Hemi-anechoic Chamber Research Articles

Latency-free real-time auralisation in hemi-anechoic chambers

Real-time auralisation allows users to interact with a simulated acoustical room, providing a more responsive way of evaluating acoustical performance than pre- recorded auralisation. Nevertheless, real-time auralisation is computationally intensive and, therefore, susceptible to latency. This paper presents a novel latency-free real-time auralisation method that enables users to auralise a simulated acoustical event with self-excitation more accurately than current methods. The method uses a loudspeaker system inside a hemi-anechoic chamber. By exploiting the inherent floor reflection of the hemi-anechoic chamber, the impulse response can be trimmed to compensate for the total latency of the loudspeaker system. The method's validity is demonstrated with measurements presenting virtual wall reflections arriving at the correct time at the listening position. The influence of the speaker placement regarding latency is discussed. This method enables users to evaluate a given response more accurately than conventionally possible.

Investigation and comparison of heat pump noise in the lab and field

The presentation covers a contribution from the publicly funded HP-QUEEN MENESA project on the characterisation of a heat pump in the laboratory with comparisons in the field at comparable operation conditions. The data in the lab are obtained from a hemi-anechoic chamber that is equipped with temperature control to allow standardized measurements at -7 °C and 7 °C, for example. In this environment spherical sound radiation pattern are determined on a movable hemi-circle microphone arrangement allowing the assessment of source location and radiation contribution as well as sound power determination at the different operation points of the heat pump. These data are compared and analysed to measurements with the same heat pump in the field at model houses at the Fraunhofer-IBP location in Holzkirchen. Furthermore, we are also investigating the transmission path from the noise of the heat pump outdoors into the building taking sound reduction index of i.e., windows into account. It is planned to connect a psychoacoustic analysis together with a questionnaire to these investigations that allows assessment of the final sound characteristics inside the building.

Validation measurement of vehicle pass-by models for dynamic urban environments

Simulation and auralization of urban traffic noise can help city planners and residents to improve existing urban areas or plan new attractive living spaces while enabling discussions about shared auditory experiences, regardless of expertise. Evaluating street noise in the context of urban areas requires careful consideration of their urban surroundings, such as landscapes, buildings, and other structures. Obtaining measurements to validate simulations may be difficult given the variety and complexity of sounds within an urban setting. Recreating the real urban scenario on a test track, e.g., including building facades and exact source and receiver positions, helps validate simulations but also poses challenges due to varying road surfaces, weather conditions, and disturbances such as industrial or fly-over sounds. This research represents a method of bringing a measurement set-up consisting of source (e.g., vehicle), receiver (e.g., pedestrian), and surfaces (e.g., walls) to the more controlled environment of a hemi-anechoic chamber. The method is intended for the validation of previous vehicle pass-by measurements by utilizing sine sweeps with walls parked at varying distances. These environments aim to deliver more insight into the relevance of diffracted sound for configuration of urban sound simulation models.

Test method for evaluating the NIOSH Noise Mapper application in a hemi-anechoic chamber

The Noise Mapper Application developed by the National Institute for Occupational Safety and Health (NIOSH) can be used to identify areas in the workplace where workers may be exposed to hazardous noise. The app was created to empower workers to measure sound levels in their workplace, make informed decisions about noise exposure, and prevent occupational hearing loss. To evaluate the maps generated by the app, a single sound source or multiple sources were used to generate steady state sound fields within the NIOSH hemi-anechoic chamber (HAC). A sound level meter (Larson Davis, Model 831C) was used to measure A-weighed equivalent continuous sound levels on a grid with a spacing of 1.5 m (5 ft). The NIOSH Noise Mapper app was then used to generate sound level contour plots with a grid spacing of 3.0 m (10 ft). The accuracy of the map was evaluated by comparing the predicted sound levels with the measured sound levels. The results show the app can estimate the sound levels to within ±1.2 dB(A) of the measured values. The test method used to evaluate the NIOSH Noise Mapper app and the results of the comparison will be discussed in this paper.

