Distribution Of Noise Sources Research Articles

Enhancing noise sources in stationary-phase zones for accurate phase-velocity estimation of high-frequency surface waves

Passive surface-wave methods are appealing tools for their ability to noninvasively obtain shear-wave velocity with high accuracy and low costs. A linear array of stations is widely adopted in urban areas for its convenient deployments and efficient utilization of ambient noise energy from stationary-phase zones (SPZs), which contributes the most to surface-wave retrieval. However, noise sources in nonstationary-phase zones may threaten the reliability of velocity estimation when they possess strong energy. To obtain reliable phase-velocity estimation of high-frequency surface waves (>1 Hz), we enhance the contribution of noise sources in SPZs by using the multichannel-coherency-weighted stack (MCWS) method for the stacking of noise crosscorrelation functions and dispersion spectra. We model a nonuniform source distribution with strong offline sources. The seismic interferometry (SI) and passive multichannel analysis of surface waves (PMASW) methods overestimate velocities at lower frequencies (<5 Hz), whereas phase velocities of short wavelengths are relatively accurate. The true velocities of short wavelengths are then set as the velocity scanning range for MCWS, and the waves whose apparent velocities lie within the scanning range would be emphasized by MCWS. Noise sources in SPZs are enhanced because the apparent velocities of waves from other noise sources are higher and not in the scanning range. After using MCWS, the phase-velocity estimations of SI and PMASW are consistent with the theoretical dispersion curve. A field data example at a crossroads demonstrates that uneven source distributions might cause serious artifacts and the feasibility of our method is again confirmed. In addition, the enhancement of noise sources in SPZs is verified through noise source distribution imaging by a matched field processing technique.

Experimental Study on the Noise Evolution of a Horizontal Axis Icing Wind Turbine Based on a Small Microphone Array

In recent years, the global energy mix is shifting towards sustainable energy systems due to the energy crisis and the prominence of ecological climate change. Wind energy resources are abundant in cold regions, and wind turbines are increasingly operating in cold regions with wet natural environments, increasing the risk of wind turbine blade icing. To address the problem of noise source distribution and the frequency characteristic variation of wind turbines in natural icing environments, this paper uses a 112-channel microphone array to acquire the acoustic signals of a horizontal axis wind turbine with a diameter of 2.45m. Using the beamforming technique, the wind turbine noise evolution law characteristics under natural icing environment were studied by field experiments, and the noise source distribution and noise increase in different frequency bands under different icing mass and positions and different angles of attack were analyzed in detail. The results show that under the leading-edge and windward-side icing, the noise source gradually moves toward the blade tip along the spanwise direction with the increase in ice mass. In addition, the total sound pressure level at 460 r/min, 520 r/min, 580 r/min, and 640 r/min are increased by 0.82 dB, 0.85 dB, 0.91 dB, and 0.95 dB, respectively for the leading-edge icing condition in comparison with the uniform icing over the windward side of the blade.

Overview of ocean ambient noise interferometry – Theory and simulation

Ambient noise interferometry is a passive acoustic technique for environment characterization. The technique uses coherent ambient sound to approximate the Green’s function between two sensors. It has previously been used in ocean acoustics to passively estimate water temperature, sound-speed structure, and mode shapes as well as for sensor localization. Since this technique utilizes the ambient noise field, whose characteristics are often unknown, understanding the effects of non-isotropic ambient sound is important for ambient noise interferometry. In this talk, an overview of the theoretical literature for noise interferometry will be presented with a specific emphasis on the effects of non-isotropic ocean noise source distributions. Additionally, simulations that explore the emergence of the Green’s function will be presented. Specifically, sound source distributions and environmental parameters such as sound speed profile will be explored. Lastly, the implications of these works on the possibilities and limitations of ambient noise interferometry will be discussed. [Work supported by the ONR.]

