Semi-anechoic Chamber Research Articles

Off-Grid DOA Estimation Based on Circularly Fully Convolutional Networks (CFCN) Using Space-Frequency Pseudo-Spectrum.

Low-frequency multi-source direction-of-arrival (DOA) estimation has been challenging for micro-aperture arrays. Deep learning (DL)-based models have been introduced to this problem. Generally, existing DL-based methods formulate DOA estimation as a multi-label multi-classification problem. However, the accuracy of these methods is limited by the number of grids, and the performance is overly dependent on the training data set. In this paper, we propose an off-grid DL-based DOA estimation. The backbone is based on circularly fully convolutional networks (CFCN), trained by the data set labeled by space-frequency pseudo-spectra, and provides on-grid DOA proposals. Then, the regressor is developed to estimate the precise DOAs according to corresponding proposals and features. In this framework, spatial phase features are extracted by the circular convolution calculation. The improvement in spatial resolution is converted to increasing the dimensionality of features by rotating convolutional networks. This model ensures that the DOA estimations at different sub-bands have the same interpretation ability and effectively reduce network model parameters. The simulation and semi-anechoic chamber experiment results show that CFCN-based DOA is superior to existing methods in terms of generalization ability, resolution, and accuracy.

Measuring infrasound outdoors with a focus on wind turbines: the benefits of a wind-shielding dome

Wind-induced microphone noise complicates infrasound measurements considerably. A wind-shielding dome for signal-to-noise ratio improvement of acoustic pressure infrasound frequencies was designed and tested. The semi-spherical shape aimed at maximizing the pressure averaging of large atmospheric turbulent eddies while keeping the structure reasonably compact. The insertion loss of the dome was measured in a semi-anechoic chamber (in absence of flow) and showed nearly full transparency in the low frequency range. In an outdoor test, wind turbine infrasound was simultaneously measured with an uncovered and a dome-covered low-frequency microphone under different wind speeds and turbulence intensities. Largest improvements of the signal-to-noise ratio were measured at high mean wind speeds for frequencies down to 0.5 Hz. The dome allowed to clearly identify the infrasonic tonal components of wind turbines that were otherwise completely covered by the wind-induced microphone noise even at low mean wind speeds. The use of the dome thus opens possibilities for more accurately measuring infrasonic immissions from e.g. wind turbines.

Simulation of deterministic tyre noise based on a monopole substitution model

This paper deals with tyre noise indoor tests in a semi-anechoic chamber and with the identification of a monopole substitution model suitable for simulating the deterministic part of the sound field generated by a rolling tyre. The first part of the paper illustrates the test procedure and the data processing methodology adopted for separating the deterministic contribution to tyre noise, i.e. that mainly related to tread pattern, from the random one. In the second part, the monopole substitution model is presented, together with the results of model identification and sound field simulation.The results reported in the paper for two commercial tyres (with the same size and similar structural characteristics, but with different tread pattern design) show how the predominance of the deterministic/random noise contributions varies with frequency and confirm the random noise spectrum to be substantially independent of the specific tread pattern. With reference to the monopole substitution model, its capability of reproducing the acoustic field radiated by a rolling tyre is assessed through the analysis of both sound spectra and space distribution.

Analysis of Electromagnetic Field Distribution Generated in an Semi-Anechoic Chamber in Aspect of RF Harvesters Testing

The results of work in the field of analysis of the distribution of electromagnetic fields generated by the antenna of an RFID reader operating in the UHF frequency band have been presented in this paper. The carried-out tests concern the efficiency of obtaining energy from the field of an electromagnetic RFID reader. To analyze the distribution of the electric field strength, the calculation procedures were elaborated on the basis of the developed mathematical dependencies in the MathCad program. The developed numerical procedures enable the synthesis of the electric field strength and power density distribution for a given antenna radiation pattern, for any suspension heights of the transmit and receive antennas and any mutual distance between them. In order to verify the correctness of calculations the test stands and measurements were made for a model antenna. The measurements were carried out in a semi-anechoic chamber.

