Non-anechoic Environment Research Articles

Robotic arm, 3D vision and geometric matching for an efficient implementation of inverse Patch Transfer Function method

The inverse patch transfer function method was developed to reconstruct the acoustic fields (velocity, pressure, intensity) at the surface of a vibrating source with a complex geometry using the concept of a virtual acoustic domain. This method can be used to manage sources with complex shapes in a non-anechoic acoustic environment, even in the presence of masking objects. However, this method is quite demanding experimentally as it requires a large number of measurement positions with good spatial positioning accuracy. This article presents a dedicated experimental platform developed to simplify and automate the entire measurement process. First, a 3D camera placed on a robotic arm constructs a point cloud characterising the source and its environment. Using a geometric matching algorithm, the CAD of the object under test is detected in the cloud of points, enabling the different coordinate systems (CAD, robot and physical part) to be matched. Secondly, the measurement points required to apply iPTF can be defined directly in the CAD and reproduced in near-real time on the test rig, regardless of the type of antenna used, providing an efficient means of applying iPTF and opening the way to many advanced applications.

Acoustic field reconstruction in the presence of masking objects

In acoustics, field reconstruction methods aim at retrieving acoustic fields (pressure, velocity and intensity) from acoustic measurements around a radiating source, which is often a vibrating structure of complex shape (pumps, engines…). If they are extensively used in laboratory conditions, their application to in situ characterization is not straightforward due to the presence of disturbing sources or masking objects, or to non-anechoic environments. The inverse Patch Transfer Function (iPTF) approach, thanks to the concept of virtual acoustic volume modelled by a finite element model, has already demonstrated its ability to deal with sources of complex shape and the presence of disturbing stationary sources in a non-anechoic acoustic environment. The objective of this article is to show how the presence of rigid masking objects can be easily and efficiently taken into account. A numerical experiment consisting of a thin, simply supported rectangular plate radiating noise in a semi-infinite acoustic field and partially masked by a rigid parallelepiped is presented. The acoustic fields identified and the directivity diagrams are compared with the reference and show that iPTF is able to cancel the presence of the masking object even if the latter completely covers the radiating plate. Finally, an industrial example consisting in an electric motor in operation is presented. Two configurations were tested: with and without the presence of a rigid object. Comparison of the results shows that the fields identified are in good agreement, demonstrating the ability of iPTF to cancel out the effect of masking objects.

Extrapolation and inversion of near-field propeller noise measurements

This paper describes an experimental method for making high resolution measurements of the tonal noise from a UAV propeller in the acoustic near-field and then methods of using this data to predict blade loading, via inversion, and the radiated ‘steady loading’ and ‘thickness’ tones, via extrapolation. The experimental method uses a traversing probe microphone to measure the acoustic pressure along an axial line outboard of the propeller tip. The acoustic pressures were then ensemble averaged over many propeller revolutions. Because of the azimuthal periodicity of the ensemble-average pressure field produced by the propeller, the measured data was used to infer the pressure on a cylinder enclosing the propeller. These measurements are compared with a noise prediction made using data from a computational fluid dynamics simulation of the same propeller and are found to be in reasonable agreement – with differences attributed to the small differences in the predicted and actual blade loading distributions. Because of the high amplitude and evanescent nature of the near-field pressure produced by a propeller, measurements were made in a non-anechoic environment and were relatively unaffected by background noise or reverberation effects. The near-field acoustic data was used as input to an inverse problem to estimate the aerodynamic loading distribution on the propeller blades. The method was applied first to the numerical data for validation and then to the experimental data as a demonstration. The reconstructed loading distributions were then used as input to a method for calculating the radiated acoustic field and the predictions show good agreement with a direct projection using the measured near-field experimental data.

