Anechoic Environment Research Articles

A novel UWB flexible antenna with dual notch bands for wearable biomedical devices.

This study presents a novel UWB flexible antenna with dual band-notched design for wearable biomedical devices. The proposed antenna is designed on Kapton Polyimide-based flexible substrate. This includes a CPW fed circular and triangle structure. The dual notched bands are realized by using two triangular-shaped spiral slots defected ground structures. The first notched band (2.4-3.7GHz) is generated for rejecting WLAN and WiMAX, the second notch (5.15-5.725GHz) is generated for rejecting HyperLAN/2. The designed UWB antenna has approximately a bandwith of 159% (2.05-14GHz) in simulation. Thus, the designed UWB antenna meets FCC standards. The antenna has an omnidirectional radiation pattern with a maximum gain of 12.7dB in 8.4GHz. The proposed antenna is fabricated with the low-cost airbrush printed technique. In this technique, a higher gain value is obtained by controlling the thickness of the conductive layer. Effect of flexibility on the antenna performance is tested for different configurations in the simulation and anechoic chamber environments. According to the results obtained, the overall performance is not affected except for the shift in frequency. Since the antenna has a UWB structure, the frequency shift that occurs in bending is at a tolerable level. The proposed UWB antenna is suitable for wearable biomedical devices, with a high UWB performance.

Experimental investigation on a side-by-side twin rotor system in pusher configuration

Growing interest in electric propulsion, together with the increased deployment of unmanned aerial vehicles, results in the consequent need for a clear understanding of the physical phenomena under propeller interactions which represent a primary noise source in multi-rotor configurations. To this purpose, the effect of rotor-to-rotor interactions of a twin pusher propeller configuration at static thrust conditions was experimentally investigated through aerodynamic and aeroacoustic measurement campaigns. The propellers tested refer to the APC-8x45MR rotor with an 8 inches diameter. The aerodynamics and aeroacoustics of several configurations were investigated using PIV technique, Pitot tube acquisitions, and microphone measurements performed in an anechoic environment. The propeller speed was varied within a typical range in the applications for both co-rotating and counter-rotating layouts. To define critical relative positions, many tip-to-tip separation distances were considered highlighting the general effects. Owing to the deformation exhibited by the rotor wakes due to their proximity, the pressure and velocity signals were conditioned to inspect the driving mechanisms responsible for the noise emission. Results provide a deeper insight into the physics involved and show different types of interaction effects on the fluid-dynamic field as a function of the propeller position, allowing the identification of the main critical configurations.

Distance discrimination thresholds of proximal sound sources in a real anechoic environment

When a sound source off the midline is located close to the head, the interaural level difference changes dramatically with distance. Although distance discrimination thresholds for frontal sound sources have been comprehensively studied, the thresholds for real lateral sound sources in the proximal region have rarely been explored. The present study measured distance discrimination thresholds via a loudspeaker located at different distances and directions, both with fixed-emission and normalized intensities referring to the head center, to confirm the contribution of near-field binaural cues in a quantitative manner. The results demonstrated that the sound level is the main cue for distance discrimination, and binaural cues promote the ability to perceive differences in sound distances for lateral sources located at 0.5 m from the head center. The thresholds for lateral sources were significantly lower than those for sources in the median plane, both with and without level cues. However, when the sound sources were located at a distance of 1.0 m, the promotion of lateral thresholds was not significant. This study provides plausible just-noticeable differences for distance discrimination at various directions and distances from the head.