Experimental investigation on the potential of the Feedback Fx-LMS Active Noise Control technology for use in industrial generator sets

Active Noise Control (ANC) has been considered a promising technology for the abatement of acoustic noise from the mid-20th century. Feedback and Feedforward ANC algorithms, based on the destructive interference principle applied to acoustic waves, have been developed for different applications, depending on the spectrum of the noise source. Feedback ANC algorithms make use of a single control microphone to measure an error signal which is then employed by an adaptive filter to estimate the noise source and generate an opposite-phase control signal. The Fx-LMS (Filtered-X Least Mean Square) algorithm is mostly adopted to update the filter. Feedback ANC systems have proven to be effective for the abatement of low-frequency quasi-steady noises; however, different challenges must be overcome to realize an effective and durable system for high-temperature application. This paper aims at experimentally assessing the feasibility of a Feedback Fx-LMS ANC system with off-line Secondary Path estimation to be used in mid-size diesel gensets for the reduction of the exhaust noise. Several solutions are proposed, including the mechanical design, the development of the Fx-LMS algorithm in the LabVIEW FPGA programming language, and the key features required to prevent parts from thermal damage and fouling. The developed prototype was implemented on a 50-kW diesel genset and tested in a semi-anechoic chamber. The noise abatement inside the exhaust pipe and at different measurement points around the machine was evaluated and discussed, showing good potential for improving the acoustic comfort of genset users.

A basic investigation into the optimization of cylindrical tubes used as acoustic stethoscopes for auscultation in COVID-19 diagnosis.

During the COVID-19 outbreak, the auscultation of heart and lung sounds has played an important role in the comprehensive diagnosis and real-time monitoring of confirmed cases. With clinicians wearing protective clothing in isolation wards, a potato chip tube stethoscope, which is a secure and flexible substitute for a conventional stethoscope, has been used by Chinese medical workers in the first-line treatment of COVID-19. In this study, an optimal design for this simple cylindrical stethoscope is proposed based on the fundamental theory of acoustic waveguides. Analyses of the cutoff frequency, sound power transmission coefficient, and sound wave propagation in the uniform lossless tube provide theoretical guidance for selecting the geometric parameters for this simple cylindrical stethoscope. A basic investigation into the auscultatory performances of the original tube and the optimal tube with proposed dimensions was conducted both in a semi-anechoic chamber and in a quiet laboratory. Both experimental results and front-line doctors' clinical feedback endorse the proposed theoretical optimization.

Remote RF excitation for small-bore MR imager at 15.2 T

We report a method for remote excitation of the RF signal for preclinical-equivalent ultra high field Magnetic Resonance Imaging (MRI). A parallel-plate waveguide together with a bio-inspired surface coil were used to perform remote excitation experiments to acquire images with a small-bore MR imager at 15.2 T. The imager bore size limits the RF coil transmitter dimensions, so the Gielis super-formula was used to design an RF coil with small dimensions. Electromagnetic simulations of the principal mode were run to study the waveguide filled with air and loaded with a saline solution-filled tube. Radiation patterns were also computed in a semi-anechoic chamber for the same scenarios as above. A saline solution-filled spherical phantom and a formaldehyde-fixed mouse phantom were used to acquire images. Radiation patterns showed an omnidirectional distribution with no side lobes, and a very smooth behaviour with almost no loss of information in the saline solution-filled tube and without. The theoretical wave impedance was calculated and compared with simulated results showing an excellent correspondence. Spherical phantom image data and simulation results of B1 were contrasted and showed an important correlation. Ex vivo mouse images were of high quality and exhibited clear delineation of anatomical structures. These imaging results are in very good agreement with the simulations. Numerical, theoretical and experimental results validate this approach, using a bio-inspired surface coil with a simple waveguide for preclinical small-bore MRI at ultra high field.

Influencing factors and rules of shielding effectiveness of electromagnetic shielding clothing sleeve

PurposeThe change rules of the shielding effectiveness (SE) of the sleeve has not been clarified, which leads to the lack of the basis for the design, manufacture and evaluation of the electromagnetic shielding (EMS) clothing.Design/methodology/approachAccording to a simplified analysis model, a series of sleeve samples with different fabrics and styles are designed and manufactured. The SE of the sleeve is tested with the proposed special test method in a semi-anechoic chamber to analyze the influence of different factors on the SE of the sleeve.FindingsThe SE is greatly reduced about 60–90% after the fabric is manufactured into the sleeve. The larger the sleeve length is, the higher the peak value of the SE is. When the sleeve length is low, the SE value is easy to appear negative. As the cuff circumference increases, the SE of the sleeve will change with the frequency band. The influence of the cuff style on the SE of the sleeve mainly depends on the cuff width and style. The larger the cuff width is, the lower the overall SE of the sleeve is. The more wrinkles there are at the cuff, the better the SE of the sleeve is.Originality/valueOur results provide a reference for the design, production and evaluation of the sleeve and the whole EMS clothing.

