Distribution Of Noise Sources Research Articles

Extracting Seismic Body and Rayleigh Waves from the Ambient Seismic Noise Using the rms-Stacking Method

The cross correlation of ambient seismic noise recordings can be used to extract the empirical Green’s functions (hereafter EGFs) between pair of receivers (Shapiro and Compillo, 2004). In the past few years, extensive research has been performed on this topic around the world in various fields including travel‐time tomography (Sabra et al. , 2005; Shapiro et al. , 2005; Yao et al. , 2006; Yang et al. , 2007; Ward et al. 2013; Young et al. , 2013), anisotropy (Guo et al. , 2012; Shirzad and Shomali, 2014), fault detection (Shirzad et al. , 2013) and retrieving body wave (Poli et al. , 2012; Lin and Tsia, 2013; Boue et al. , 2014). Depending on various parameters including the quality of data and its frequency content, different processing approaches have been developed to obtain noise correlation. In the most approaches developed, the stacking was further assumed to be nonindependent on a particular phase. The main scope of these approaches is to remove the deterministic signals (e.g., earthquakes) and also extract the coherent part of ambient seismic noise (Stehly et al. , 2006; Pedersen et al. , 2007; Bensen et al. , 2008). In general, the extracted EGFs are dominated by the surface‐wave signal, even if two stations have a large interstation distance. Additionally, uneven noise source distribution may affect the extracted interstation EGFs of surface waves and consequently dispersion measurements (see Stehly et al. , 2006; Tsai, 2009; Yao and van der Hilst, 2009). Body waves may also be emerged by storm and/or swell interference (Landes et al. , 2010; Obrebski et al. , 2013). Obrebski et al. (2013) indicated that body and surface waves in random wavefields have different patterns resulted from distinctive amplification of ocean wave‐induced pressure …

Cylindrical acoustical holography applied to full-scale jet noise.

Near-field acoustical holography methods are used to predict sound radiation from an engine installed on a high-performance military fighter aircraft. Cylindrical holography techniques are an efficient approach to measure the large and complex sound fields produced by full-scale jets. It is shown that a ground-based, one-dimensional array of microphones can be used in conjunction with a cylindrical wave function field representation to provide a holographic reconstruction of the radiated sound field at low frequencies. In the current work, partial field decomposition methods and numerical extrapolation of data beyond the boundaries of the hologram aperture are required prior to holographic projection. Predicted jet noise source distributions and directionality are shown for four frequencies between 63 and 250 Hz. It is shown that the source distribution narrows and moves upstream, and that radiation directionality shifts toward the forward direction, with increasing frequency. A double-lobe feature of full-scale jet radiation is also demonstrated.

Statistical properties of the seismic noise field: influence of soil heterogeneities

Seismic noise is generally considered as a reproducible and temporarily stationary natural source of energy. Based on the recording of seismic noise by a dense temporary network installed at various sites in the city of Dushanbe we have investigated the variations of the 3D average squared soil displacement on different time scales. This allows studying the statistical features of the soil motion due to the seismic noise wave field properties and its dependency on the near-surface geology. A selfaffine random medium is the most appropriate description of the heterogeneous material properties of the geological structures. In particular, the intrinsic properties show a clear dependency on the local shallow subsoil conditions with consolidated soils being characterized by a nearly ballistic behavior for short time scales. A distinct multi-fractal behavior, being characterized by a crossover to a diffusive character for longer time scales, indicates a long-range anti-correlated motion which can be seen as a mixture between uncorrelated noise and white noise. Although in a strict sense the seismic wave field is not completely isotropic, an ultimate pre-condition for a diffusive wave field, the deviations compared to a uniform distribution are rather small, meaning that the emergence of the Green's function is effective for all network sites after a sufficient self-averaging process that is provided by the scattering and the random spatial-temporal noise source distribution. By analyzing the transition in the dynamical character of the energy propagation process allows studying seismic noise from a different perspective. Complementary to a 1D description of the wave propagation for a diffusive field (e.g. Kawase et al. 2011) this enables quantifying also the 2D variability and the multiple scattering required to produce diffusive wave field.