Evaluation of the noise impact of flap-tip fences installed on laminar wings

The aeroacoustics of laminar wings and the noise abatement provided by flap-tip fences are studied experimentally on a scaled model installed in the closed test section of a subsonic wind tunnel. The activity has been developed under an international project funded by the European Union through the H2020 Framework Program. The model reproduces an innovative regional aircraft mounting aft-engines and adopting the Natural Laminar Flow concept. Both take-off and landing settings are tested for several combinations of wind tunnel speeds and angles of attack; these configurations being the most critical from the viewpoint of airframe noise generation. A baseline configuration without flap-tip fences is also tested for a comparative study. Noise sources are identified by measuring pressure fluctuations through a phased microphone array. Data are processed using both Conventional Beamforming and CLEAN-SC algorithms to retrieve the sound source maps and the integrated spectra over the areas of interest. Directivity effects are investigated as well by moving the microphone array in different axial positions corresponding to different aircraft polar directivity angles. The wind tunnel data are eventually extrapolated to full scale and projected to flight condition allowing the analysis of the results in terms of Effective Perceived Noise Level (EPNL). Tests provided an extensive characterization of the acoustic behavior of the analyzed model demonstrating the capability of measurements carried out in a non-anechoic environment, to provide reliable data. The longitudinal traversing of the microphones array allowed us to compute, even though qualitatively, the EPNL and thus demonstrated the feasibility of a procedure that can be a reference for the design of future aeroacoustic tests. The effectiveness of the flap-tip fences to successfully reduce the flap side-edge noise has been definitely verified both through the analysis of the acoustic maps retrieved from the beamforming investigation and by the estimation of the overall variation of the EPNL with respect to a baseline reference configuration without low-noise devices.

Experimental Moving Target Imaging in a Nonanechoic Environment With Linear Sampling Method

In this letter, imaging of moving targets with qualitative microwave imaging methods (Q-MWMs) is addressed. The problematic side of Q-MWM is the requirement of background measurement. To eliminate this necessity, the total electric field measured at different time instants (say Etot n and Etot l are the total electric fields measured at the nth and lth time instants, respectively) is applied to Q-MWM. For these input data, the output of Q-MWM can be assumed to be the summation of the indicators at these time instants (i.e., Etot n - Etot l produces the differential indicator Inl = In + Il, where In and Il are the indicators at the nth and lth time instants, respectively). Using this information for all possible time pairs, an equation system is set for indicator values at different time instants. Solving this equation system, the indicator of Q-MWM for each measurement time is obtained, without taking any background measurement. The performance of the proposed algorithm is verified for the linear sampling method (LSM), which is an example of Q-MWM, with the 3-D and 2-D [both transverse magnetic (TM) and transverse electric (TE)] experimental measurements performed in a nonanechoic environment.

A high capacity tunable retransmission type frequency coded chipless radio frequency identification system

This article presents a 12-bit frequency coded chipless RFID system in the frequency range of 3 to 6 GHz. The system consists of a fully printable chipless tag and a pair of high-gain reader antennas. The tag also incorporates its own antennas to improve the read range. Information is encoded into frequency spectrum using a multi-resonant circuit. The circuit consists of multiple microstrip U and L-shaped open stub resonators patterned in a unique configuration. The proposed configuration aids in capturing more data in a reduced space as well as tunable frequency operation. Tag and reader antennas utilize techniques such as stepped impedance feeding line, defective partial ground plane, and stair-step patch structure to achieve wide-band impedance bandwidth in miniature size. The results of the wireless measurements in the non-anechoic environment show that the proposed system has a reading range of more than 20 cm. The presented system possesses great potential for low-cost short-range inventory tracking.

Microwave Imaging of Electrical Wires With MUSIC Algorithm

Microwave imaging of electrical wires with multiple signal classification (MUSIC) algorithm is considered. The motivation arises from safety/security applications where concealed electrical wires are required to be remotely sensed in a quasi-real-time setting. The algorithm is first reanalyzed in terms of scattering parameters to better incorporate experimental microwave measurements. Later, a simple strategy, where the spectral cutoff is selected based on the expected signal-to-noise level, is introduced to enhance imaging capabilities. The reformulated algorithm is experimentally verified with limited aperture measurements performed with an antenna array in a nonanechoic environment. The results demonstrate that the algorithm can rapidly reconstruct images that clearly reveal geometrical features of electrical wires.

Analysis of Artifacts on Chipless RFID Backscatter Tag Signals for Real World Implementation

Chipless radio-frequency identification (RFID) has been studied as an alternative technique to conventional chipped RFID. However, chipless RFID tag backscatter signals are affected by motion and especially when they are attached to different objects. In this paper, we performed a comprehensive analysis of the backscattered chipless tag signal in two different stages. At the first stage, we performed an analysis of the backscattered signal in the nonanechoic environment when the frequency-coded tags were moving at different stepped positions. Then we continued to identify the response of signal where the chipless tag was attached to highly scattering objects. The results showed the variation in amplitude and shifting in the frequency of tag resonant when the tag was moving at different stepped positions. The variation was more prominent when the tag was attached to random objects. In order to address the issue, we conducted an experiment on orientation insensitive ground plane tag to solve the frequency shifting problem and proposed a robust detection method based on mother wavelet for the detection of tag IDs. The overall results show that the proposed technique reduces uncertainty in frequency-based chipless RFID tag signal response due to the variation in amplitude. The technique also helps in detecting the tag data bits with better resolution and has a good arrangement between time and frequency resolution for high-frequency signal components. The throughput of 90% is obtained even when the tag is attached to highly scattering objects. Finally, to validate the performance, a ground plane tag is implemented and the proposed algorithm is experimentally demonstrated.