The combined effect of source directivity and binaural listening on near-field binaural speech transmission index evaluation

The binaural speech transmission index (BSTI) is mainly affected by the effect of binaural listening, background noise, and acoustic conditions in a given space, as well as source characteristics such as directivity. However, little attention has been paid to the combined effect of source directivity and binaural listening on the BSTI in near-field regions. This study thus investigates this issue using BSTI measurements in an anechoic environment to exclude the influence of acoustic conditions. Three directional loudspeakers are adopted as the sources, and the impulse responses under different radiation directions in the horizontal plane are measured in an anechoic chamber. These impulse responses are then convolved with the near-field head-related transfer functions of a KEMAR manikin to obtain the binaural impulse responses, which are further used to calculate the BSTIs. The results show that the BSTI decreases considerably with an increasing deviation of the axial direction of the source from the listener (i.e., an increasing radiation angle) due to the directivity pattern of the source, which is almost independent of the azimuth and the distance between the source and listener. Finally, the BSTI correction functions of the source radiation angles are established with third-order polynomial regression for the three sources and can be integrated into the azimuth-dependent BSTI model to yield a comprehensive model that considers the source radiation angle as well as the azimuth and distance between the source and listener.

Aeroacoustic characteristics of supersonic offset jets

The offset jet configuration is one where the jet is discharged at some distance from a solid surface. Although the geometric configuration may look simple, the flow may involve several complexities. In propulsion systems, the high-speed jet generated from the rear engine of an aircraft, flowing nearby the fuselage, can be treated as an offset jet. In this work, an experimental investigation of the interaction noise due to circular high-speed offset jets is performed in an anechoic environment at different nozzle pressure ratios and offset ratios (height of the jet centerline above the plate per nozzle width). A large horizontal plate placed over a height-adjustable stand is used as the offset plate. Acoustic characteristics such as overall sound pressure level and the directivity pattern of free and offset jets are compared for different nozzle pressure ratios. The effect of offset ratio on noise characteristics is also investigated. Flow visualization is also carried out to understand the shock structure and its noise generation mechanism. Acoustic characteristics reveal that noise levels are higher for an offset jet compared to a free jet. Sound pressure levels for offset ratio 0.5 are lower than those for other offset ratios. The noise levels are higher for offset ratio 1.0 due to the presence of feedback tone. Schlieren visualization studies also corroborate the above characteristics.

A Bayesian model for human directional localization of broadband static sound sources

Humans estimate sound-source directions by combining prior beliefs with sensory evidence. Prior beliefs represent statistical knowledge about the environment, and the sensory evidence consists of auditory features such as interaural disparities and monaural spectral shapes. Models of directional sound localization often impose constraints on the contribution of these features to either the horizontal or vertical dimension. Instead, we propose a Bayesian model that flexibly incorporates each feature according to its spatial precision and integrates prior beliefs in the inference process. The model estimates the direction of a single, broadband, stationary sound source presented to a static human listener in an anechoic environment. We simplified interaural features to be broadband and compared two model variants, each considering a different type of monaural spectral features: magnitude profiles and gradient profiles. Both model variants were fitted to the baseline performance of five listeners and evaluated on the effects of localizing with non-individual head-related transfer functions (HRTFs) and sounds with rippled spectrum. We found that the variant equipped with spectral gradient profiles outperformed other localization models. The proposed model appears particularly useful for the evaluation of HRTFs and may serve as a basis for future extensions towards modeling dynamic listening conditions.

Spatial release of masking in children and adults in non-individualized virtual environments.

The spatial release of masking (SRM) is often measured in virtual auditory environments created from head-related transfer functions (HRTFs) of a standardized adult head. Adults and children, however, differ in head dimensions and mismatched HRTFs are known to affect some aspects of binaural hearing. So far, there has been little research on HRTFs in children and it is unclear whether a large mismatch of spatial cues can degrade speech perception in complex environments. In two studies, the effect of non-individualized virtual environments on SRM accuracy in adults and children was examined. The SRMs were measured in virtual environments created from individual and non-individualized HRTFs and the equivalent real anechoic environment. Speech reception thresholds (SRTs) were measured for frontal target sentences and symmetrical speech maskers at 0° or ±90° azimuth. No significant difference between environments was observed for adults. In 7 to 12-year-old children, SRTs and SRMs improved with age, with SRMs approaching adult levels. SRTs differed slightly between environments and were significantly worse in a virtual environment based on HRTFs from a spherical head. Adult HRTFs seem sufficient to accurately measure SRTs in children even in complex listening conditions.