Source and processing effects on noise correlations

Abstract We quantify the effects of spatially heterogeneous noise sources and seismic processing on noise correlation measurements and their sensitivity to Earth structure. Using numerical wavefield simulations and adjoint techniques, we calculate interstation correlations and sensitivity kernels for arbitrarily distributed noise sources where—as in the real Earth—different frequencies are generated in different locations. While both heterogeneous noise sources and processing can have profound effects on noise correlation waveforms, narrow-band traveltime measurements are less affected, in accord with previous analytical studies. Sensitivities to Earth structure depend strongly on the source distribution and the processing scheme, and they reveal exotic frequency dependencies that go beyond the well-known frequency scaling of the Fresnel zone width. Our results indicate that modern full waveform inversion applied to noise correlations is not possible unless one of the following measures is taken: (1) properly account for noise source distribution and processing, or (2) limit measurements to phase or time shifts in narrow frequency bands. Failure to do so can lead to erroneous misfits, tomographic artefacts, and reduced resolution.

Investigation of Twin Jet Aeroacoustic Properties near a Hybrid Wing–Body Shield

In preparation for wind-tunnel acoustic experiments of a hybrid wing–body vehicle with two jet engine simulator units, a series of twin jet aeroacoustic investigations was conducted leading to increased understanding and risk mitigation. An existing twin jet nozzle system and a fabricated hybrid wing–body aft deck fuselage are combined for a 1.9% model scale study of jet nozzle spacing and jet cant angle effects, elevon deflection into the jet plume, and acoustic shielding by the fuselage body. Linear and phased array microphone measurements are made, and data processing includes the use of the deconvolution approach for the mapping of acoustic sources. Closely spaced twin jets with a 5 deg inward cant angle exhibit reduced noise levels compared with their parallel flow counterparts at similar and larger nozzle spacings. A 40 deg elevon deflection into the twin jet plume, which is required for hybrid wing–body ground rotation, can significantly increase upstream noise levels (more than 5 dB overall sound pressure level) with only minimal increases in the downstream direction. Lastly, deconvolution approach for the mapping of acoustic sources processing can resolve the noise source distribution of multiple shielded jet sources within the limits of the incoherent source assumption.

Noise Source Distribution of Coaxial Subsonic Jet-Short-Cowl Nozzle

The noise source distribution of a short-cowl coaxial jet operating at different velocity ratios is described in this work. This was motivated by an ongoing research about noise prediction of coaxial jets through Acoustic Analogy with purposes of industrial engine application. This research has been carried out between Universidade Federal de Uberlândia (UFU), Brazil and the Institute of Sound and Vibration Research (ISVR) at Southampton University, UK. The numerical approach employed is originally based on Lighthill Acoustic Analogy. This technique, although likely known, is associated with an improved energy transfer time-scale, used in the turbulence two-point correlation function, in order to enhance the source model. The source model is coupled with the aerodynamic calculation of flow through turbulence quantities evaluated by using a standard k-e turbulence modeling. The Computational Fluid Dynamics data have also been used to provide complementary information about the coaxial jet noise production mechanisms. Experimental data were used in order to corroborate the results from the current model. Good agreement has been found, showing that high and low frequency contributors to the radiated noise for low velocity ratio are aggregated in a region about seven to ten secondary diameters downstream, while at higher velocity ratios sources are continuously spread from about one up to ten secondary diameters from the jet exit.