A design methodology for quiet and long endurance MAV rotors

Over the last 10 years, the use of micro air vehicles has rapidly covered a broad range of civilian and military applications. While most missions require optimizing the endurance, a growing number of applications also require acoustic covertness. For rotorcraft micro air vehicles, combining endurance and covertness heavily relies on the capability to design new propulsion systems. The present paper aims at describing a complete methodology for designing quiet and efficient micro air vehicle rotors, ranging from preliminary aerodynamic prediction to aeroacoustic optimization to experimental validation. The present approach is suitable for engineering purposes and can be applied to any multirotor micro air vehicle. A fast-response and reliable aerodynamic design method based on the blade-element momentum theory has been used and coupled with an extended acoustic model based on the Ffowcs Williams and Hawkings equation as well as analytical formulations for broadband noise. The aerodynamic and acoustic solvers have been coupled within an optimization tool. Key design parameters include the number of blades, twist and chord distribution along the blade, as well as the choice of an optimal airfoil. An experimental test bench suitable for non-anechoic environment has been developed in order to assess the benefit of the new rotor designs. Optimal rotors can maintain high aerodynamic efficiency and low acoustic signature with noise reductions in the order of 10 dB(A).

Free-Space Materials Characterization by Reflection and Transmission Measurements using Frequency-by-Frequency and Multi-Frequency Algorithms

The knowledge of the electromagnetic constitutive properties of materials is crucial in many applications. Free-space methods are widely used for this purpose, despite their inherent practical difficulties. This paper describes an affordable free-space experimental setup for the characterization of flat samples in 1–6 GHz in a non-anechoic environment. The extracted properties are obtained from the calibrated Scattering Parameters, using a frequency-by-frequency solution or a multi-frequency reconstruction. For the first, we describe how the Time-Domain Gating can be implemented and used for filtering the signals. For the latter, a weighting factor is introduced to balance the reflection and transmission data, allowing one to have a more favorable configuration. The different role of transmission and reflection measurements on the achievable results is analyzed with regard to experimental uncertainties and different noise scenarios. Results from the two strategies are analyzed and compared. Good agreement between simulation, measurement and literature is obtained. According to the reported results for dielectric materials, there is no need of filtering the data by a Time-Domain Gating in case of the multi-frequency approach. Experimental results for Polymethylmethacrylate (PMMA) and Polytetrafluorethylene (PTFE) samples validate both the setup and the processing.

Active binaural localization of multiple sound sources

Sound source localization serves as a significant capability of autonomous robots that conduct missions such as search and rescue, and target tracking in challenging environments. However, localization of multiple sound sources and static sound source tracking in self-motion are both difficult tasks, especially when the number of sound sources or reflections increase. This study presents two robotic hearing approaches based on a human perception model (Wallach, 1939) that combines interaural time difference (ITD) and head turn motion data to locate sound sources. The first method uses a fitting-based approach to recognize the changing trends of the cross-correlation function of binaural inputs. The effectiveness of this method was validated using data collected from a two-microphone array rotating in a non-anechoic environment, and the experiments reveal its ability to separate and localize up to three sound sources of the same spectral content (white noise) at different azimuth and elevation angles. The second method uses an extended Kalman filter (EKF) that estimates the orientation of a sound source by fusing the robot’s self-motion and ITD data to reduce the localization errors recursively. This method requires limited memory resources and is able to keep tracking the relative position change of a number of static sources when the robot moves. In the experiments, up to three sources can be tracked simultaneously with a two-microphone array.

Method for material characterization in a non-anechoic environment

This paper presents a characterization method for extracting the reflection coefficient of materials and the real part of their permittivity. The characterization is performed in a real environment, as opposed to the classical measurement methods that require an anechoic chamber. In order to reduce the effects of the multipath propagation, a free space bistatic measurement was performed at different distances material-antennas in far field. By using a Teflon sample and a commercial absorbing material sample, measurements have been performed in order to validate the characterization technique.