Numerical modeling of axial fan noise in fuel-cell electric truck: from component to full-vehicle analysis

This paper presents a comprehensive approach to predict the noise generated by an axial fan in electric vehicles, which is more noticeable due to the absence of combustion engines. Such cooling fans require stronger cooling performance due to increased demand for heat dissipation, resulting in even higher noise levels. A numerical model for fan noise in a fuel-cell electric vehicle (FCEV) is proposed with two steps: component-level and full-vehicle level models. At component level, Lattice Boltzmann Method (LBM) is employed for aeroacoustics simulation, determining the component sound power level (SWL). At full vehicle level, statistical energy analysis (SEA) is utilized to estimate the sound pressure level (SPL) inside the vehicle cabin, particularly at the driver's ear locations. The noise sources in the full vehicle model, were characterized from the component level SWL, as monopole sources. The model is validated by comparing the numerical predictions with the experimental measurements of the SWL in a hemi-anechoic chamber and the SPL in the cabin. Good agreements were obtained between numerical results and measurement results, demonstrating the effectiveness of the model. This allows for faster and more accurate virtual NVH design validations on large and complex systems where prototype measurements are difficult to conduct.

Creating noise contour maps utilizing the NIOSH Noise Mapper App (A pilot study)

High noise levels can lead to physical and psychological stress, reduced productivity, and permanent hearing loss in workers. Traditional noise monitoring techniques indicate that mine workers, for example, consistently have a higher level of noise exposure when compared to workers in other heavy industries. It is critical to conduct risk assessments, identify noise sources, and control them to prevent occupational noise-induced hearing loss (ONIHL). The National Institute for Occupational Safety and Health (NIOSH) has recently developed the smartphone-based NIOSH Noise Mapper Application, which generates a noise contour map to enable individuals, workers, and hearing conservation professionals to apply the hierarchy of controls for optimized prioritization and focused targeting of the most effective hearing conservation interventions. Preliminary testing of the Noise Mapper app has been carried out in a hemi-anechoic chamber at the NIOSH Mining Laboratory in Pittsburgh. The results demonstrate that the Noise Mapper app can quickly and cost-effectively generate adequate noise contour maps to identify noise hazards and relevant noise-generating machinery to prioritize and target the most effective interventions. However, additional testing is needed to evaluate the Noise Mapper app's performance in real-world environments and its ability to measure multiple sound sources. This new tool supplements, but does not, replace traditional methods of creating noise contour maps.

Numerical and Experimental Investigation of a Volumetric Resistance Blower Performance and Its Optimization for Portable Computing Device Applications

Abstract In this paper, we present our results on a relatively new kind of blower called Volumetric Resistance Blower (VRB) for cooling of portable computing platforms like laptop computers. The VRB performance was modeled numerically and compared to traditional bladed blowers. The sources of noise, dominant in bladed blower, are absent in case of VRB, because it uses a continuous porous disk instead of discrete blades. Thus, even though at iso-rpm, VRB yielded lower flowrate, its iso-acoustic performance could be superior. Hence, further analysis was crucial to quantify the potential benefit. The acoustics experiments for bladed blower and VRB were conducted in a hemi-anechoic chamber in accordance with ECMA-74 and ECMA TR/99 standards. Iso-acoustics pressure versus volume flowrate plot for both bladed blower and VRB are compared. VRB was found to have superior performance as compared to bladed blower. The volume flowrate at open flow condition for bladed blower and VRB are comparable, but as back pressure increased the flowrate yielded by VRB kept increasing and at stagnation condition, VRB showed around 79% higher static pressure. In the second part of the work, the experimentally validated numerical model for VRB was used for numerical optimization using a design of experiments (DOE) approach and varying the geometrical parameters. Rotor distance (minimum distance from the axis of rotation of impeller to the cutwater surface) was found to be the most important parameter, and an optimum value was found. A second DOE elucidated the optimal rotor hub center location in the two-dimensional space inside the casing as when the rotor is tucked back into the casing as much as possible and when the rotor distance is above 20.15 mm. A partial P–Q curve is generated (up to 20 Pa) for optimal geometry configuration. Based on the numerical and experimental evidence, VRB is found to have the potential to replace traditional bladed design in portable computing devices. In addition, due to absence of blades, it creates lower tonal noise, giving a much more comfortable experience to the end user.

