Anechoic Research Articles

High-efficient, polarization-insensitive, wide-angle, compact metamaterial energy harvester for S-band and C-band applications

This article proposes a polarization-insensitive compact Metamaterial (MM) energy harvester that can be used seamlessly in the S-band and C-band frequencies. It is important to focus on the limitations of many current designs of harvesters, which need to be overcome. The existing devices are large and often operate in a single frequency band, while their Energy Harvesting (EH) efficiency is low. The proposed harvester solves these problems using smart technology, using a rectangle strip with two gaps, each containing 50 Ω resistors for efficient energy collecting. Also, four hexagonal ring resonators are embedded into the cross-dumbbell configuration, connecting them with strip lines. Smaller rectangular rings surround these hexagonal rings, each with gaps labelled g1–g4. Despite its sophisticated design, the size of this harvester is (10 × 10) mm2 only. This harvester operates at frequencies of 3.5 GHz and 5.5 GHz, demonstrating remarkable absorption responses across varying polarizations and incident angles in both transverse electric (TE) and transverse magnetic (TM) modes. The simulation results indicated impressive energy harvesting efficiencies of 97% at 3.5 GHz and 98% at 5.5 GHz. In addition, experiments in an anechoic chamber with a 3 × 3 array (30 × 30) mm2 were used to confirm the efficiencies empirically. The simulated and measured results showed a strong correlation, confirming the reliability of the proposed design. The proposed MM harvester is distinguished by its high efficiency, polarization-insensitive behaviour, and compactness, making it very promising for many applications in EH.

Echo-free quality factor of a multilayer axion haloscope

We report a methodology to determine the quality factor (Q) of wavy dark matter detectors equipped with multilayered resonators, including Fabry-Pérot interferometers and the so-called dielectric haloscope. An anechoic chamber enables the observation of the resonance frequency and its amplitude for an unlimited series of layers for the first time, which is conveniently filtered. The frequency-normalized power enhancement measured in a Dark Photons and Axionlike Particles Interferometer (DALI) prototype is a few hundred per layer over a sweep bandwidth of a few dozen of megahertz. In light of this result, this scaled-down prototype is sensitive to axions saturating the local dark matter density with a coupling to photons between gaγγ≳10−12 and gaγγ≳few×10−14 GeV−1 at frequencies of several dozens of gigahertz once cooled down to the different working temperatures of the experiment and immersed in magnetic fields ranging from about 1 to 10 T; while the sensitivity of the full-scale DALI is projected at gaγγ≳few×10−15 GeV−1 over the entire 25–250 μeV range since Q≳104 is expected. Published by the American Physical Society 2024

Measurements and analysis of the radar cross section of chaff dipoles

AbstractA free space test method to measure the Radar Cross Section (RCS) of single dipole elements is presented. The method is such that dipoles can be measured over a large range of frequencies, in free space and without the need of an anechoic chamber. The setup employs with a Vector Network Analyser (VNA) synchronised to a rotating turntable that controls the position of the transmitting and receiving antennas and the polarisation of the irradiated electric field. Measurements are collected for chaff dipoles made of copper wire, microwire and aluminised glass, from 5 to 11 GHz, to allow a comparison of chaff RCS properties with respect to chaff material and method of fabrication. Measurements of the copper wire chaff also allowed a comparison with existing theoretical predictions of chaff reflectivity. An analysis and a comparison of the results is presented. Results show that the experimental trends of the chaff RCS curves agree with the theoretical RCS curves for perfectly conducting wires. They also show that copper wire provides the strongest reflective chaff followed by the aluminised glass and the microwire.