Analysis of a grid anti-islanding impedance detection system in presence of uniform distributed noise

The incomplete knowledge of the power distribution grid status and in particular the impossibility to constantly monitor the number of subjects (feeder) able to inject current autonomously at a known time can represent a hazardous condition for the main utility provider operators’ safety. Whenever an unexpected islanding condition is induced on the utility grid, every subject injecting power on the grid should stop such injection switching to a safety shut down condition. Due to the presence of multi injecting sources on the utility grid it is possible that one or more feeders do not recognise such event and continue to supply energy to a portion of the utility grid, keeping the main provider operators unaware of such condition. Such situation is the islanding condition. The present work aims, starting from a previous paper Fort et al. (2010) at assessing the robustness of an anti-islanding detection system in presence of a uniform distributed noise sources on the grid. The anti islanding detection system is based on the injection of an interharmonic component on the grid. The single phase case is considered. In this paper it is considered the optimum tradeoff between the harmonic distortion injection of a component which is required to be both meaningful from a signal to noise ratio standpoint and low enough not to affect the overall grid THD. The aim of this paper is to prove that such device will constitute a low cost, stand alone, and reliable automatic detection system for grid disconnection detection, to be supplied in conjunction with devices like the photovoltaic (PV) inverters in order to minimise their islanding non-detection zone.

A numeric evaluation of attenuation from ambient noise correlation functions

The ambient noise correlation function (NCF) calculated between seismic stations contains, under appropriate conditions, accurate travel time information. However, NCF amplitudes are highly debated due to noise source intensity and distribution, seismic intrinsic attenuation, scattering, and elastic path effects such as focusing and defocusing. We prove with various numerical simulations that the NCFs calculated for a uniformly dispersive medium using the coherency method preserve accurate geometrical spreading and attenuation decay. We show that for a wide range of noise source distributions, the coherency of the noise correlation functions matches a Bessel function decaying exponentially with a specific attenuation coefficient. Conditions needed to obtain these results include averaging over long enough time intervals, a uniformly distributed seismic network, and a good distribution of far‐field noise sources. We also show that the estimated attenuation coefficient corresponds to the interstation and not the noise‐source‐to‐receiver structure.

Relationship between Spatial Distribution of Noise Sources and Target Scatterings Observed in the 2010 Sea Trial of Ambient Noise Imaging

An aspherical lens with an aperture diameter of 1.0 m has been designed and fabricated to develop a prototype system for ambient noise imaging (ANI). A sea trial of silent target detection using the prototype ANI system was conducted under only natural ocean ambient noise at Uchiura Bay, in November of 2010. It was verified that the targets are successfully detected under natural ocean ambient noise, mainly generated by snapping shrimps. In this study, we surveyed the relationship between the spatial distribution of noise sources and the target scattering captured by the ANI system. The observation using a pair of tetrahedron arrays was conducted at the same time as the sea trial. The estimated source positions were spread when the noises arrived from the sea bottom. Some of the sources were around the barge, and other sources were around fish preserves. On the other hand, the source positions were coincident with the barge when the noises arrived from the sea surface. The calculated scattering fields of the target showed sharp directivities. The locations of noise sources, where the ANI system can capture target scatterings with high intensities, were roughly determined at the barge around the sea surface.

The influence of nonuniform ambient noise on crustal tomography in Europe

AbstractAmbient‐noise seismology is of great relevance to high‐resolution crustal imaging, thanks to the unprecedented dense data coverage it affords in regions of little seismicity. Under the assumption of uniformly distributed noise sources, it has been used to extract the Green's function between two receivers. We determine the imprint of this assumption by means of wave propagation and adjoint methods in realistic 3‐D Earth models. In this context, we quantify the sensitivity of ambient‐noise cross‐correlations from central Europe with respect to noise‐source locations and shear wave‐speed structure. We use ambient noise recorded over 1 year at 196 stations, resulting in a database of 864 cross‐correlations. Our mesh is built upon a combined crustal and 3‐D tomographic model. We simulate synthetic ambient‐noise cross‐correlations in different frequency bands using a 3‐D spectral‐element method. Traveltime cross‐correlation measurements in these different frequency bands define the misfit between synthetics and observations as a basis to compute sensitivity kernels using the adjoint method. We perform a comprehensive analysis varying geographic station and noise‐source distributions around the European seas. The deterministic sensitivity analysis allows for estimating where the starting crustal model shows better accordance with our data set, and gain insight into the distribution of noise sources in the European region. This highlights the potential importance of considering localized noise distributions for tomographic imaging, and forms the basis of a tomographic inversion in which the distribution of noise sources may be treated as a free parameter similar to earthquake tomography.

