Anechoic Test Chamber Research Articles

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.

Experimental Study of Omnidirectional Scattering Characteristics of Complex Scale Targets Based on Coded Signals

To investigate the omnidirectional geometric scattering characteristics of an underwater vehicle and the target detection performance of phase coded (BPSK) signals, acoustic scattering tests were carried out in an anechoic chamber using the Suboff scale model. To mitigate the overlapping interference of the direct wave on the scattering wave in the limited test space, physical suppression with an “anechoic cloak” and direct wave cancellation were proposed. Target echo and reflection wave tests at different offset angles were carried out, and the accuracy of the BPSK signal in acquiring highlight features and the feasibility of anechoic chamber tests were verified through comparison with theoretical range profiles. A series of echo and omnidirectional scattering characteristics were obtained through the experiment and simulation, which verified the effectiveness of the low-frequency submarine model detection (there were still strong scattering waves at the dimensionless frequency ka = 1.88). Comparison tests of CW, LFM, and BPSK signals were carried out, and the measured data proved that the BPSK signal had the advantages of low sidelobe, high resolution, and noise resistance in target detection. The acoustic scattering test method designed in this study and the omnidirectional scattering characteristics obtained can be used as a reference for semi-physical target acoustic scattering simulations and practical multistatic detection.

Development of a weather robust microphone configuration for sonic boom measurments

This paper discusses ongoing development and testing of ground-based, weather-robust microphone measurement systems in conjunction with preparation for community testing of NASA's X-59 low-boom aircraft. Prior efforts [Anderson et al., Proc. Mtgs. Acoust. 42, 040005 (2022)] resulted in the refinement of a ground-plate setup by investigating varied windscreen and plate diameter and thickness. The most recent goal has been to make the setup more compact and easier to manufacture. This paper discusses the results of additional tests performed on updated designs meant to find the balance between compactness and performance. Anechoic chamber testing was performed to show ground plate and windscreen performance and to compared results against prior versions. Outdoor tests included measurements during different wind conditions and over different ground surfaces to examine low-frequency wind noise reduction and ground impedance effects. Results discussed during the presentation suggest the new design strikes an acceptable compromise between compactness, manufacturing ease and robustness, and performance. [Work supported by NASA Langley Research Center through Analytical Mechanics Associates]

Multirotor broadband noise modulation

Rotor broadband noise spectra are typically analyzed over time scales on the order of one or more rotor periods. However, modulation of the broadband noise spectrum with the blade passage frequency (BPF) has been shown to be significant for noise levels and perception of wind turbines and helicopters. In contrast, time-varying broadband noise has not been extensively studied for aircraft with many rotors, such as unmanned aerial vehicles (UAVs) or advanced air mobility aircraft. In this work, significant broadband noise modulation was measured in flight and anechoic chamber tests of hexacopter UAVs at various observer angles. This modulation is aperiodic with the BPF such that the modulation amplitude varies substantially between blade passages, even when the BPFs are controlled to be nearly constant between all rotors at all times. Furthermore, the azimuthal phasing between rotors greatly affects the measured modulation, such that the modulation of multiple rotors may be less than or greater than for a single rotor, depending on the phase offsets. The effects of phase variations on acoustic interactions between rotors is studied by comparing the sum of the modulation of individual rotors to the modulation of those rotors operating simultaneously. This is done not only using measurements, but also noise predictions made using PSU-WOPWOP. These results contribute understanding to how the noise modulation of rotors sum together, including the resulting directivity and aperiodicity.

Low profile high gain RHCP antenna for L-Band and S-Band using rectangular ring metasurface with backlobe suppression.