The Impact of Standing Waves on Localization Ability of Low-Frequency Sound Sources

This paper studies the effect of room modal resonances on the localization of very low–frequency sound sources. A subjective listening test is conducted with 20 participants in an anechoic chamber, where the listener must detect the direction of the sound source for pure sinusoids at 31.5, 50, and 80 Hz. A synthetic standing wave pattern modeling a room resonant effect is created with two additional sound sources located at the left and right sides of the listener. Results show that the perception of low-frequency direction is negatively impacted by the node of the standing wave, even when the standing wave has a relatively low level, whereas the antinode does not have as strong of an effect. The second experiment conducted indicates that variations in the presentation level does not impact the effect of the standing wave. The results of this study suggest that in the low-frequency spectrum, direction judgement is not so strongly a question of this auditory system’s ability but more so of the acoustical properties of the listening environment.

The effect of the molting cycle on the acoustic characteristics of clicks emitted by Litopenaeus vannamei

In penaeid shrimp, the mandibles are responsible for emitting click sounds when they collide during food intake, which has been used to assess feeding activity by passive acoustic monitoring. However, the acoustic parameters of the clicks could be affected by variations in the thickness of mandible cuticle during the molting cycle. Thus, the aim of this study was to evaluate the effects of the molting cycle on the acoustic characteristics of clicks emitted during the feeding activity of Litopenaeus vannamei fed commercial pelleted diet. Shrimp with an average weight of 8.75 ± 1.05 g were individually maintained in 24 tanks in a recirculating water system. A total of 12 animals had their feeding activity individually recorded soon after ecdysis (postmolt group), while another 12 were recorded in the middle of their molting cycle (intermolt group). Recordings took place in anechoic chambers with a hydrophone connected to a digital recorder (sampling rate of 192 kHz), and the click acoustic parameters were characterized in Raven® 1.5 Pro software. After the recordings, the shrimp had their mandibles removed for length measurement and histological analysis of the mandibular cuticle. Body (weight and length) and mandibular measurements did not differ between shrimp groups, but those in the postmolt stage showed significantly lower cuticle thickness. The acoustic characteristics of the clicks were affected by shrimp molting stages, with significantly lower maximum energy (dB) and maximum frequency (kHz) in postmolt shrimp, probably related to their lower mandibular cuticle thickness. These results may contribute to the assessment of shrimp feeding behavior associated with the molting cycle, as well as to the optimization of algorithms controlling acoustic automatic feeders in farming systems.

Highly isolated electrically compact UWB MIMO antenna for wireless communications applications

A multiple-input multiple-output Ultra-Wide Band (UWB) antenna with a compact size of 0.8λ x 0.7λ is proposed. The presented structure is constituted with patch geometry on a planar surface. The radiator consists of three counter U-shaped conducting strips united with a patch structure where two are positioned nearby and the third geometry covers the remaining conducting strips. A corrugated design has been created on a ground plane in which zigzag pattern slots are engineered to receive high isolation of more than 20 dB. The configured antenna radiates over a wide frequency spectrum of 3.10 GHz–11.85 GHz. It offers a fractional bandwidth of 195 % with a gain of 2.80 dBi, 3.08 dBi, and 2.72 dBi for targeted resonances. The antenna radiation patterns and MIMO diversity parameters are also presented. The acceptable values of ECC (<0.05 abs), MEG (−6 dB to −8 dB), CCL (<0.02 bits/sec/Hz) were found. The fabricated model was tested inside an anechoic chamber environment to receive the measured response with optimum accuracy through multiple iterations. The measured response shows a strong correlation with numerically computed responses finding the presented antenna suitable for WiMAX, WLAN (max protocol) and radar applications.

Wide angle wide band polarization insensitive metamaterial absorber for C, X and Ku band application with hexagonal packaging