Analysis of aerodynamic and aeroacoustics of full scale landing gear

The aerodynamic noise of a landing gear is an important source of airframe noise. The analysis of its noise characteristics plays an important role in the design of a low-noise landing gear. Based on the FL-52 acoustic wind tunnel test technology, the coupled scale adaptive model and the acoustic disturbance equation, the results on aerodynamic noise of a full-scale landing gear model are analyzed. The high-fidelity model includes transverse strut, torsion arm, piston rod, wheel and other parts. The characteristics of static pressure distribution, power spectrum density of pulsating pressure, aerodynamic noise source distribution and directivity of overall sound pressure level are analyzed. The noise characteristics of the far-field microphone are compared with the local microphone installed in the wheel cavity. In this way, we characterize the directivity of pure tone in the wheel cavity and understand its contribution to the far-field noise. The results show that the aerodynamic noise of the landing gear can be quantified accurately by the hybrid numerical method. The pure tone has two frequencies inside and outside the wheel of the landing gear: 560 Hz and 960 Hz. The peak of the loudest sound pressure level reaches 136 dB, and the pure tone radiates to the surface of the non-separation area of the wheel of the landing gear. However, the wall pressure spectrum of the points located in the turbulence region shows a wide-frequency characteristic, and there is no obvious pure tone. From the point of view of the far-field noise directivity, the forward noise of the landing gear is larger than the rear noise, and there is a small overall sound pressure level area at the points of 65 and 110 degrees respectively. When the monitoring points are far away, the far-field noise of the landing gear shows the characteristics of wide frequency, and no obvious pure tone appears. The method can provide the technical support for predicting the aeroacoustics of a landing gear and designing a low-noise land gear.

Using the three-station interferometry method to improve urban DAS ambient noise tomography

Distributed acoustic sensing (DAS) is a novel seismological observation technology based on the fiber-optic sensing method, and can transform existing urban fiber-optic cables into ultra-dense array for urban seismological researches, thus opening abundant opportunities for resolving fine details of near surface structures. While high frequency ambient noise recorded on DAS has been applied in surface wave tomography, it is often difficult to extract a clear dispersion curve for the data recorded by urban internet cable because of the effect of precursor signals on noise correlation functions due to uneven distribution of noise sources, and weak coupling between the cable and the solid earth. In this study, we investigate the performance of the three-station interferometry method for improving the noise cross-correlation functions of the linear array. We applied this method to a DAS dataset acquired in an urban area, suppressed the precursor signal, improved the measurement of the dispersion curve, and constructed a 2D S-wave profile that reveals the hidden fault beneath the city. We also observed that the convergence of noise cross-correlation functions with weak coupling was significantly accelerated using this method. We employed this method to improve the signal quality of surface waves at far offset for the long segment, thus obtaining a more accurate dispersion curve. In conclusion, the three-station interferometry is an effective method to enhance the surface wave signal and suppress the precursor signal retrieved from the data recorded by urban internet cable, which could help in providing high resolution images of shallow structures in built-up areas.

Deep structure of the Verkhnetovskaya kimberlite pipe in the Arkhangelsk diamondiferous province according to passive seismic and radiological methods

AbstractThe exploration of kimberlite pipes is difficult. However, information about their deep structure is necessary for tasks such as prospecting, exploration and development of kimberlite pipes. Therefore, it is necessary to increase the efficiency of deep structure research. In addition, pipes in different territories differ in their properties. The latter requires the study of the peculiarities of the manifestation of pipes in various areas. The Verkhnetovskaya pipe is the only kimberlite object currently known in the Chernoozerskaya area of the Arkhangelsk diamondiferous province. This indicates the need for a comprehensive study of this pipe. The main reason it is difficult to study the pipes of the Arkhangelsk diamondiferous province is the large overburden thickness. In order to increase the efficiency of prospecting, we used a set of methods: microseismic sounding method, passive seismic interferometry, H/V method, gamma spectrometry and emanation mapping. The analysis showed that a new field measurement technique was tested in this research, resulting in more stable data. The main improvement in field measurements is the determination of the optimal accumulation time of microseism when implementing the microseismic sounding method. In addition, the location of the stations in the implementation of passive interferometry made it possible to minimize the influence of the azimuthal distribution of ambient noise sources. The studies made it possible to construct the geophysical image of the investigated pipe and the surrounding medium. As a result, it was shown that the northwestern side has a vertical structure, while according to the drilling data, the side has a uniform slope. In particular, the presence of previously unknown lateral channels was shown. The proposed methodology made it possible to obtain important information with minimal time and technical costs, confirming the applicability of the proposed methodology in the Chernoozerskaya area.