In this reported work a single feed, miniaturized, dual layer, and low profile antenna is presented for 1.575GHz frequency band. The proposed antenna offers high gain, lower noise bandwidth, with better sensitivity and range. The ground choke technique is used for back lobe suppression. The prototype is fabricated on FR 4 substrate using manual fabrication technique. This offers an inexpensive and readily available fabrication. Therefore, fabricated antenna is small size, low cost, easily fabricated and tested for satellite communication. The antenna comprises of two layers, containing a patch radiator and a Metasurface layer with 3x3 rectangular ring resonators. The layers are separated using foam with a 12mm width. The proposed prototype is radiating at 1.575GHz and 2.33GHz with an overall dimension of 85.6 x 68.4 x 15.204 mm. The developed antenna provides a gain of 5.9 dBi. The simulated results are verified using VNA and anechoic chamber testing. Moreover, the developed antenna has been successfully tested for L-Band Satellite communication in real time scenario without any LNA. Higher Gain due to Metasurface increase the efficiency of the system. The promising results indicate the aptness of the developed antenna for real-world applications of L-Band and S-Band.

AeroFeathers: Feathered airfoils inspired by the quiet flight of owls

Owls are well-known as the quietest flying birds. Recent studies show that they achieve noise-less flight because of their specialized feather design. Their feathers have three important components: a velvety surface texture, comb-like hooks on their leading edge, and jagged fibrous fringes on their trailing edges. Some researchers have attempted to imitate these design features within airfoil designs to reduce aeroacoustic noise emitted by fixed wing and vertical lift vehicles; however, such features cannot be easily fabricated using traditional manufacturing processes. In this student-led project funded by NASA’s University Student Research Challenge, we leverage our established success at additively manufacturing fibrous structures to introduce a new method to 3-D print velvety textures, fibrous fringes, and flexible edge serration using low-cost material extrusion-based printing methods. By altering the printer’s G-code, we create customized feathered airfoils with individual design parameters, including serration thickness and fringe density. We conduct acoustic tests in an anechoic chamber and aeroacoustic tests in an acoustic wind tunnel to determine the effect of each parameter on the overall acoustic signature of the airfoil. We analyze the data and explain the role of owl feather-like features on the noise reduction performance of propeller blades and scaled fixed-wing airfoils.

Preparation and characterization of Y-shaped fractal frequency selective fabric

In order to explore the electromagnetic transmission performance of frequency selective fabrics (FSFs) with fractal units, six samples of Y-shaped fractal parameters with different sizes were prepared by computer embroidery technology, and tested using anechoic chamber test method. The effects of unit type, polarization mode, incident angle and unit size parameters on the transmission performance of different samples were respectively analyzed in depth. The results show that for the aperture type and patch type FSFs, the incident angle has a certain influence. When the incident angle of the two groups of samples is 0°, the resonance peak is the largest. When the incident angle increases to 45°, there are two resonance peaks of different amplitudes in the range of 4 GHz ∼ 20 GHz. The size and spacing of the unit pattern have both visible effects on the transmission coefficient, but the former has a greater impact on the transmission coefficient. There is a certain tension and contraction between the base fabrics during the preparation of the flexible FSFs, which are controlled to make the sample size more accurate. In addition, in order to reduce the calculation time and improve the design efficiency, two kinds of patch type and aperture type with the same size were simulated and optimized using HFSS algorithm. Comparison of measured and simulated results verify the effectiveness of simulation design.

Improved Optical Path Structure for Symmetric Demodulation Method in EFPI Fiber Optic Acoustic Sensors Using Wavelength Division Multiplexing.

This paper presents a novel improvement in the optical path structure of a three-wavelength symmetric demodulation method applied to extrinsic Fabry-Perot interferometer (EFPI) fiber optic acoustic sensors. The traditional approach of using couplers to construct the phase difference in the symmetric demodulation method is replaced with a new approach that combines the symmetric demodulation algorithm with wavelength division multiplexing (WDM) technology. This improvement addresses the issue of a suboptimal coupler split ratio and phase difference, which can affect the accuracy and performance of the symmetric demodulation method. In an anechoic chamber test environment, the symmetric demodulation algorithm implemented with the WDM optical path structure achieved a signal-to-noise ratio (SNR) of 75.5 dB (1 kHz), a sensitivity of 1104.9 mV/Pa (1 kHz), and a linear fitting coefficient of 0.9946. In contrast, the symmetric demodulation algorithm implemented with the traditional coupler-based optical path structure achieved an SNR of 65.1 dB (1 kHz), a sensitivity of 891.75 mV/Pa (1 kHz), and a linear fitting coefficient of 0.9905. The test results clearly indicate that the improved optical path structure based on WDM technology outperforms the traditional coupler-based optical path structure in terms of sensitivity, SNR, and linearity.