Abstract In this paper wide angle wide band polarization insensitive Metamaterial absorber (MA) has been proposed. The proposed unit cell structure consists of hexagonal concentric ring along with hexagonal packing. The structure is fabricated on FR4 substrate. The advantage of hexagonal packing is that it covers less area as compared to square packing. The unit cell structure is scaled w.r.t hexagonal ring and resistance of the substrate. The absorptivity achieved range from 6.54 to 14.85 GHz having Bandwidth (BW) of 8.31 GHz. The metamaterial behaviour of the structure is proven by plotting real and imaginary pat of permittivity and permeability. Absorption mechanism is shown by plotting surface current distribution at the top and bottom of the surface. The proposed MA structure is analysed for normal and oblique incidence. From the normal incidence it was found that structure has wide angle polarization insensitive. The absorptivity of the fabricated structure is measured inside the anechoic chamber. The simulated and measured plot is compared and found that both the results are very close to each other. At last, the proposed and already reported MAs are compared and from the observation it was found that proposed unit cell have larger BW compact in size, polarization insensitive and have hexagonal packing. The proposed MA covers partial C band, full X band and partial Ku band. The Proposed MA found practical applications for satellite communication, Wi-Fi devices, cordless telephone, military, air traffic control, defence tracking, vehicle speed detection.

Reflectivity characterization of in-flow acoustic floor treatment for the NASA Langley 14- by 22-Foot Subsonic Tunnel

An experimental campaign was performed to assess acoustic modifications to the NASA Langley Research Center 14- by 22-Foot Subsonic Tunnel. Recent acoustic testing campaigns in this facility have emphasized the reduction of aerodynamic noise due to flow scrubbing across acoustically treated floor baskets. However, the measures implemented to address this noise contaminant have resulted in the installation of floor basket coverings of high acoustic reflectivity. Therefore, a controlled testing campaign was performed to identify a suitable acoustic basket configuration in terms of both acoustic absorptivity and reduced flow scrubbing noise. This testing campaign was conducted in two phases: the first in an anechoic chamber facility on a single acoustic basket panel to identify a notional configuration of acceptable absorptivity, and the second in a complete floor basket installation checkout test in the NASA Langley 14- by 22-Foot Subsonic Tunnel. Results show that the newly implemented floor treatment exhibits suitable performance in both desired categories of acoustic absorptivity and reduced flow scrubbing noise. This campaign has also validated the effectiveness and usefulness of the anechoic chamber test configuration as a means of assessing acoustic reflections at oblique angles of incidence.

Implementation of an experimental set-up for insertion loss measurements of sonic crystal acoustic screens

The use of metamaterials for the implementation of noise control devices in the context of environmental acoustics is a topic that has gained more and more attention in recent years. The complex design of such devices and the difficulty of their construction in real-world applications complicates the acquisition of experimental data related to their acoustical performance. Numerical simulation tools are used to simulate the behaviour of these novel noise control devices in their design phase and help to validate the designs by comparing them with experimental data. This data, usually acquired under controlled conditions, holds significant importance in confirming the increasingly precise numerical models that have been developed over the years to simulate the behaviour of these physical systems. In this paper, an experimental set-up that enables acquisition of acoustical performance data for a particular metamaterial-based noise control device, consisting of networks of isolated scatterers, is presented. This prototype possesses several distinctive characteristics that render it particularly interesting, including its adaptability, ease of use, speed of obtaining results, and, particularly, its economic viability in relation to the tests that can be conducted in an anechoic chamber.

Sparse recovery by genetic algorithms for acoustic testing applications

Various acoustic testing applications involve solving inverse problems from acoustic measurements, such as acoustic imaging and duct mode identification. In these problems, the physical quantity to be inferred is sparse, which allows applications of certain sparse recovery techniques, such as the compressive sensing. But such methods usually involve very complex theory, which therefore set a high threshold for beginners. By contrast, this work proposes a similar but much simpler method based on the well-known genetic algorithm, which borrows only a few easily understood concepts from nature evolution and can rely on easily accessed software/toolboxes. The performance of the developed method is demonstrated by two acoustic testing experiments in an anechoic chamber, i.e., acoustic imaging and duct mode identification.