Comparison of microphone array techniques and standardized methods for sound power estimation

The sound power level can be estimated using several well-established techniques available in the market. However, the variety of methods can be overwhelming when choosing among them due to environmental restrictions, standard requirements, and the necessary expertise to acquire valid data. In this study, we aim to compare the results obtained with different techniques, either standardized, as described in ISO 3744 in hemi anechoic rooms and ISO 9614-1, engineering method, using sound intensity, and non-standardized microphone array methods, as optimized acoustic beamforming and acoustic holography algorithms. The sound power levels of an omnidirectional sound source with pink noise as input and a printer were measured in a hemi-anechoic chamber using all the techniques. In addition, the measurement with the omnidirectional sound source were carried in a meeting room to check the effect of room and background noise. Considering the results of the sound intensity method as the baseline, all the overall sound power level results varied within the uncertainty stipulated by ISO 9614-1 for both sources. The results in one-third octave bands were within the uncertainty for all bands except one case for both sources in the case of refined beamforming.

The Effectiveness of Least Mean Squared-Based Adaptive Algorithms for Active Noise Control System in a Small Confined Space

Active noise control (ANC) is a technique applied to eliminate an unwanted sound by superposing a signal of equal amplitude and opposite phase, sometimes defined as an anti-noise signal, computed through an adaptive algorithm. The study described herein aims to evaluate and compare the performance of some of the most popular algorithms based on the least mean squares (LMS) approach applied to a multichannel active noise control system in a small, enclosed space. The comparison is conducted through an experimental evaluation of the ANC algorithms’ performance, carried out on a tractor cabin in a hemi-anechoic chamber, generating the unwanted sound field using a dodecahedron sound source placed outside the enclosure, emitting narrowband and broadband signals. The experimental analysis and the comparison with the results obtained in a free field condition have made it possible to show certain practical limitations when implementing the algorithms. The results show that the feed-forward systems allow for greater stability, avoiding the acoustic feedback from the control loudspeakers to the reference microphone when this is outside the cabin, while the feedback system is the slowest configuration to converge, requiring an internal modeling of the reference signal. With random signals, the feed-forward systems concentrate their performance in the range above 500 Hz, while the feedback system becomes ineffective.

Fan-noise reduction of data centre telecommunications’ server racks, with an acoustic metamaterial broadband, low-frequency sound-absorbing liner

An acoustic metamaterial is evaluated successfully as a means to reduce axial fan noise in an IT server rack application. The continuing growth in demand for cloud-based memory storage is outpacing sustainable energy management which still requires the axial fan as a fundamental component in temperature control technologies. As chip temperatures increase, additional numbers of more powerful fans are required to run faster with the result that data centres have become unsafe working environments due to the acoustic output exceeding prescribed occupational noise level limits. Noise isolation using soundproof cabinets is expensive and impractical in data centres which seek to reduce floor space and communication losses. Instead, in this work, a 15 mm deep acoustic liner solution is proposed being the approach taken in the aeronautics sector to reduce aeroengine noise which, similarly, is a flow duct problem. The acoustic metamaterial adopted here is the recently developed Segmented Membrane Sound Absorber (SeMSA) which can be tailored for low frequency, broadband and tonal noise in a thin form-factor and which can be used in a grazing flow configuration. An equivalent circuit model developed for the SeMSA allows its design to be optimised to attenuate a typical server fan’s frequency spectrum. An array of manufactured rectangular SeMSA cells was embedded in a “deflector plate”, which is an existing component in a server rack. The deflector plate was implemented in a custom-built fan sound power rig, tested in an hemi-anechoic chamber and compared to a hardwall case and to PU foam. A relative overall decrease of 2.5 dBA was achieved for the SeMSA deflector plate liner which corresponds to a 43.9% decrease in overall sound emissions, which using a material thickness of only 15 mm, signifies an almost twofold increase in the time required for hearing damage to occur. A 3 dBA drop was achieved at the blade passing frequency of 620 Hz, while relative drops of 1–2.5 dBA were achieved from 2000–8000 Hz.