Extracting surface wave attenuation from seismic noise using correlation of the coda of correlation

AbstractExtracting surface wave travel time information from the cross‐correlation (CC) of seismic ambient noise has been a great success and remains fast growing. However, it is still challenging to exploit the amplitude content of the noise CC. Although spatial average is able to constrain somewhat meaningful attenuation using noise CC amplitudes, clear bias is observed when spatially varying attenuation is estimated with the traditional noise CC calculation methods. Perhaps the key lies in the development of novel techniques that can mitigate the effect of the uneven distribution of natural noise sources. In this paper, we propose a new method to use the correlation of the coda of correlation of noise (C3) for amplitude measurement. We examine the ability of the method to retrieve surface wave attenuation using data from selected line array stations of the USArray. By comparing C3‐derived attenuation coefficients with those estimated from earthquake data, we demonstrate that C3 effectively reduces bias and allows for more reliable attenuation estimates from noise. This is probably because of the fact that the coda of noise correlation contains more diffused noise energy, and thus, the C3 processing effectively makes the noise source distribution more homogeneous. When selecting auxiliary stations for C3 calculation, we find that stations closer to noise sources (near the coast) tend to yield better signal‐to‐noise ratios. We suggest to preprocess noise data using a transient removal and temporal flattening method, to mitigate the effect of temporal fluctuation of the noise source intensity, and to retain relative amplitudes. In this study, we focus our analysis on 18 s measurements.

Three-dimensional vocal tract modeling of fricatives /s/ and /sh/ for post-glossectomy speakers

Production of fricatives involves a narrow supraglottal constriction along the vocal tract. Air flows through the constriction, and generates turbulent noise source(s) by impinging on some obstacles downstream. In post-glossectomy speakers, the production of /s/ and /sh/ is often problematic. It is mainly caused by the tongue surgery, which changes tongue properties such as volume, motility, and symmetry, preventing the tongue from creating proper constrictions. The purpose of this study was to gain some insights on how the vocal tracts of abnormal /s/ and /sh/ are shaped and what are their corresponding acoustic consequences. Based on cine magnetic resonance images, we built 3-D vocal tract models for /s/ and /sh/ from two post-glossectomy speakers (one with abnormal /s/ and the other with abnormal /sh/). Due to the missing part of the tongue, the reconstructed vocal tracts are asymmetric with either an air-flow bypass or a side branch formed near the constrictions. Two coupled physics submodels are included in the 3-D FEM acoustic simulation: incompressible potential flow for the mean air flow and aeroacoustics for the distributed noise sources. The resulting acoustic spectra and acoustic roles of air flow bypass or side branch will be discussed. [This study was supported by NIH R01CA133015.]

Speech intelligibility among modulated and spatially distributed noise sources

At a cocktail party, listeners are faced with multiple, spatially distributed interfering voices. The dominant interfering voice may change from moment to moment and, consequently, change in spatial location. The ability of the binaural system to deal with such a dynamic scene has not been systematically analyzed. Spatial release from masking (SRM) was measured in simple spatial scenes, simulated over headphones with a frontal speech source. For a single noise at 105°, SRM was reduced if that noise modulated (10 Hz square wave, 50% duty cycle, 20 dB modulation depth), but, for two noises in symmetrical locations, SRM increased if the noises were modulated in alternation, suggesting that the binaural system can "switch" between exploiting different spatial configurations. Experiment 2 assessed the contributions of interaural time and level differences as a function of modulation rate (1-20 Hz). Scenes were created using the original head-related impulse responses and ones that had been manipulated to isolate each cue. SRM decreased steeply with modulation rate. The combined effects of interaural time and level differences were consistent with additive contributions. The results indicate that binaural sluggishness limits the contribution of binaural switching to speech understanding at a cocktail party.