Noise reduction of automobile cooling fan based on bio-inspired design

This study aims to minimize the noise generated by automobile cooling fans. Fan blade structures with ridged surfaces based on bio-inspired principles are 3D printed and used to replace the conventional fan blades. The effect of the bio-inspired ridge structures on the noise reduction of the cooling fan is demonstrated by orthogonal experiments in a semi-anechoic chamber. Experimental results show that with an increase in the rotational speed, the effect of the surface textures on the acoustic performance of the cooling fan becomes more significant. For example, at a fan speed of 1750 r/min, all the bio-inspired blade designs reduce noise compared with the original fan and, in particular, the sound pressure level is reduced by 3.83 dB(A) for the design with a ridge width of 4 mm and a ridge pitch of 15 mm. Through variance analysis of the measured noise, the ridge pitch distance has the most significant impact on noise reduction under low speed conditions whilst, under high speed conditions, the ridge width has the most significant influence. In addition to the experimental studies, computational fluid dynamics (CFD) simulations of the cooling fan are carried out to explain the mechanism of noise reduction for the ridged fan blades. When the fan runs, the horseshoe vortexes generated by the ridge structures disturb the flow of the boundary layer, reduce the influence of the fluid flow on the boundary layer, and delay the transition of the fan blade laminar flow to turbulence. It is also seen that there is a reduction of the intensity of the fan blade trailing edge vortices and the scale of the secondary vortices, thereby achieving the overall aim of noise reduction. This research has significance in the noise reduction design of automobile cooling fans.

Flexible electrostatic transducer with tuned acoustic impedance for improved sensing of body-and water-borne sounds

Acoustic transducers that are designed for airborne sound suffer from impedance mismatches when used in different media. These mismatches result in lowered energy density at the transducer and increased corruption by external noises. Coupling layers can mitigate mismatches, but restrict real-world applicability through limited bandwidth, thickness- dependent sensitivity, and bonding issues. We introduce a self-powered, flexible, electrostatic transducer with a highly tunable acoustic impedance and wide frequency bandwidth, without matching layers. The transducer can be precisely tuned to impedance values in the range of 1 to 2.5 MRayls, which includes skin, fresh and saltwater, and most plastics. Metal-coated microstructures on the surface of an elastomer create small gaps with the charged polymer electret film that are compressed by mechanical vibrations in the elastomer and induce a change in the voltage output. The transducer exhibits a sensitivity greater than 2V/N and an SNR improvement of 35 dB in highly noisy environments. Major factors that affect the output of the transducer are analyzed, including the dimensions of the microstructures, elastomer thickness, and properties of the electret polymer. The acoustic properties, such as sensitivity and improved rejection of environmental noise, are experimentally evaluated through a body phantom in simulated noise environments in a semi-anechoic chamber.

Squeak Noise Prediction of a Door Trim Panel Using Harmonic Balance Method

<div class="section abstract"><div class="htmlview paragraph">Squeak and rattle noise in a vehicle’s interior is perceived as an annoying sound by customers. Since persistent noise (e.g. engine, wind or drive train noise) has been reduced continuously during the last decades, the elimination of sounds, which have their origin in the vehicle’s interior components, is getting more important. Therefore, noise prediction based on simulation models is useful, since design changes can be realized at lower costs in early virtual development phases. For this task, linear simulation methods are state of the art for the identification of noise risk, but in general without knowing if a sound is audible or not. First approaches have been developed based on the Harmonic Balance Method to predict squeak noise and assess their audibility. This paper presents vibroacoustic measurements at a door trim panel for squeaking and non-squeaking configurations. Vibrations are excited harmonically by a force controlled low noise shaker. The system response is measured in a semi-anechoic chamber by acceleration sensors and audibility is assessed. Additionally, a 3D finite element model is built and the Harmonic Balance Method using a dry friction law is applied to predict the acoustic behavior. Finally, the simulation results are compared to the measurements. A good agreement between simulation and experiment can be observed.</div></div>

NORMALIZED SITE ATTENUATION MODELING OF INDOOR FACILITIES USED FOR ELECTROMAGNETIC COMPATIBILITY TESTING

Models of Anechoic and semi-anechoic chambers for electromagnetic compatibility tests are proposed. Normalized site attenuations in the frequency range of 30 to 1000 MHz in both cases are determined using full electromagnetic simulation in HFSS software environment. The received results can be used as a reference in the validation tests of anechoic and semi-anechoic facilities.