Optimized signal processing for microphone arrays containing continuously-scanning sensors

The paper presents guidelines for the optimal treatment of signals from microphone arrays that include continuously-scanning sensors. The signals from the scanning sensors are non-stationary due to the time-varying source-sensor distance and the traversing of an acoustic field with spatially-varying statistics. Quasi-stationarity has been sought in past studies by dividing the signals into blocks and applying a frequency-dependent window within each block. Optimal schedules versus frequency for block size and block overlap are proposed that enhance the spatial resolution while containing the computational cost. The guidelines are applied to the acoustic fields emitted by an impinging-jets source and a subsonic turbulent jet in isolation and integrated with a shielding plate. The noise source distributions are obtained through a direct spectral estimation approach in which the error between the modeled and measured far-field statistics is minimized. The ability of the continuous-scan paradigm, coupled with the proposed treatment of the signal, to provide high-definition noise source maps is demonstrated.

Extraction of surface-wave phase velocities from ambient noise in the presence of local noise sources based on matched-field processing

Passive surface wave methods have gained much attention from geophysics and civil engineering communities with the advantage of lower costs and deeper penetration depths compared to active seismic methods. The successful extraction of surface waves from ambient noise relies on the assumption of a homogeneous random distribution of noise sources. In urban environments, there are many strong noise sources located in localized areas (i.e., local sources) that violate the ideal noise distribution conditions, which causes errors in surface-wave dispersion extraction. To solve this problem, we propose a new passive surface wave analysis method in the presence of strong local noise sources (local-source passive surface wave analysis, LSPSW). We use the matched field processing (MFP) algorithm to capture noise source distributions and then extract reliable surface wave phase velocities. The MFP algorithm can accurately estimate the distribution of noise sources located either close to or far from the receiver array. Synthetic and field examples demonstrate the feasibility of using LSPSW to estimate phase velocities in complicated noise environments with a pseudo-linear array. Comparisons with other passive surface wave methods (seismic interferometry, the spatial autocorrelation method, pseudo-linear array analysis of passive surface waves based on beamforming) further illustrate the superiority of the MFP-based LSPSW method.

Leveraging large eddy simulations to assess noise source imaging of a controlled supersonic jet

Noise source imaging based on phased array measurements is an essential tool in the aeroacoustic analysis of new nozzle designs, especially at full-scale. This investigation aims to assess the capability of a deconvolution-based beamforming technique to accurately estimate the changes in noise sources for model-scale heated military jets when fluid inserts are used for noise control. This goal is achieved by performing well-validated Large Eddy Simulations (LES) to complement the experimental measurements. The LES data is segregated into its hydrodynamic, acoustic and thermal components using Doak’s Momentum Potential Theory (MPT). The near-field MPT-derived components are subjected to Spectral Proper Orthogonal Decomposition (SPOD) to compare with the frequency-dependent noise source maps obtained directly from experiments. It is shown that fluid inserts alter the naturally occurring Kelvin-Helmholtz (K-H) instability in the jet shear layer, which leads to a change in the directivity of the noise radiated in the near-field. The upstream shift in the noise source distribution resulting from the modified K-H instability is accurately captured by the deconvolution-based source imaging technique using just the far-field measurements. These changes in source locations as a function of frequency are documented.