Spatial release from masking in anechoic and reverberant environments

Listening with both ears provides children with access to binaural and monaural cues that are helpful for understanding speech in competing babbles. Specifically, when the target and masker are spatially separated, children can gain an intelligibility benefit which is known as spatial release from masking (SRM). Recent work [Peng et al., 2021 JASA] suggested that school-age children demonstrated immature SRM using binaural cues that are distorted by reverberation. In this follow-up study, we further investigate the effect of reverberant distortion on individual auditory spatial cues, namely binaural and monaural head shadow cues. We compare SRM between adults and school-age children with typical hearing using the novel measure of minimum angle of separation (MAS) between target and masker, for which individual achieves a 20% intelligibility gain, in both virtually simulated anechoic and reverberant environments. MAS was measured in both binaural and monaural hearing conditions, as well asymmetric versus asymmetric masker displacement to probe access to various auditory cues of interest. Preliminary results show that adults demonstrate similar SRM in anechoic and in reverberation of T60 = ∼0.6 s, but better (smaller MAS) when additional spatial cues become available. Reverberation effect on children’s SRM will be discussed. [Work supported by NIH-NIDCD.]

Auditory detection probability of propeller noise in hover flight in presence of ambient soundscape.

Unmanned aerial vehicles are rapidly advancing and becoming ubiquitous in an unlimited number of applications, from parcel delivery to people transportation. As unmanned aerial vehicle (UAV) markets expand, the increased acoustic nuisance on population becomes a more acute problem. Previous aircraft noise assessments have highlighted the necessity of a psychoacoustic metric for quantification of human audio perception. This study presents a framework for estimating propeller-based UAV auditory detection probability on the ground for a listener in a real-life scenario. The detection probability is derived by using its free-field measured acoustic background and estimating the UAV threshold according to a physiological model of the auditory pathway. The method is presented via results of an exemplar measurement in an anechoic environment with a single two- and five-bladed propeller. It was found that the auditory detection probability is primarily affected by the background noise level, whereas the number of blades is a less significant parameter. The significance of the proposed method lies in providing a quantitative evaluation of auditory detection probability of the UAV on the ground in the presence of a given soundscape. The results of this work are of practical significance since the method can aid anyone who plans a hovering flight mode.

Experimental Demonstration of Test Cap Design for Space Transponder Applications

Antenna test caps allow the testing of transmit and receive functionality of complete end-to-end radio frequency (RF) space systems in flight configuration outside anechoic environment. The test caps are made of two off-centered cylinders. The upper cylinder contains a probe, corresponding port, as well as a tuning screw. The lower cylinder contains absorbing material, the antenna under test. A full design and its experimental validation are demonstrated for a Martian X-band radio science payload.

Experimental Validation of an Emission Test Method for Source Stirred Reverberation Chamber

This article reports on the experimental validation of a method to predict radiated emissions of equipment under test in a reverberation chamber where the multiple monopole sources stirring technique is implemented. The method is validated for three different case studies: a loop, a slot in a metal enclosure, and a realistic device comprised of the chassis of a power supply for a workstation. The reconstruction of the free space electromagnetic emissions starting from measurements in a reverberation chamber is compared with measurements in an anechoic environment. A statistical analysis of the uncertainty due to a positioning error of the field sample measurement points is also proposed.