Acoustic radiation prediction using panel contribution analysis combined with scale modeling

Panel contribution analysis can be used to predict machinery sound radiation, component contributions, and the effect of sound reduction treatments. The machinery is first discretized into a collection of panels or patches. Volume velocities are measured for each patch with the machinery operating, and transfer functions are measured between patches and receiver locations with the machinery turned off. In this work, transfer functions are measured using a scale model. Then, the sound pressure level produced by the generator set is predicted using a combination of panel contribution analysis and scale modeling where a 1/2 scale model is used to measure the acoustic transfer functions. It is demonstrated that panel contribution analysis can be used to predict sound pressure levels in the far field even using a relatively small hemi-anechoic chamber. In addition, the procedure successfully assesses the efficacy of barrier treatments.

Round-robin testing on Sound power level measurement for Reference Sound Source and Office printer

Round-robin testing for two samples, Reference Sound Source in accordance with ISO 6926 and Office printer with electrophotographic engine, were executed by seven testing laboratories in Japan on 2020. All tests were executed with parallelepiped measurement surface in hemi anechoic chamber in accordance with testing standard for engineering-grade sound power determination, ISO 3744:2010. The results show that sample standard deviation for RSS is better than printer. Standard deviations for overall A-weighted sound power level for two samples are better than combined standard uncertainties calculated with reference example of standard deviations in ISO 3744 (1.5 dB for Reference Sound Source as standard deviation of operating/mounting condition is negligibly small, 1.6 dB for printer as stable operating/mounting condition 0.5 dB). This paper also indicates tips for those who would conduct round-robin testing to obtain valid results by obviating incorrect operations and malfunctions of printers or similar equipment from the experience of some round-robin tests.

Experimental Study of the Performance of a Propeller with Trailing-Edge Serrations

Experimental studies were conducted at a hemi-anechoic chamber to investigate the effectiveness of trailing-edge (TE) serration designs for reducing aerodynamic noise of a propeller. The propeller noise was measured under different speeds in both the vertical and horizontal directions. The thrust force produced by the propeller was also measured at rotational speeds. The flaplet, concave and sawtooth shapes of serration were integrated on the TE of baseline propeller to obtain three different serrated propellers. The results show that the flaplet, concave and sawtooth propellers improve the thrust force more than 10% on the basis of baseline propeller. Three types of TE serration all managed to attenuate large amount of noise, while the sawtooth serrations are more effective than flaplet and concave serrations in a wider frequency range. Then, the serration distribution length (DL) of sawtooth propeller was trimmed to other three different values to obtain the 3/4-DL, 1/2-DL and 1/4-DL propellers. The 3/4-DL propeller exhibited best comprehensive performance, by an average of 2.7 and 3.0 dB noise reduction in tested two positions and simultaneously generated overall 6.7% higher thrust force than did the baseline propeller. The morphological and parametric effects of TE serration in current studies are more significant in a relative high Reynolds number (1.4 × 106).

Effect of the uneven blade spacing on the noise annoyance of axial-flow fans and side channel blowers

Several rotors of axial-flow fans and side channel blowers with optimal circumferential spacing have been tested in a hemi-anechoic chamber; such rotors have been designed by means of an existing method based on the minimization of the tonal noise peaks prominence from the broadband spectrum. The resulting SPL spectra contain different components: tonal ones, which are a well-known cause of annoyance (intended as an undesired feature of the received noise related to the short-term exposure), broadband ones, which could have a positive masking effect, and further components due to the electric motor. As expected, the blade spacing strongly affects the tonal noise part of the spectrum, which is mainly of aeroacoustic origin; in axial-flow fans, it also affects the low-frequency broadband part related to the leakage flow. The present study has also confirmed the good consistency between measured spectra and theoretically predicted ones even for side channel blowers.The characteristic curves of the tested machines are not significantly affected, but the quality of the radiated noise considerably changes as the spacing non-uniformity increases: tonalness decreases and roughness increases, fluctuation has not a systematic behavior, and no significant variation in other perceived characteristics may be noticed. In order to quantify such perception, the psychoacoustic parameters commonly employed for automotive cooling fans have been computed.The tonalness decrease leads to a significant annoyance reduction, while the increase in roughness seems of minor importance from a noise annoyance standpoint. Nevertheless, in strongly asymmetric rotors, the change in the perceived acoustic signature may be relevant and an unaware listener could interpret the roughness increase as a potential mechanical malfunction.As for generality of the present discussion, it should be considered that the tested rotors have been optimized in order to reduce the tonal noise peaks prominence, which obviously results in a reduction in the perceived tonalness. On the contrary, spacing obtained by means of different criteria could yield different variations in tonalness, and the same likely happens to loudness. Instead, the rise of peaks at shaft frequency harmonics is a direct consequence of the uneven spacing which should sistematically yield a roughness increase.