Modelling of courtyard acoustics using the nodal discontinuous Galerkin finite element method to validate a newly developed PML

Evaluating the acoustics of courtyards has gained increasing attention from a sustainable urban design perspective, which requires numerical predictions tools for estimation of noise pollution and assessment of the influence of acoustic conditions. Accurate modeling of the acoustics of a courtyard is scarce because of the variety of factors to include: meteorological effects, building material properties, and geometry of the inner façades. We consider the nodal Discontinuous Galerkin finite element method framework in a 3D static and uniform medium and use it to model the sound propagation in a courtyard. A new and efficient Perfectly Matched Layer formulation is implemented at the open boundary of the courtyard model to absorb outgoing acoustic waves and this way limiting the size of the computational domain. The numerical results are validated against experimental measurements carried out in a scaled model of a courtyard with an elementary design to implement an accurate corresponding numerical model. The experiments were conducted in an anechoic chamber where the meteorological effects are controlled, and the courtyard facades are modelled as flat surfaces with known material properties.

LCV Vehicle in-cab noise prediction and reduction using SEA based simulation approach

With increase in consumer demand vehicle OEMs are developing quieter vehicles at a design stage using acoustic simulation tools to reduce development cycle times avoiding costly proto builds and test iterations. Present work describes application of statistical energy analysis (SEA) simulation approach for in cab noise prediction and noise reduction through sound package optimization for LCV vehicle. In first step complete vehicle in-cab SEA model is prepared with baseline sound package concept. It consists of comprehensive definition of structural panels, interior and exterior cavities. Noise control treatment is modelled as multi-layer poro-elastic materials. Airborne noise excitations for sources engine, transmission, intake and exhaust muffler are applied. Source strengths for above sources were derived from dyno-coupled powertrain noise tests in the anechoic chamber. Load case of rated speed was considered neglecting tire and wind noise during simulation. Contribution analysis was carried out to find out sensitivity of different panels on interior noise at various frequencies. It was observed that bottom middle floor panel and rear glass panel were major noise contributors. Noise mitigation was carried out by optimizing floor panel sound package and intake air filter within given space constraints. These modifications resulted in overall 2 dB in-cab noise reduction

Setting up light and noise maps to define the contours of a reserve of darkness and silence in Sherbrooke, Québec, Canada

Urban sprawl brings cities closer to natural areas, making them cohabit is a significant environmental challenge. Densification can also increase light and noise pollution, negatively affecting human and biodiversity well-being. Grounded on a collaboration between a college and a university, this work combines brightness and noise maps to establish the contours of a reserve of darkness and silence in Sherbrooke's Mont-Bellevue Park (Québec, Canada). The light and sound environments of the city are mapped by the research team using two devices mounted on vehicles or bicycles: a multispectral and multidirectional brightness sensor and a sound level meter. Long-term and fixed-point light and noise measurements were also conducted to develop the mobile measurement protocol. Wind tunnel measurements in an anechoic chamber were finally used to determine the effect of travel speed on measured noise levels, showing that speeds of less than 15 km/h ensure unbiased acoustic measurements using a windscreen. Perspectives include using the Noise Capture application to provide crowdsourced and additional noise levels at fixed measurement points. The open-source Noise Modelling tool will provide us with calculated environmental noise maps to be compared with measured data.

Study on noise barrier shape for reducing Shinkansen noise using scale model experiment

Since Shinkansen viaducts are aging, a large-scale renovation project has been planned. In this project, aged noise barriers of various shapes are scheduled to be removed and replaced. After replacing the noise barriers, further noise reduction is required. Therefore, it is desirable to verify the relationship between shapes of noise barriers and noise reduction effects. Against this background, the scale model experiments were conducted in anechoic chamber, focusing on the heights, the edge shapes of noise barrier and the differences in the area where sound absorbing materials were applied to the noise barrier. In these experiments, noise levels outside a viaduct were evaluated when the conditions of the noise barrier were changed. As one of the results, it is shown that applying sound absorbing material as large an area as possible is effective in reducing noise, regardless of the differences of edge shapes. Based on the experiments, it is thought that noise barriers with simple shape for easy maintenance can be realized by effective use of sound absorbing materials for replacing the aged noise barriers.