An Extra Low-Mass Harmonic Radar Transponder for Insect Tracking Applications

The design, construction, and performance of a harmonic radar transponder with a total mass of less than 500 μg is presented. The transponder is intended for insect tracking applications and consists of very fine wire and a small Schottky diode. It is designed for fundamental and harmonic frequencies of 10 GHz and 20 GHz, respectively. Compared to existing harmonic radar transponders, this transponder is easy to construct because the loop inductor can be implemented with a simple bend in the dipole conductor without degrading performance. Through careful design optimization, the conversion loss of the transponder is not impacted by the measures taken to minimize its mass. The expected harmonic power versus the transmitted power is estimated based on the link analysis between the transmitter and receiver of the radar, with the link analysis itself being performed via calculation, harmonic balance simulation, and full-wave simulation. The link analysis simulation predicted a received power of -66.4 dBm for a transmitted power of +22 dBm and a range of 2.4 m. The measured received power level at the harmonic frequency, obtained from the broadside of the transponder in an anechoic test chamber, is approximately -70 dBm, which agrees well with the link analysis. Simulated and measured transponder radiation patterns are also compared and show good agreement. Low-mass transponders such as this enable tracking of smaller insects without reducing their lifespan or compromising their ability to fly at natural altitudes and ranges.

Stealthy Configuration Optimization Design and RCS Characteristics Study of Microsatellite

Firstly, the radar cross section (RCS) test results of the stealthy microsatellite of TianXun-1(TX-1) in the anechoic chamber are compared with the RCS numerically simulated by the physical optics method, and the accuracy of the physical optical method is verified. On this basis, in order to improve the radar stealth performance of the microsatellite, a satellite stealth configuration optimization design method is proposed with the multi-prismatic stealth configuration of TX-1 as the initial configuration, and two olive stealth satellite configurations are obtained. By comparing the RCS simulation and radar detection probability of the optimized Olive-A and Olive-B satellite stealth configurations in multiple directions, it is demonstrated that the stealth performance of the Olive-B configuration is better. Finally, the anechoic chamber test is conducted on the metallic Olive-B model, and the test results show that the test results and simulation results of Olive-B are in good agreement, which again verifies that the stealth performance of Olive-B is better than that of TX-1 and Olive-A.

Design of a Transparent System for Mutual Coupling Reduction of Microstrip Array Antennas with Confined Water

Herein, a new method for minimizing mutual coupling of an antenna array with confined water is proposed. In multiantenna systems, mutual coupling reduction is needed to sustain array signal processing efficiency. There are currently a variety of ways to suppress coupling, including defected ground, electromagnetic (EM) bandgaps, and advanced structures. The water absorption characteristic is devised in this innovative approach to minimize mutual coupling without a permanent alteration in the structure. Surface current between two antennas is cancelled using permittivity and water depth in a new configuration. A plexi‐glass confined water is added between the propagating elements to achieve this aim. The rectangular microstrip patch of transparent conducting components serves as the antenna array. Using scattering theories, numerical EM calculations, and anechoic chamber tests, the suggested technique shows a 15 dB decrease in coupling at center frequency (7 GHz). Other propagating functions have also been improved. The suggested framework can be used in multifunction communication networks, aerial vehicle antennas, solar panels, and naval communications because of its optical transparency.