Assessment of surface roughness for a ‘silent’ aircraft

AbstractBoundary-layer noise produced by rough surfaces is a potential contributor to airframe noise. In this paper, an attempt is made to assess the surface roughness noise for a conceptual Silent Aircraft design SAX-40 using a prediction model described in previous theoretical work (Liu and Dowling(12)). Estimates of three idealised test cases show that surface roughness could produce a relatively high noise level and enhance the trailing-edge noise somewhat. Roughness height and roughness density are two significant parameters which affect surface roughness noise, with roughness height having the more dominant effect. The distribution of roughness noise sources decreases in the streamwise direction on the SAX-40 surface. Two candidate rough surfaces are selected for SAX-40 to keep surface roughness noise at a negligible level and to meet an aggressive noise target.

On the dependence of the noise amplitude on the correlation length of inlet temperature fluctuations in PWRs

The dependence of the amplitude of the neutron noise in PWRs on the radial correlation length of the noise source, that is the inlet temperature fluctuations, is investigated. The motivation for this work comes from the recent observation that the noise amplitude has increased significantly in certain Spanish PWRs. The reason of this increase has not yet been satisfactorily identified, although there are a few assumptions. One new hypothesis, put forward in this paper, is that a coolant flow with more coherent characteristics, which has actually been observed, could explain the increase of the noise amplitude. A more coherent flow leads to a longer spatial (radial) correlation length of the inlet temperature fluctuations, which in turn, in case the APSD of the temperature noise is unchanged, will lead to the increase of the neutron noise APSD. The mechanism behind this phenomenon is the same as the reason of the failure of the traditional noise based method for the determination of the Moderator Temperature Coefficient (MTC), and is related to the characteristics of the spatially randomly distributed noise sources. In this paper the relationship between the radial correlation length of the inlet temperature fluctuations and the neutron noise amplitude is investigated quantitatively and the increase of the noise amplitude with the increased spatial correlation of the inlet temperature fluctuations is demonstrated.

Green's function retrieval from the CCF of random waves and energy conservation for an obstacle of arbitrary shape: noise source distribution on a large surrounding shell

SUMMARY For imaging the earth structure, the cross-correlation function (CCF) of random waves as ambient noise or coda waves has been widely used for the estimation of the Green’s function. We precisely study the condition for the Green’s function retrieval in relation to the energy conservation for a single obstacle of arbitrary shape. When an obstacle is placed in a 2-D homogeneous medium, the Green’s function is written by a double series expansion using Hankel functions of the first kind which represent outgoing waves. When two receivers and the scattering obstacle are illuminated by uncorrelated noise sources randomly and uniformly distributed on a closed circle of a large radius surrounding them, the lag-time derivative of the CCF of random waves at the two receivers can be written by a convolution of the antisymmetrized Green’s function and the autocorrelation function of the noise source time function. We explicitly derive the constraint for the Hankel function expansion coefficients as the sufficient condition for the Green’s function retrieval. We show that the constraint is equal to the generalized optical theorem derived from the energy conservation principle. Physical meaningofthegeneralizedopticaltheorembecomesclearwhentheHankelfunctionexpansion coefficients are transformed into scattering amplitudes in the framework of the conventional scattering theory. In the 3-D case, the Green’s function is written by a double series expansion using spherical Hankel functions of the first kind and spherical harmonic functions. When two receivers and the scattering obstacle are illuminated by noise sources randomly and uniformly distributed on a closed spherical shell of a large radius surrounding them, we explicitly derive the constraint for the spherical Hankel function expansion coefficients for the Green’s function retrieval and the energy conservation. We note that the derivation of the constraint does not assume that two receivers are in the far field of the scattering obstacle.

