this paper, a recently proposed eigenvalue-based inverse method for estimating locally-reacting impedance at measurement system resonance frequencies is validated. The validation uses data from three samples measured in an impedance tube. The method is also used to estimate complex-valued, frequency-dependent impedances at frequencies below 100 Hz in a measured reverberant room and in a simulated reverberation chamber.

Effect of reverberation chamber volume on result of qualification procedure for loudspeaker and loudspeaker positions for airborne sound insulation measurements

Measurement of sound insulation from airborne sound is carried out by determining the level of sound power radiated by the test item. In laboratory practice, this translates into measuring the sound level in the transmitting and receiving chambers and measuring the reverberation time in the receiving chamber. When performing these measurements, significant fluctuations in the sound level for low frequencies can be observed, which is related to the occurrence of standing waves inside the chambers. To ensure proper level averaging, as well as repeatability and reproducibility of measurements ISO 10140-5 describes a qualification procedure for sources and source positions. This article presents the effect of increasing the volume of the source chamber on the results of the qualification procedure for sources and source positions for floor airborne sound insulation measurements.

The frame around a test specimen and its influence on the measured sound absorption coefficient

For most practical cases of reverberation room measurements of sound absorption coefficient according to ISO 354:2003, the standard requires the edges of the test specimen to be covered by a reflective frame. The purpose of this requirement is to keep the edges of the test specimen from absorbing sound. While this statement is consistent with practical measurement results at medium to high frequencies, it has been shown that the low frequency performance of a high-absorbing test specimen worsens when a frame is not in place. To further investigate this phenomenon, a series of reverberation room measurements have been carried out on several variations of a porous absorber test specimen. The goal of the measurements has been to determine whether the size and dimensions of the test specimen affect the test results, and if any such affections are dependent on the presence of a frame. Additionally, the sound absorption area of the frame itself has been measured to elucidate whether the frame is likely to give a significant contribution to the total sound absorption area of a framed test specimen. The results will be presented and discussed in this paper.

An applicability discussion of the in-situ measurement method using the ensemble averaging technique for sound absorption characteristics

In our former papers, 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 named it EA-method, for short. So far, we have 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: i.e. two-microphone and PU-sensor. In this paper, the applicability of the EA-method is discussed onto three types of absorption: porous, panel, and resonator. At first, the outline of the EA-method is summarized and measurement mechanism is explained. Then, the robustness as well as the reproducibility of the method are discussed based on accumulated results of measurements conducted in various environments including reverberation rooms and in-situ conditions.

Investigation of sound field diffuseness in an irregularly-shaped room based on FMBEM incidence directivity analysis

Several methods to evaluate sound field diffuseness have been proposed in terms of the isotropy of traveling sound waves. In a previous paper, we investigated the characteristics of two kinds of diffusion indices in a rectangular room, based on the incidence directivity analysis employing the FMBEM. The present study applies the incidence directivity analysis to an irregularly-shaped reverberation room for measurement of sound absorption, and investigates the effects of room shapes and absorbing surfaces on the diffusion indices. The numerical results show that the indices tend to be higher in an irregularly-shaped room than in a rectangular room, and those decline remarkably with increasing the absorption coefficient of the walls. Moreover, the tendencies are more clearly seen in the diffusion indices for the 1/3-octave bands than for single frequencies.

Optimization of speech intelligibility in street-facing rooms of adult care home based on acoustic simulation

Due to the limited availability of urban land in China, many adult care facilities have resorted to constructing roadside living spaces. However, this has led to a significant issue of traffic noise disturbance. Besides, many rooms lack of control over indoor reverberation. As a result, the communication efficiency of the residents might be greatly impacted. This study aims to investigate the impact of traffic noise level, room size, and surface sound absorption on speech transmission index (STI), based on a typical room layout and conducted using acoustic simulation. The simulation results indicated that in order to achieve the required STI over 0.79 for elderly residents, it is imperative to control the road noise interference below 40 dBA and ensure that the room reverberation time under 0.42 s. To achieve this, it is recommended that for a road with a noise emission level of 72 dBA, the distance between the roadside room and the road should not less than 10 m. Additionally, windows with Rw+Ctr over 28 dB without road sound barriers (over 20 dB with sound barriers) should be installed. Besides, the room area needs to be controlled within 20 m2 and an acoustic ceiling with NRC>0.65 should be installed.