Air Support

The arrival of 5G NR consumer technology has brought about mmWave frequencies and integrated antennas to transfer these signals, resulting in the need for over-the-air (OTA) measurements in the development phase. Reproducible OTA measurements under lab conditions require the use of anechoic test chambers which can be exceptionally large. To perform OTA measurements in limited spaces a new generation of T&M solutions with innovative space-saving features has been developed

Cadence Clarity 3D Transient Solver

In this paper, we introduce the feature of Cadence Clarity 3D Transient Solver, a system-level simulation solution that solves electromagnetic interference (EMI) system design issues up to 10x faster than legacy 3D field solvers and offers unbounded capacity. Built on massively parallel solver technology, the Clarity 3D Transient Solver handles workload levels that previously required time-consuming and expensive anechoic test chambers to test prototypes for electromagnetic compatibility (EMC) compliance

A Metawindow with Optimised Acoustic and Ventilation Performance

Crucial factors in window performance, such as natural ventilation and noise control, are generally conceived separately, forcing users to choose one over the other. To solve this dualism, this study aimed to develop an acoustic metamaterial (AMM) ergonomic window design to allow noise control without dependence on the natural ventilation duration and vice versa. First, the finite element method (FEM) was used to investigate the noise control performance of the acoustic metawindow (AMW) unit, followed by anechoic chamber testing, which also served as the validation of the FEM models. Furthermore, FEM analysis was used to optimise the acoustic performance and assess the ventilation potential. The numerical and experimental results exhibited an overall mean sound reduction of 15 dB within a bandwidth of 380 to 5000 Hz. A good agreement between the measured and numerical results was obtained, with a mean variation of 30%. Therefore, the AMW unit optimised acoustic performance, resulting in a higher noise reduction, especially from 50 to 500 Hz. Finally, most of the AMW unit configurations are suitable for natural ventilation, and a dynamic tuned ventilation capacity can be achieved for particular ranges by adjusting the window’s ventilation opening. The proposed designs have potential applications in building acoustics and engineering where natural ventilation and noise mitigation are required to meet regulations simultaneously.

Fast Spherical Near-Field to Far-Field Transformation for Offset-Mounted Antenna Measurements

In spherical near-field measurements, the number of measured samples grows with the radius of the minimum sphere enclosing the antenna under test (AUT). This leads to unnecessary long acquisition times for antennas mounted in offset positions with respect to the measurement sphere. In this letter, a general technique is proposed to reduce the sampling rate requirements of offset mounted antennas. The approach is based on centering the spherical wave expansion (SWE) over the AUT by a shift in the coordinate system to compensate for the offset. The purpose of this shift is to reduce the number of significant spherical waves required to represent the AUT field. In the subsequent steps of the algorithm, the coordinate system is brought to its original location to preserve the efficiency and probe-correction capabilities of the traditional spherical near-field to far-field transformation technique. This is performed by a proper translation of the SWE coefficients between both coordinate systems. Higher order probe correction is also supported intrinsically. The proposed algorithm is numerically tested to assess its performance. Electromagnetic simulation and anechoic chamber tests are used to validate it, showing reductions in measurement times with low transformation errors.

DETERMINATION OF SOUND TRANSMISSION COEFFICIENT OF GYPSUM PARTITION WALLS INSULATED BY CELLUBOR™

One of the biggest problems of using public spaces for constructions, such as high-rise buildings, offices, hospitals, and hotels, is noise caused by sound transmission through the lightweight partition walls used to divide volumes. In this study, experiments were carried out to determine the sound transmission coefficient of lightweight gypsum partition walls insulated by CelluBor. Experiments were performed in an anechoic test chamber consists of two separate cells. One cell was used as the sound source, and the other one was used as the receiver cell. A total of 12 samples were used in the experimental studies, which were categorized into four separate groups. The main features that distinguished these groups from each other were the profile height used in the samples and the number of gypsum board layers used on each side of the profiles. In addition, one blank (no insulation) sample, one half filled, and one fully filled with insulation material were used for each test group. The test samples were placed between two cells. The sound volume of the sound source was measured separately in dB in both cells, and the sound transmission coefficients of 12 different samples were determined. It was observed that the most efficient results were obtained only when half of the profile height was filled with the CelluBor material. The use of a composite material consists of boron and waste materials such as CelluBor as sound insulation material in lightweight partition walls can affect the sound transmission coefficient greatly.