Geometry-Independent Excitation of Dark Modes Using Dipole Moment Transitions

Despite numerous potential advantages of subradiative dark modes due to an extremely high quality ( ${Q}$ ) factor, the practicality still depends on the consistency of excitations for a tight fabrication tolerance. This article proposes a novel approach to adaptive transitions between different dipole moments to allow for consistent excitation of Fano and electric quadrupole resonances that are extremely sensitive to geometric errors. The proposed geometry of each metamolecule consists of four strips printed on a thin dielectric film with four gaps between the strips, and the geometric error is described as structural deformation by adjusting the strip lengths and gap sizes in the vertical direction. In each deformed state, current and magnetic-field distributions are analyzed to interpret operating principles, and multipole moments are calculated based on the multipole expansion using the current distributions. The results demonstrate that the consistent excitation of the electric quadrupole moment is enabled based on a simultaneous transition between toroidal and electric dipole moments, and the Fano resonance is consistently excited by the magnetic-to-electric dipole moment transition. For verification, original and deformed metasurfaces are fabricated, and their transmission and reflection coefficients are measured in a semianechoic chamber to experimentally validate the proposed dipole moment transitions. The results confirm that the proposed approach has great potential to bring advances in recent sensing and energy harvesting applications where an extremely high ${Q}$ -factor is essential.

High Directivity Reference RF Source and its Radiated Emission Model for Site Correlation

This article presents a configurable high directivity radio frequency reference noise source and its radiated emission (RE) simulation model. This noise source is a representative of actual programmable logic controller (PLC) and input output (IO) products with a high-speed gigabit backplane. It is designed to produce a reference noise signature from 30 MHz to 6 GHz with RE levels close to the class A electromagnetic radiation disturbance limits of CISPR 11 and CISPR 22. An RE model f this noise source was established to investigate the design factors. Multiple emissions' scans were performed in two separate semianechoic chambers located in different facilities in order to test the repeatability of this noise source. The measurement results indicated that the noise source has very good repeatability and that it can serve as a reference noise source. The results were also used to evaluate the maturity of the simulation technology as well as to quantify the difference between chambers.

An approach for indoor prediction of the pass-by noise of a vehicle based on the time-domain equivalent source method

An approach based on the time-domain equivalent source method (TESM) is developed for the indoor prediction of the pass-by noise of a vehicle in a small-scale semi-anechoic chamber. In the approach, a vehicle drives on a chassis dynamometer, and a number of microphones are distributed around the vehicle to measure the pressure radiated by the vehicle in the near field. Based on the idea of TESM, the sound sources generated by the vehicle are modeled by a series of time-domain equivalent sources, and the sound radiated from these sources to the microphones is depicted by the half-space Green’s function that takes the reflections from the rigid ground into account. To evaluate the pass-by noise, the strengths of equivalent sources are first solved by using the measured pressures as the input of TESM, and then the pressures on the target points specified by the ISO 362-3 Standard are reconstructed by the superposition of sound fields generated by the equivalent sources. Finally, the sound pressure level (SPL) of the pass-by noise can be calculated from the reconstructed pressures. In particular, the proper arrangement scheme for the equivalent sources is investigated in numerical simulations to ensure the prediction accuracy. Numerical and experimental results demonstrate that the proposed approach can effectively predict the SPL of the pass-by noise of a vehicle for both constant-speed and accelerated test conditions in a small-scale semi-anechoic chamber.

Analysis of an automobile door closure vibroacoustic response

The acoustic response of a car door latch has been shown to directly impact the customers’ perceived quality and value evaluation of the automobile. This work introduces an experimentally validated computational model of three door latch components. The transient sound pressure level response of the three door latch components during door closure was collected in a semianechoic chamber using a three-element condenser microphone array. Postprocessing methodologies such as sound pressure level versus 1/3 octave band and continuous wavelet transform analysis were performed. This provided an in-depth analysis on the overall acoustic response and identification of dominant frequencies corresponding to four specific impact events during latch operation. Computational finite element analysis of the closure system using a rigid body, and explicit dynamic and transient structural acoustic analyses provided additional insights into the latch component interactions and the acoustic response generated empirically. Recorded average sound pressure level, frequency decomposition, and impact reaction forces are presented in addition to a comparison between the acoustic response for two different door closure speeds. It was found that an increased door closure speed increased the response sound pressure level, decreased damping of the primary impact, and decreased the frequency bandwidth of the response, thereby generating an acoustic response that would be perceived as noisier, less safe, and less secure by customers. These findings provide additional insights into the primary impact acoustic response of an automotive door latch during closure. The methodology introduced in this work allows automotive engineers to perform future work with modified latch components to further improve the psychoacoustic response of the automotive car door latch, further increasing the value evaluation of the automobile.