Systematic validation of the PAAShA shielding prediction method

Based on classical diffraction theories with modifications and extensions in analytical formulations and numerical implementations, a new code has been developed at NASA for the prediction of aircraft noise shielding, named as Propulsion Airframe Aeroacoustic Shielding Attenuation (PAAShA). The code is developed primarily for aircraft system noise predictions, although it may also be useable in other applications with acoustic shielding. The requirements for this code are driven by the need for a robust, capable code to use with NASA’s Aircraft Noise Prediction Program (ANOPP) for aircraft integration and system noise research. The requirements are met and include capabilities to use a wide range of aircraft geometries, rapid calculation times consistent with aircraft system noise problems, and the flexibility to model realistic noise source characteristics and distributions. The accuracy and robustness of the method are demonstrated in this paper with a set of problems, including a cylinder, a finite plate, a symmetrical two-dimensional airfoil, and a full three-dimensional hybrid wing body aircraft model tested in a wind tunnel. This range of problems demonstrates both smooth and sharp edge diffraction capabilities for a wide range of frequencies and low Mach number flow effects at low angles of attack. Predictions are shown to be accurate to within 1–4 dB over a wide range of the most significant frequencies and directivity angles. This is determined by comparing with data, which have experimental uncertainties, particularly at high frequencies, high angles, and source characteristics. The accuracy diminishes for geometries that include a significant reflection component, which is not calculated by the code. Accuracy can also be somewhat diminished for high azimuthal angles. Accurate modeling of the noise source, particularly its frequency and directivity characteristics, is essential to obtaining accurate results.

Subsonic jet noise source location as a function of nozzle exit boundary layer

Over the years, there have been numerous studies on determining subsonic jet noise source locations, typically plotted as Strouhal number as a function of distance from the nozzle exit. A comparison of the results of various studies yields a spread of about two nozzle diameters in measured source location. This work examines how boundary layer thickness, which can vary from nozzle to nozzle, could be the cause of observed differences in different studies in subsonic jet noise source location. Source location measurements of unheated jets from ASME nozzles, which have comparably thinner nozzle exit boundary layers, and conical nozzles, which have comparably thicker nozzle exit boundary layers, are compared. These results are substantiated with the use of schlieren flow visualization and velocity profile measurements. It is found that the nozzles with thinner nozzle exit boundary layers have noise source distributions that are 0.25–2 diameters upstream of those with thicker nozzle exit boundary layers. Thinner nozzle exit boundary layers result in higher growth rates of instability waves, increasing mixing and thereby moving noise sources upstream.

Common-midpoint two-station analysis of estimating phase velocity using high-frequency ambient noise

Two-station analysis is a frequently-used technique in seismology to determine interstation phase velocities of surface waves but it usually suffers from several drawbacks arising from 2π ambiguity and a narrow frequency band. This paper introduces a common-midpoint two-station analysis (CMP-TS) method to suppress the spurious energy due to 2π ambiguity and broaden the frequency band for high-frequency (>1 Hz) passive surface wave imaging. To demonstrate the advantages of the proposed method in estimating phase velocity, the authors compared results obtained by the CMP-TS method and the Multichannel Analysis of Passive Surface waves (MAPS) using synthetic data with homogeneous and inline noise-source distributions. For homogeneous noise-source distribution, phase-velocity dispersion curves obtained by both methods can match with theoretical curves, but the CMP-TS method could enhance the resolution of images of dispersion energy by about 50% compared with the MAPS method. As for inline noise-source distribution, phase velocities generated with the MAPS method can match with theoretical ones while phase velocities measured by the proposed method are underestimated. By modifying the source phase ambiguity term in the proposed method, the authors corrected the systematic measurement bias of phase velocities and concluded that the CMP-TS method is superior to the MAPS method in accuracy and resolution. Results from two field examples further demonstrated the capability of the CMP-TS method in estimating phase velocity with higher image resolution than the MAPS method. Finally, the results showed that it is reasonable for the proposed method to pick phase velocities using the one-wavelength criterion in near-surface applications.