Three-Dimensional Sound Field Reconstruction and Sound Power Estimation by Stereo Vision and Beamforming Technology

The size of the sound field reconstruction area has an important influence on the beamforming sound source localization method and determines the speed of reconstruction. To reduce the sound field reconstruction area, stereo vision technology is introduced to continuously obtain the three-dimensional surface of the target and reconstruct the sound field on it. The fusion method can quickly locate the three-dimensional position of the sound source, and the computational complexity of this method is mathematically analyzed. The sound power level can be estimated dynamically by the sound intensity scaling method based on beamforming and the depth information of the sound source. Experimental results in a hemi-anechoic chamber show that this method can quickly identify the three-dimensional position of the moving source. When the depth of the moving sound source changes, the estimated sound power is more stable than the sound pressure on the microphone.

Vibro-Acoustic Response in Vehicle Interior and Exterior Using Multibody Dynamic Systems Due to Cleat Impacts

This paper presents the structural vibration and acoustic radiation of a multibody vehicle system in interior and exterior under cleat impact excitations. The entire structure model, which consists of a hyper-elastic tire, a 17-inch lightweight wheel, a rotatable hub, a high-strength knuckle, a control arm and a spring-damper suspension, is a quarter vehicle to simulate transient response when passing through a cleat. The normal acceleration of a vibrating structure surface is applied as an acoustic noise source. A structural-acoustic coupling is employed to consider the interactive effects of the coupled system. The influence of the structure-borne noise can be confirmed by the frequency-correlation analysis between vibration acceleration and acoustic pressure. A sound pressure level (SPL) for near-field radiation is calculated by the finite element method (FEM). In addition, acoustic characteristics such as sound intensity in the passenger cabin are calculated through the boundary element method (BEM). The cleat impact experiment in a hemi-anechoic chamber is presented and validated in detail.

Auditory Localization and Spatial Release From Masking in Children With Suspected Auditory Processing Disorder.

We sought to investigate whether children referred to our audiology clinic with a complaint of listening difficulty, that is, suspected of auditory processing disorder (APD), have difficulties localizing sounds in noise and whether they have reduced benefit from spatial release from masking. Forty-seven typically hearing children in the age range of 7 to 17 years took part in the study. Twenty-one typically developing (TD) children served as controls, and the other 26 children, referred to our audiology clinic with listening problems, were the study group: suspected APD (sAPD). The ability to localize a speech target (the word "baseball") was measured in quiet, broadband noise, and speech-babble in a hemi-anechoic chamber. Participants stood at the center of a loudspeaker array that delivered the target in a diffused noise-field created by presenting independent noise from four loudspeakers spaced 90° apart starting at 45°. In the noise conditions, the signal-to-noise ratio was varied between -12 and 0 dB in 6-dB steps by keeping the noise level constant at 66 dB SPL and varying the target level. Localization ability was indexed by two metrics, one assessing variability in lateral plane [lateral scatter (Lscat)] and the other accuracy in the front/back dimension [front/back percent correct (FBpc)]. Spatial release from masking (SRM) was measured using a modified version of the Hearing in Noise Test (HINT). In this HINT paradigm, speech targets were always presented from the loudspeaker at 0°, and a single noise source was presented either at 0°, 90°, or 270° at 65 dB A. The SRM was calculated as the difference between the 50% correct HINT speech reception threshold obtained when both speech and noise were collocated at 0° and when the noise was presented at either 90° or 270°. As expected, in both groups, localization in noise improved as a function of signal-to-noise ratio. Broadband noise caused significantly larger disruption in FBpc than in Lscat when compared with speech babble. There were, however, no group effects or group interactions, suggesting that the children in the sAPD group did not differ significantly from TD children in either localization metric (Lscat and FBpc). While a significant SRM was observed in both groups, there were no group effects or group interactions. Collectively, the data suggest that children in the sAPD group did not differ significantly from the TD group for either binaural measure investigated in the study. As is evident from a few poor performers, some children with listening difficulties may have difficulty in localizing sounds and may not benefit from spatial separation of speech and noise. However, the heterogeneity in APD and the variability in our data do not support the notion that localization is a global APD problem. Future studies that employ a case study design might provide more insights.