A Study on the Development of Data COMMUCATION MODULE within Construction

Purpose: As for the safety related technologies in the field of construction and site, and in order to implement an optimal communication transmission and reception related environment, an environment which can help prevent a loss of data ought to be structured. Hence, the purpose of this paper is to identify and remove the noise elements which cause the loss of data via empirical experiments, examine the elements which can help improve communication performance within the range of radio regulations, and develop the communication module appropriate for the relevant environment. Method: The validation methodology of this paper is as follows. A comparison validation test of the customized communication environment compared to the existing communication equipment related application environment, a validation test as to whether a loss of data occurs between the AR devices, a correlation validation test between the occurrence of a loss of data due to a noise increase, a validation and development methodology for identifying the correlation between the transmission distance and the amount of data transmission and reception related variation via the validation test for the impact of data transmission are presented. Results: Since the data transmission and reception volume relatively increases in the customized communication related environment compared to the existing communication equipment related application environment for a certain period of time, it is apparent that the data transmission and reception volume between the AR devices is reduced by causing a network delay due to the overload of network equipment. Hence, it is expected that, in order to address such problems, a communication module appropriate for the environment will be developed, and the noise will be reduced via the validation. Conclusion: As it is necessary to derive the customized values for the communication equipment applicable for the construction site through this study, it is necessary to calculate the quantitative values for the improvement of the data transmission and reception volume during the radio wave amplification, and it has also been derived that it is necessary to derive the quantitative values between a loss of data due to the generation of noise. In the future studies, it is considered that a comparative validation test will be needed in an indoor and outdoor test environment in which an interference from the existing communication environment occurs and an anechoic testing environment in which an interference does not occur whatsoever from the existing communication environment.

Frequency-Spectra-Based High Coding Capacity Chipless RFID Using an UWB-IR Approach.

A novel methodology is proposed to reliably predict the resonant characteristics of a multipatch backscatter-based radio frequency identification (RFID) chipless tag. An ultra-wideband impulsion radio (UWB-IR)-based reader interrogates the chipless tag with a UWB pulse, and analyzes the obtained backscatter in the time domain. The RFID system consists of a radar cross-section (RCS)-based chipless tag containing a square microstrip patch antenna array in which the chipless tag is interrogated with a UWB pulse by an UWB-IR-based reader. The main components of the backscattered signal, the structural mode, and the antenna mode were identified and their spectral quality was evaluated. The study revealed that the antenna-mode backscatter includes signal carrying information, while the structural mode backscatter does not include any tag information. The simulation findings were confirmed by experimental measurements obtained in an anechoic chamber environment using a 6-bit multipatch chipless RFID tag. Finally, the novel technique does not use calibration tags and can freely orient tags with respect to the reader.

The "Missing 6 dB" Revisited: Influence of Room Acoustics and Binaural Parameters on the Loudness Mismatch Between Headphones and Loudspeakers.

Generations of researchers observed a mismatch between headphone and loudspeaker presentation: the sound pressure level at the eardrum generated by a headphone has to be about 6 dB higher compared to the level created by a loudspeaker that elicits the same loudness. While it has been shown that this effect vanishes if the same waveforms are generated at the eardrum in a blind comparison, the origin of the mismatch is still unclear. We present new data on the issue that systematically characterize this mismatch under variation of the stimulus frequency, presentation room, and binaural parameters of the headphone presentation. Subjects adjusted the playback level of a headphone presentation to equal loudness as loudspeaker presentation, and the levels at the eardrum were determined through appropriate transfer function measurements. Identical experiments were conducted at Oldenburg and Aachen with 40 normal-hearing subjects including 14 that passed through both sites. Our data verify a mismatch between loudspeaker and binaural headphone presentation, especially at low frequencies. This mismatch depends on the room acoustics, and on the interaural coherence in both presentation modes. It vanishes for high frequencies and broadband signals if individual differences in the sound transfer to the eardrums are accounted for. Moreover, small acoustic and non-acoustic differences in an anechoic reference environment (Oldenburg vs. Aachen) exert a large effect on the recorded loudness mismatch, whereas not such a large effect of the respective room is observed across moderately reverberant rooms at both sites. Hence, the non-conclusive findings from the literature appear to be related to the experienced disparity between headphone and loudspeaker presentation, where even small differences in (anechoic) room acoustics significantly change the response behavior of the subjects. Moreover, individual factors like loudness summation appear to be only loosely connected to the observed mismatch, i.e., no direct prediction is possible from individual binaural loudness summation to the observed mismatch. These findings – even though not completely explainable by the yet limited amount of parameter variations performed in this study – have consequences for the comparability of experiments using loudspeakers with conditions employing headphones or other ear-level hearing devices.