Case study: Evaluation of noise reduction in frequencies and sound reduction index of construction with variable noise isolation

Intense sound levels produced by engineering equipment have become an acute issue. As most of engineering equipment require air supply, exhaust and good ventilation, it is not possible to control the noise by covering them with tight hoods. Louver with blades covered with acoustic materials and gaps that enable free circu- lation of air are used to this end. Three louver configurations were tested in the semi-anechoic chamber: bare metal louver blades, louver with blades covered with 20-mm-thick polystyrene foam slabs on both sides, and louver with blades covered with 15-mm-thick glass wool slab. According to the test results, louver with blades covered with glass wool slab demonstrated the best noise attenuation characteristics. The reduction of equiv- alent sound pressure level subject to blade inclination angle was from 10.8 to 12.5 dB. Sound pressure level reduction by louver with blades covered with poly- styrene foam slabs was weaker: the reduction of equivalent sound pressure level was from 5.4 to 8.4 dB. Louver with blades not covered with any acoustic material demonstrated the least noise attenuation result from 1.9 to 3.9 dB

NVH Optimization Analysis of Permanent Magnet Synchronous Motor by Rotor Slotting

With the development of new energy vehicles, the NVH (Noise, Vibration, Harshness) performance of the permanent magnet synchronous motors (PMSM) for vehicles has attracted more and more attention. The rotor slotting optimization analysis is the critical issue in the NVH performance study of PMSM. In this paper, the theoretical formula of the PMSM radial electromagnetic force is presented. Based on which, the spatial order and frequency order characteristics of the radial force of a 6-pole 36-slot PMSM are analyzed. Firstly, electromagnetic simulation of the motor is carried out, and the specific force components, which may cause an NVH problem, is extracted. Secondly, the harmonics are calculated with a method of freezing the relative magnetic permeability. Thirdly, the motor is optimized by rotor slotting to reduce the amplitudes of these harmonics, and the effect on electromagnetic noise is analyzed in theory and simulation. Finally, the NVH test in original state and after rotor slotting state was performed in the semi-anechoic chamber. The accuracy of the theory and simulation was verified based on the comparison of the measured noise data. This paper provides a new NVH optimization idea, this method can quickly locate the fundamental problem location of electromagnetic NVH, provide a fast channel for electromagnetic NVH optimization from simulation to verification, and improve the optimization efficiency of NVH.

Bi‐static delay‐Doppler reference for cooperative passive vehicle‐to‐X radar applications

Automotive radar systems are indispensable for advanced driver assistance systems and traffic safety. Besides existing monostatic radar techniques, bi-static radar sensing like passive coherent location offers additional options to improve the radar visibility of vulnerable road users. Regarding the testing and evaluation of signal processing algorithms including parameter estimation, it is essential to provide electromagnetically shielded and reproducible measurement conditions, in addition to field tests in real traffic scenarios. This study describes the possibility to emulate relevant performance parameters for bi-static radar scenarios in the frequency range from 1 GHz to 6 GHz in a metal-shielded semi-anechoic chamber. Of special interest are the bi-static angle between transmitter, target, and receiver, and the resulting bi-static Doppler frequencies of a realistic vehicular traffic scenario. According to the concept of cooperative passive coherent location, Doppler scattering measurements are presented and compared to electromagnetic simulations. The authors find promising agreement between measured and ground truth data in the delay-Doppler spectrum.

A Flower Shaped Frequency Selective Surface for WLAN Screening Applications

This communication presents a single flower shaped resonator frequency selective surface (FSS) unit cell with stable oblique angles of incidence for both horizontal and vertical polarizations. The proposed FSS structure which can impede the frequencies from 3GHz to 6GHz at 3dB insertion loss is presented. The proposed FSS comprise an improved decagon to form a flower-shaped structure that exhibits single narrow stop band characteristics. The flower shaped FSS acts like a band-stop filter with center frequency of 4.8 GHz and notch band frequency ranging from 4.2GHz to 5.3GHz at 10dB insertion loss and 320MHz bandwidth at 20dB insertion loss. The designed FSS can use for shielding of public safety WLAN frequency. We present the equivalent circuit model and the surface current distribution to illustrate the resonance characteristics of the miniaturized FSS. To validate the design and simulation results, the proposed FSS is fabricated and tested in a semi-anechoic chamber