Linear array analysis of passive surface waves combined with mini-Sosie technique

SUMMARY Mini-Sosie, which is convenient and environmentally friendly with high energy output, is gaining increasing attention as a seismic exploration source. Passive surface wave survey using ambient noise have become a powerful tool for exploration in urban areas due to the advantages of being efficient and non-destructive. A 2-D or pseudo-1D array that can attenuate phase velocity overestimation due to directional noise are the optimal choice, but such arrays are limited by the complex environment of the experimental site on the one hand and the need to ensure that the medium beneath the array meets the assumption of lateral isotropy on the other. However, the fully 1-D linear array as an alternative lacks the ability to suppress the directional effect of the noise source. In this study, we develop a novel approach for linear array analysis of passive surface waves combined with mini-Sosie technique, called LAPSS. We use the mini-Sosie technique with fixed-frequency impact in the in-line direction to provide the theoretical phase velocity values at several frequency points as a reference for the biased dispersion image obtained from the linear array analysis, so as to estimate the azimuth of the noise source and to perform the correction of the biased dispersion image. We present the detailed workflow of LAPSS and compare the performance of LAPSS with PLAS (which has proven to be superior to the frequency–wavenumber method, spatial autocorrelation method, refraction microtremor and multichannel analysis of passive surface waves) in achieving unbiased dispersion image with different noise source distributions through synthetic tests. The results show that the accuracy of the two methods is comparable, but LAPSS greatly improves the computational efficiency, has stronger generalizability and depends on weaker assumptions. Finally, a field experiment is conducted to verify the feasibility and effectiveness of LAPSS in obtaining unbiased dispersion images using a fully linear array.

Using global optimization methods for three-dimensional localization and quantification of incoherent acoustic sources.

Complex acoustic systems typically present three-dimensional distributions of noise sources. Conventional acoustic imaging methods with planar microphone arrays are unsuitable for three-dimensional acoustic imaging, given the computational demands and the incapability to explicitly account for the presence of multiple sources. This paper proposes the use of global optimization methods to solve these shortcomings. An experiment with three incoherent speakers proved that this method can accurately determine the three-dimensional location and the respective sound level of each individual source. In addition, super-resolution is achieved beyond half the Rayleigh resolution limit.

Dipole source-based virtual three-dimensional imaging for propeller noise

The increasing attention to aerodynamic noise radiation from propellers has driven the interest in noise source identification and promoted the development of urban aerial vehicles over the last decades. Conventional acoustic imaging algorithms primarily assume monopole-type sources, but modern aeroacoustic theories have recognized that the dipole-type loading noise is dominant for propellers. To obtain physically consistent noise source distributions, we propose a moving dipole source-based beamforming method. The method is developed in the time-frequency domain to recover the time history of three-dimensional mapping for source strength and directivity, different to other techniques. The capability of the proposed method to identify noise sources is assessed by considering a two-bladed propeller benchmark. Instead of directly conducting experiments, we use results from high-fidelity numerical simulations, validated by measurements, to perform a noise source analysis. In this way, it is easier to verify the obtained source locations and directions. The results show that the noise sources are located on the trailing edge around the tip of the propeller, and the directivity varies with frequency. The proposed method exhibits competitive advantages in correctly tracing sources over conventional monopole-based methods. The proposed method can recover the three-dimensional noise sources for a high-speed rotating propeller. The consistency between the flow field and acoustic imaging results confirms the accuracy of the proposed method and suggests its potential in industrial applications.

Peak-time sensitivity kernels for noise cross-correlation envelopes.