Freehand System for Antenna Diagnosis Based on Amplitude-Only Data

This article presents a portable system for freehand antenna diagnosis and characterization based on amplitude-only data. The amplitude-only samples are acquired by moving a handheld probe, which is tracked by a motion capture system, in front of the antenna under test (AUT) aperture. The acquired measurements are processed using the phaseless sources reconstruction method to compute an equivalent current distribution on the AUT aperture. Finally, the radiation pattern of the AUT can be obtained by evaluating the corresponding radiation integrals. Unlike previous work, the use of amplitude-only data avoids the need of a phase reference, paving the way to the diagnosis and characterization of antennas under operational conditions. This fact, together with the handheld capabilities, makes the system very convenient for measurements of already deployed and onboard antennas. Moreover, these amplitude-only acquisitions also simplify the required hardware. The system has been validated through measurements in a wide frequency range from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Ka$ </tex-math></inline-formula> -band up to 300 GHz. Despite that one cannot expect the same degree of accuracy that can be achieved under laboratory conditions (including an anechoic environment and highly accurate positioners), the system shows excellent capabilities to detect malfunctions, such as wrong amplitude/phase distributions, as well as a fair estimation of the far field.

A categorization of marginally stable thermoacoustic modes based on phasor diagrams

Phasor diagrams are utilized to plot and analyze thermoacoustic modes in a duct with ideal closed/open acoustic boundary conditions that contains a velocity-sensitive flame. Inspection of phasor diagrams that represent fluctuations of velocity, pressure, heat release rate and characteristic wave amplitudes at the flame elicits characteristic features of marginally stable, intrinsic thermoacoustic (ITA) modes: the sign of velocity fluctuations and the sign of the gradient of pressure fluctuations change across the flame. These sign changes result from a reversal of direction of the velocity phasor across the flame, affected by unsteady heat release exactly out-of-phase with respect to upstream velocity fluctuations and of sufficient strength. Unlike alternative methods proposed for the identification of ITA modes, the proposed categorization does not involve a parameter sweep, but relies on an analogy with the structure of ITA modes in an anechoic environment. The phasor diagram also elucidates that continuous transitions from acoustic to ITA modes and vice versa can be achieved by an increase or decrease of the gain of the flame transfer function. The representation of acoustic wave propagation in terms of phasors facilitates the formulation of a compact dispersion relation of the thermoacoustic configuration under consideration. The value of the transitional gain of the flame transfer function, where acoustic/ITA transitions occur, may be deduced easily from the dispersion relation. A plot of solution branches of acoustic and ITA modes illustrates how the respective mode transitions, and how they can be related to the well-known quarter wave modes of a closed/open resonator. It is observed that under variation of transfer function gain and phase, marginally stable eigenmodes may occur at almost any frequency, not only at frequencies that correspond to the quarter-wave modes.

EM-Based TDOA Estimation of a Speech Source via Gaussian Mixture Models in Noisy and Anechoic Environments