Aeroacoustic measurements for an axial fan in a non-anechoic environment

Determination of the aeroacoustic emission from an axial fan in a non-anechoic environment is a challenging experimental task given ambient noise and acoustic reflections from surrounding objects. Successful strategies to address this task for a representative nine and three blade fan are presented. An array consisting of ten microphones was constructed and placed in the upstream region of the axial fans to measure the fan acoustic signature at ten distinct locations. A novel delay and sum (DS) beamforming technique (that allows precise time delays to be established by the use of cross correlation techniques) was applied to the microphone outputs in order to separate the fans’ acoustic emissions from the ambient noise and reflections from the facility walls. A numerical simulation was developed to represent the experimental facility and the measurements. The numerical simulation indicated that the extraneous noise can be satisfactorily separated from the fan noise using the array measurements and post processing the acoustic data with the present DS beamforming technique.

Normalization-Free Chipless RFIDs by Using Dual-Polarized Interrogation

A reliable encoding/detection scheme for chipless radio frequency identification (RFID) tags, free from any normalization procedure, is presented. The key strategy of the present approach consists in storing the information in the difference between vertically and horizontally polarized reflection coefficients of a completely passive tag. The measured reflection coefficients are preemptively filtered in the time domain to remove most of harmful effects due to the antenna coupling and environment multipath, and finally they are subtracted to obtain the differential response. A couple of chipless tag configurations suitable for providing the desired spectral response are presented. The resonators consist of an artificial impedance surface comprising either concentric rectangular loop resonators or square loop resonators loaded with stubs. The presented approach is experimentally verified in a non-anechoic environment, and its robustness is proved. This calibration-free approach could pave the way to practical applicability of chipless RFID tags in realistic scenarios with unknown response.

Inverse Patch Transfer Functions Method as a Tool for Source Field Identification

Many methods to detect, quantify, or reconstruct acoustic sources exist in the literature and are widely used in industry (near-field acoustic holography, inverse boundary element method, etc.). However, the source identification in a reverberant or nonanechoic environment on an irregularly shaped structure is still an open issue. In this context, the inverse patch transfer functions (iPTF) method first introduced by Aucejo et al. (2010, “Identification of Source Velocities on 3D Structures in Non-Anechoic Environments: Theoretical Background and Experimental Validation of the Inverse Patch Transfer Functions Method,” J. Sound Vib., 329(18), pp. 3691–3708) can be a suitable method. Indeed, the iPTF method has been developed to identify source velocity on complex geometries and in a nonanechoic environment. However, to obtain good results, the application of the method must follow rigorous criteria that were not fully investigated yet. In addition, as it was first defined, the iPTF method only provides source velocity while wall pressure or intensity should also give useful information to engineers. In the present article, a procedure to identify wall pressure and intensity of the source without any additional measurement is proposed. This procedure only needs simple numerical postprocessing. Using this new intensity identification, the influence of background noise, evanescent waves, and mesh discretization are illustrated on numerical examples. Finally, an experiment on a vibrating plate is shown to illustrate the iPTF procedure.

Fast measurement system and super high directional resolution head-related transfer function database

Head-related transfer functions (HRTFs) describe the acoustical transmission process from a point sound source to two ears in the free field. They are vital to the researches of binaural hearing and virtual auditory display. Measurement is the most important approach to obtain HRTFs, but it is time-consuming. To accurate HRTF measurement, a computer-controlling measurement system is designed. The system consists of multiple sound sources at different elevations with a finest interval of 5° and a horizontal turntable with an azimuthal resolution of 0.1°. It is able to work in non-anechoic environment and suitable for both mannequin and human subject measurement. A practical example indicates that the system allows for measuring far-field HRTFs at 493 source directions within half an hour. By using the system, a super high directional resolution HRTF database for KEMAR mannequin is established. The database includes 3889 pairs of head-related impulse responses (HRIRs, the time domain counterpart of HRTFs) at distance of 1.0 m, elevation interval of 5° (from -45° to 90° ), and azimuthal interval of 2.5°. Each HRIR is 1024-point length at 96 kHz sampling frequency and 24-bit quantization. The database is applicable to the research on binaural hearing and virtual auditory display.