Vision Emulation Methods for Intelligent Vehicle Radiated Immunity Test in Anechoic Chamber

Lane departure warning system (LDWS) application helps enable a safer driving experience for drivers. This application provides additional information about the movement of the vehicle to the driver through front view system, and can warn the driver of dangerous situations through lane departure warning. The warning information is provided through varies sensors such as cameras, analogue and digital interfaces, machine-vision processors. This means that every sensor and every process is safety-relevant and has to be comprehensively tested under the electromagnetic environment. A method to activate and conduct LDWS installed in Vehicle is put forward to test the electromagnetic immunity performance. A virtual vision emulation system which can activate LDWS warning repeatedly and continuously is built and based on the principle of camera imaging. Moreover, it is proved to be feasible for various vehicle by the proper adjustment. With this system, the radiated immunity test for LDWS and other applications based on front camera of intelligent vehicle can be performed in the anechoic chamber.

Vision emulation methods for intelligent vehicle radiated immunity test in anechoic chamber

Lane departure warning system (LDWS) application helps enable a safer driving experience for drivers. This application provides additional information about the movement of the vehicle to the driver through front view system, and can warn the driver of dangerous situations through lane departure warning. The warning information is provided through varies sensors such as cameras, analogue and digital interfaces, machine-vision processors. This means that every sensor and every process is safety-relevant and has to be comprehensively tested under the electromagnetic environment. A method to activate and conduct LDWS installed in Vehicle is put forward to test the electromagnetic immunity performance. A virtual vision emulation system which can activate LDWS warning repeatedly and continuously is built and based on the principle of camera imaging. Moreover, it is proved to be feasible for various vehicle by the proper adjustment. With this system, the radiated immunity test for LDWS and other applications based on front camera of intelligent vehicle can be performed in the anechoic chamber.

Design and Verification of an Integrated Free-Standing Thick-Screen FSS Radome

Frequency selective surface (FSS) radomes can effectively improve the stealth of flying weapons. In this letter, an integrated free-standing thick-screen FSS radome is designed using the CST simulation software. The bandpass characteristic is achieved by Y-shaped apertures with periodic distribution around the central axis of the radome. Based on a model of a round-head FSS radome with a built-in phased array antenna, we have analyzed the bandpass transmittance and boresight error of the radome. The copper shell was prepared by a spin forming process. The FSS radome was fabricated by a laser robot system with multiple degrees of freedom and a rotary platform. The results of the microwave anechoic chamber test show that the thick-screen bandpass FSS radome has a good electromagnetic performance with a passband transmittance of about 80% and a stopband transmittance of less than 10%.

Comparison of Three Different Styles of Submarine Sails on Electromagnetic Scattering under the Detection of Airborne Radar

This paper studies radar cross section of submarine sails on the water. Under the detection of the enemy’s airborne radar, considering the motion of the submarine, the sail model of radar pitch angle incidence range analysis is established. By using CATIA software, the 3D models of AKULA sail, SUBOFF sail and VICTOR sail are built. On the basis of the physical optics method and the equivalent currents method, the scattering characteristics of sails RCS(radar cross section) are simulated under X radar band. Through the microwave anechoic chamber test, this paper verifies the accuracy of the combination of the physical optics method and the equivalent electromagnetic flow method. The influence of the distance from the airborne radar to the sail on the pitch angle of the electromagnetic wave is discussed, with the elevation angle of the radar antenna varies. Then, we illustrate the characteristics of circumferential direction RCS of the sail under different pitch angles. Finally, the mean RCS of the sail at a given pitch angle is simulated. The results show that the AKULA sail is considerably superior to SUBOFF sail and VICTOR sail on stealth performance with the pitch angle less than 4° . But when the pitch angle exceeds 10°, the SUBOFF sail can be given priority.