The propagation delay difference of a speech signal transmitted from the source to microphones, also known as time difference of arrival (TDOA), embodies the information of speech source position. The TDOA estimation plays a vital role in diverse systems such as teleconferencing and far-field speech recognition since the TDOA is a key parameter impacting quality of restored speech signals. This paper is devoted to estimating the TDOA of one speech source on a frame by frame basis in noisy and anechoic environments. First, we propose two variants of Gaussian mixture model to represent the speech signal received by a microphone pair, assuming Gaussianity of the signal and modeling speech sparsity by the speech presence probability (SPP). Second, after estimating the noise parameter in advance and formulating the speech parameters using the maximum likelihood principle, the proposed Gaussian mixture models are reduced to being dependent only on two unknowns, i.e. TDOA and SPP. Third, following these two models, we present two distinct estimators to estimate the TDOA and the SPP iteratively based on the expectation maximization algorithm. The proposed two estimators are free from the ad hoc parameter selection which is required in many classical approaches. Simulation results show that the TDOA estimated by them could be more accurate than that of the state-of-the-art GCC variants in a wide range of frames with specific SPP values. More importantly, the automatically estimated SPP which can be served as voice activity detection in a soft manner encodes the information of the TDOA estimation accuracy. In a speech frame, the estimated SPP with a large value indicates the estimated TDOA with small error. For example, when the SPP is larger than 0.76 and 0.87 in the two proposed estimators, respectively, the TDOA estimation error could be at most 19% of that in the worst case.

Detection Method for Single-Beam Echo Sounder Based on Equivalent Measurement

Acoustic instruments like the single-beam echo sounder are basic tools for marine scientific research; consequently, the effective detection and calibration of the equipment are crucial to ensure the quality of marine data. Considering that the detection of full-range precision index of a single-beam echo sounder cannot be carried out in a laboratory sink, this paper proposes a new detection method based on equivalent measurement. In an anechoic environment, a circuit is used to simulate the long-distance propagation of the sound wave. The full-range equivalent measurement of the single-beam echo sounder is realized within the limited range of the laboratory sink. Experimental verification proves that the proposed method is feasible and provides a new technical approach for the detection of marine acoustic instruments.

3-D Gait Abnormality Detection Employing Contactless IR-UWB Sensing Phenomenon

Gait disorder diagnosis and rehabilitation is one area where human perception and observation are highly integrated. Predominantly, gait evaluation comprises technological devices for gait analysis, such as dedicated force sensors, cameras, and wearable sensor-based solutions; however, they are limited by insufficient gait parameter recognition, postprocessing, installation costs, mobility, and skin irritation issues. Thus, the proposed study concentrates on the creation of a widely deployable, noncontact and nonintrusive gait recognition method from impulse radio ultrawideband (IR-UWB) sensing phenomenon, where a standalone IR-UWB system can detect gait problems with less human intervention. A 3-D human motion model for gait identification from IR-UWB has been proposed with embracing spherical trigonometry and vector algebra to determine knee angles. Subsequently, normal and abnormal walking subjects were involved in this study. Abnormal gait subjects belong to the spastic gait category only. The prototype has been tested in both anechoic and multipath environments. The outcomes have been corroborated with a simultaneously deployed Kinect Xbox sensor and supported by a statistical graphical approach Bland and Altman (B&A) analysis.

Evaluation of head-tracked binaural auralizations of speech signals generated with a virtual artificial head in anechoic and classroom environments

In order to realize binaural auralizations with head tracking, BRIRs of individual listeners are needed for different head orientations. In this contribution, a filter-and-sum beamformer, referred to as virtual artificial head (VAH), was used to synthesize the BRIRs. To this end, room impulse responses were first measured with a VAH, using a planar microphone array with 24 microphones, for one fixed orientation, in an anechoic and a reverberant room. Then, individual spectral weights for 185 orientations of the listener’s head were calculated with different parameter sets. Parameters included the number and the direction of the sources considered in the calculation of spectral weights as well as the required minimum mean white noise gain (WNGm). For both acoustical environments, the quality of the resulting synthesized BRIRs was assessed perceptually in head-tracked auralizations, in direct comparison to real loudspeaker playback in the room. Results showed that both rooms could be auralized with the VAH for speech signals in a perceptually convincing manner, by employing spectral weights calculated with 72 source directions from the horizontal plane. In addition, low resulting WNGmvalues should be avoided. Furthermore, in the dynamic binaural auralization with speech signals in this study, individual BRIRs seemed to offer no advantage over non-individual BRIRs, confirming previous results that were obtained with simulated BRIRs.