Free Space Radiation Pattern Reconstruction from Non-Anechoic Measurements Using an Impulse Response of the Environment

The objective of this paper is to investigate a methodology, which can extract approximate results for the free space radiation pattern from non-anechoic measurements. Using an impulse response both in the time and angular domains of the non-anechoic measurement environment, the free space pattern of the device under test is estimated. The purpose of this paper is, as opposed to what has been stated in some papers, to show that a deconvolution based technique is feasible for reflection compensation in non-anechoic measurements. The proposed method can also be applied at a single frequency as illustrated in this paper. Simulated data has been used to illustrate the applicability of this new technique and its improved performance over the conventional FFT-based methods.

차량 내부에서의 모노폴 안테나 방사패턴 연구

본 논문에서는 차량 내부에서 발생되는 안테나 방사패턴의 변화를 관찰하기 위해 모노폴 안테나를 사용하여 차량 내부에서 안테나의 방사패턴을 측정하였다. 다수의 반사파가 존재하는 좁은 공간에서는 안테나의 특성이 변해 무선 통신 환경에 영향을 끼칠 수 있다. 그러므로 실제 차량 내부에서 제공되는 무선 통신 서비스인 GPS 와 Wibro 동작 주파수 대역에서 안테나 방사패턴에 대한 모의실험과 측정실험을 수행하였다. 모의실험은 광선추적법을 기반으로 하였으며 측정실험은 각 동작 주파수 대역을 중심 주파수로 갖는 두 쌍의 모노폴 안테나를 제작하여 자유공간과 차량 내부에서 주파수에 따라 방사패턴을 측정하였다. 모의실험과 측정실험의 결과를 바탕으로 차량 내부와 자유공간에서의 안테나 방사패턴을 비교하였을 때 안테나의 전방향성 특성을 유지되며, 차량 내부에서 안테나의 수신전력이 자유공간에서의 결과보다 증가하는 것을 확인 할 수 있었다. 또한 주파수가 증가할수록 자유공간과 차량 내부에서의 수신전력 차이가 증가하는 것을 확인하였다. 【In this paper, the antenna radiation pattern inside the vehicle is presented for observing the change occurring in non-anechoic environment using monopole antenna. The environment for radio communication can be affected by antenna characteristic variation inside the small space existing multi reflection waves. To perform simulation and measurement about antenna radiation pattern, the radio frequency bands for GPS and Wibro services expected to use inside the vehicle is selected. The simulation is based on the ray-tracing method and the radiation pattern is measured inside the vehicle and in free space using monopole antenna having those frequency bands. In accordance with the measurement and simulation results, when two antennas are put inside the vehicle, the omni-directional characteristic of the antenna is maintained but the received power is increased than free space case and the difference between the each case of measurements is increased as the operating frequency is increased.】

Generation of Free Space Radiation Patterns From Non-Anechoic Measurements Using Chebyshev Polynomials

Generally, antenna radiation patterns are measured in an anechoic condition so that the presence of undesired reflections does not skew the final measurements. The objective of this communication is to search for methodologies which can extract meaningful approximate results from non-anechoic measurements. In other words, how to process the reflection contaminated data and extract valuable information when one does not have a good anechoic room and performs measurements with various undesired reflections present in the data. The Chebyshev polynomials are used to extract from the data collected under non-anechoic environment and generate a radiation pattern for the same antenna radiating in free space conditions. The problem with this method is that some <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</emphasis> information is required about the environment. Measured data has been used to illustrate the applicability of this new methodology and its improved performance over the FFT based methods.

Identification of source velocities on 3D structures in non-anechoic environments: Theoretical background and experimental validation of the inverse patch transfer functions method

In noise control, identification of the source velocity field remains a major problem open to investigation. Consequently, methods such as nearfield acoustical holography (NAH), principal source projection, the inverse frequency response function and hybrid NAH have been developed. However, these methods require free field conditions that are often difficult to achieve in practice. This article presents an alternative method known as inverse patch transfer functions, designed to identify source velocities and developed in the framework of the European SILENCE project. This method is based on the definition of a virtual cavity, the double measurement of the pressure and particle velocity fields on the aperture surfaces of this volume, divided into elementary areas called patches and the inversion of impedances matrices, numerically computed from a modal basis obtained by FEM. Theoretically, the method is applicable to sources with complex 3D geometries and measurements can be carried out in a non-anechoic environment even in the presence of other stationary sources outside the virtual cavity. In the present paper, the theoretical background of the iPTF method is described and the results (numerical and experimental) for a source with simple geometry (two baffled pistons driven in antiphase) are presented and discussed.