The envelope of the time-lagged cross-correlation of an underwater noise field between two hydrophones can under certain conditions be used as a proxy for active acoustic receptions between the two locations enabling the study of ocean variability. Previous work looked at the sensitivity of cross-correlation peak amplitudes with respect to the distribution of the noise sources. The present study examines the sensitivity of the cross-correlation envelope peak times with respect to changes in the sound-speed distribution. A wave-theoretic scheme allowing for finite-frequency calculations in two and three dimensions, combined with the Born approximation for perturbations of the Green's function and the peak arrival approach, is used to obtain sensitivity kernels with respect to environmental (sound-speed) changes. These kernels provide a way to infer ocean structure from the cross-correlation peaks, considered as observables on their own and valid even in cases where the cross-correlation function does not approximate the time-domain Green's function between the two receivers. The sensitivity behavior is studied for different propagation conditions and noise-source distributions, ranging from spatially distributed uncorrelated noise sources to point sources, such as individual ships. Deviations from linearity are addressed and discussed.

Numerical and Experimental Investigations of Modal Coherence Characteristics for the Determination of Broadband Noise in Flow Ducts

This paper presents a detailed investigation about the modal coherence characteristics of turbomachinery noise. The modal coherence characteristics of ducted noise at high frequencies are investigated by establishing three kinds of ducted sound source distribution models. The results are consistent with the acoustic experimental results of a single-stage axial fan. The results show that acoustic modes of different orders are mutually incoherent, and radial modes within the same circumferential mode have a great degree of coherence. This paper successfully conducted an acoustic measurement on turbomachinery noise by rotating microphone arrays. The acoustic experimental results show that the coherence of some modes has little impact on the measurement of incident acoustic waves in the flow duct, while it leads to about 3 dB deviation in the prediction of the reflected modal sound power and has little influence on the spectral shape of reflected acoustic waves. It was demonstrated that the experimentally measured modal coherence functions give an inner sense of the spatial noise source distribution and enable the investigation of mode scattering within the propagation. Based on the modal coherence characteristics obtained in this paper, more theoretical and methodological studies can be carried out for the turbomachinery broadband noise test.

Rod–Airfoil Interaction Noise at Different Rod Cross Sections and Airfoil Angles

Vortex–body interaction noise from a rod–airfoil model has been investigated experimentally in an aeroacoustic wind tunnel. The model configuration varies by changing either the rod cross section (circular, square, rhombic, and elliptical) or the install angle of the airfoil at three directions (attack angle , sweep angle , and lean angle ) independently. This paper aims to assess the effects of these parameters on the characteristics of vortex–body interaction noise. A near-field microphone array was applied to locate the noise sources on the rod–airfoil model, and a far-field microphone array determined the noise level and radiation directivity. The results suggest that, in the test range, both the elliptical rod cross section and the change of the airfoil install angles can visibly reduce vortex–body interaction noise compared to the baseline rod–airfoil containing the circular rod and the airfoil at zero angles. The interaction noise level decreases with the increase of the eccentricity of the elliptical rod. Of the angles, the most effective way is the change of the sweep angle , which can achieve a noise reduction of 8 dB when reaches 20 deg, but the attack angle has more influence in reducing the vortex shedding frequency. In addition, the noise source distribution of different frequencies also changes with the variation of rod–airfoil configurations. Flow visualization using particle image velocimetry was conducted to analyze the noise change mechanism. The findings from this study are beneficial for engineers in designing low-noise components in several aeronautical and industrial applications.

Research on an Anechoic Wind Tunnel in the Design Phase by Numerical Simulation

As an important facility for acoustic measurement and research in air medium, anechoic wind tunnel requires high acoustic performance. In this paper, the flow field and acoustic characteristics of anechoic wind tunnel were studied by modeling and simulation. The velocity and pressure distribution of the whole flow field in the wind tunnel are obtained by simulation calculation, and the noise source distribution in the anechoic wind tunnel is studied by the broadband noise source method, which provides a reference for the design, construction and optimization of the anechoic wind tunnel.

An assessment of uncertainties in VS profiles obtained from microtremor observations in the phased 2018 COSMOS blind trials

Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 (VS10), 30 (VS30), 100 (VS100), and 300 (VS300) meters and show its usefulness in quantitative comparisons of VS profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.