The potential use of a NIOSH sound level meter smart device application in mining operations.

Many mobile sound measurement applications (apps) have been developed to take advantage of the built-in or fit-in sensors of the smartphone. One of the concerns is the accuracy of these apps when compared to professional sound measurement instruments. Previously, a research team from the National Institute for Occupational Safety and Health (NIOSH) developed the NIOSH Sound Level Meter (SLM) app for iOS smart devices. The team found the average accuracy of this app to be within ±1 dBA when using calibrated external microphones with a type 1 reference device and measuring pink noise at levels from 65 to 95 dBA in 5-dBA increments. The studies were conducted in a reverberant noise chamber at the NIOSH Acoustics Laboratory in Cincinnati. However, it is still unknown how this app performs in measuring industrial/mining sound levels outside of a controlled laboratory environment. The current NIOSH study evaluates the NIOSH SLM app to measure sound levels from a jumbo drill (a large mining machine). The study was conducted in a hemi-anechoic chamber at the NIOSH Pittsburgh Mining Research Division and followed by a field evaluation in an underground metal mine. Six different iOS smart devices were used with two types of external microphones chosen from previous studies to measure sound levels during jumbo drill operations, and the results were compared with a reference device. Results show that the average sound levels measured by the NIOSH SLM app are within ±1 dBA of the reference device both in the laboratory and field. However, the type of operation being performed, the selection and use of external microphones, distance from a noise source, and environmental factors (e.g., air movement) may all influence the accuracy of the app's performance. Although additional validation is still needed, the results from this study suggest a potential for using the NIOSH SLM app, with calibrated external microphones, to measure sound levels in mining operations.

Common problems associated with reverberation rooms in sound testing laboratories

Reverberation rooms have proven to be a very useful tool in acoustic laboratories for measuring the sound absorption of acoustic materials, airborne and impact sound transmission through building partitions, as well as measuring the overall radiated sound power level of a wide variety of sound sources. The primary benefit of a reverberation room is that the measurement can be made in much less time than with alternative environments (e.g., anechoic or hemi-anechoic chambers, or via acoustic intensity). The primary disadvantage of the reverberation room measurements is that all information regarding the directivity of the acoustic source is lost. This paper discusses some of the design and acoustical performance problems that are often found in reverberation rooms that could have been avoided with proper foresight. Issues that will be discussed include location relative to nearby noise and ground vibration sources, room size and shape, space planning for microphone locations, low frequency absorbers, and the importance of temperature and humidity control. Specific guidelines that should be followed when planning a new reverberation room will be provided.

Anechoic and hemi-anechoic chamber performance qualification—Review of recent standards changes

Historically, anechoic and hemi-anechoic chamber qualification has been governed by ISO 3745 Annex A. In 2017, a revised version of ISO 26101 was published. A simultaneous update to ISO 3745 Annex A, now refers to the method described in ISO 26101. Many differences exist between the previous versions of ISO 3745 Annex A and the new ISO 26101 method including curve fit, traverse direction and quantity, traverse length, measurement spatial resolution, and others. These changes will impact qualification distances and frequency ranges for anechoic and hemi-anechoic chambers. A comparison between the previous method and the new method will be presented. The impacts of those changes on existing chambers and new chamber designs will be discussed.