Basic examinations of the sound transmission loss measurement using two cardioid microphones

This paper reports results of an experiment on sound transmission loss measurement using the C-C method. This method is an inexpensive and stable method for measuring the sound pressure, the particle velocity, and the sound intensity. The measurements are carried out in accordance with JIS A 1441-1:2007 based on ISO 15186-1:2000, and the C-C method is applied for measuring the sound intensity. Comparing the measured results with those of the sound pressure level difference method, comparable results are obtained above 250 Hz. Naturally, the precision of the measurements depends on the microphone calibration used in the C-C method. In this report, we employ two calibration methods using an acoustic tube and an anechoic chamber, and also exhibit comparison of the results obtained by these methods.

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.

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).

Feasibility of an acoustic liner applied to a Fenestron: experimentation

A helicopter's anti-torque system is a significant contributor to radiated noise. The Fenestron, used on Airbus Helicopters, allows to mitigate the anti-torque noise by masking effect and modulation of the blade distribution. However, unlike aircraft engine nacelles, it does not contain acoustic liners inside the duct to absorb the noise radiated by the rotor blades. This paper describes: first, the (aero-)acoustic tests applied to different acoustic liners optimised to be integrated into a 1/3-scale fenestron mock-up, in order to provide a specific impedance (measurement in impedance tube and aeroacoustic bench); second, the configuration of a mock-up installed in an anechoic room, with an acoustic antenna; third, the acoustic pressure mitigation obtained for several diffusers equiped with liners (variation of RPM and pitch for the purpose of the experiment). It appears that the tested liners are likely to provide a high absorption coefficient in a large frequency range and a significant attenuation when installed in the Fenestron mock-up, in accordance with preliminary simulations. Another paper completes the study with a preliminary numerical assessment of the effect of an absorbing treatment introduced inside the collector or the diffuser, and a design optimization applied to different acoustic liner types, adapted to the mock-up.

Multirotor drone noise: numerical approach for aeroacoustics and in-flight noise prediction

Nowadays, multirotor drone usage has become widespread for various purposes: amateur or professional videos, building inspection, civil security, military defense, etc. But this usage increase has also seen growing concerns about community noise annoyance in inhabited areas, acoustic detectability for security and military purposes, or remote wildlife monitoring. Predicting the drone noise can now be accurately achieved thanks to computational aeroacoustics (CAA). The present work deals with the aeroacoustic simulation and measurement of a quadcopter drone and in-flight condition prediction. The CAA simulation of a single rotor is first compared with test bench measurements in an anechoic room. Comparison for the full drone is then derived from this single-rotor comparison. Finally, the study is further extended to in-flight comparisons, in order to show that CAA simulation can help design quieter drones in real operating conditions.

Feasibility of an acoustic liner applied to a Fenestron: simulation

A helicopter's anti-torque system is a significant contributor to radiated noise. The Fenestron, used on Airbus Helicopters, allows to mitigate the anti-torque noise by masking effect and modulation of the blade distribution. However, unlike aircraft engine nacelles, it does not contain acoustic liners inside the duct to absorb the noise radiated by the rotor blades. This paper describes: first, the FEM modelization of the noise radiation for a 1/3-scale fenestron mock-up at the 1st BPFs, in static conditions; second, the assessment of the effect of an absorbing treatment introduced inside the collector or the diffuser and defined by its acoustic specific impedance; third, the design optimization applied to different acoustic liner types, adapted to the diffuser of the mock-up. Because of the limited duct length, forward and afterward radiations interact, generating interference pressure patterns. Several types of liners (conventional and unconventional), designed to deliver a high absorption, appear adapted to dimensional constraints: SDOF or DDOF type, with a perforated plate above cavities or based on the concept LEONAR ("Long Elastic Open Neck Acoustic Resonator"). Another paper completes the study with (aero-)acoustic tests, applied to elementary samples to validate the impedance target, and performed in an anechoic room with the mock-up.