Comparative Study on Aerodynamic Noise of Dimensionally Similar Locomotive Models of High Speed Trains

This study is focused on the locomotive of high speed train. First, wind tunnel test is used to verify the credibility of numerical results. Then, in order to compare the difference of aerodynamic noise generated by different size locomotive, dimensionally similar models of 1/8th, 1/12th and 1/15th scale are studied by using numerical simulation, including stationary aerodynamic characteristics, fluctuation characteristics of unsteady flow, noise source distribution on surface and far-field noise spectral characteristics. Based on the result of pressure fluctuation, it is noted that the difference between 1/15th and 1/8th scale model is larger in individual parts, including the separation zone in the roof and the edge of window. In addition, according to the far-field noise calculation, the result of 1/12th scale model is better than 1/15th scale model. From the results mentioned above, 1/15th or much smaller scale model should not be used in wind tunnel test as possible.

The Research of Cylindrical Noise Source Distribution Using Near-Field High-Resolution Location Methods Based on MVDR Algorithm

According to the lack of cylinder-model noise source localization technology based on near-field focused beam-forming, the paper aimed to find a near-field high-resolution method of the cylindrical noise source which is applied to measure the model as cylinder-models. This method established a near-field measurement model of noise sources using cylindrical measurement surface, and applied MVDR algorithm to noise sources localization. Reconstruct the cylinder measurement surface through the joint amplitude-phase compensation to achieve the noise source high-resolution localization and relative intensity estimates in the near-field. Verify the availability of this method through the simulations, which provided a theoretical basis for the next practical engineering application of the cylinder-model noise source localization based on near-field focused beam-forming.

Research on the High Resolution Underwater Acoustic Image Measurement Technology based on STMV Method

Abstract The technology of underwater acoustic image measurement is a passive positioning technology with high precision in the near field. It can be used to measure the spatial distribution of radiation noise sources. The resolution of conventional underwater acoustic image measurement technology for the low frequency noise sources is poor. In order to improve the resolution, the STMV focused beamforming algorithm with high resolution was proposed in this paper. STMV beam-forming is a coherent subspace signal processing method, which is usually used in the high resolution processing field in the far field. In this paper, the weight of STMV beam-forming was revised so that it could be used in the underwater acoustic image measurement in the near field. The formulas of the weights and space spectrum are given through the theoretical deduction. The algorithm was validated by the related simulations and experiments. The results showed that the resolution of low frequency sources and the performance of underwater acoustic image measurement were improved in the method.

Noise Radiation from a Continuous Mold-Line Link Flap Configuration

The results of an experimental study of the noise from a Continuous Mold-Line Link (CML) flap are presented. Acoustic and unsteady surface pressure measurements were performed on a main element wing section with a half-span CML flap in NASA Langley's Quiet Flow Facility. The acoustic data were acquired with a medium aperture directional array (MADA) of microphones. The Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) method is applied to determine the spatial distribution and strength of the noise sources over the surface of the test model. A Coherent Output Power (COP) method which relates the output from unsteady surface pressure sensors to the output of the MADA is also used to obtain more detailed characteristics of the noise source distribution in the trailing edge region of the CML. These results are compared to those obtained for a blunt flap to quantify the level of noise benefit that is achieved with the CML flap. The results indicate that the noise from the CML region of the flap is 5 to 17 dB lower (depending on flap deflection and Mach number) than the noise from the side edge region of the blunt flap. Lower noise levels are obtained for all frequencies. Spectral analysis of the noise from the cove region of the CML and blunt flap models also reveal a spectral peak in the high frequency range that is related to noise scattering at the trailing edge of the main element. The peaks in the CML and blunt flap cove noise spectra are close in level and often exceed blunt side edge noise. Applying a strip of serrated tape to the trailing edge of the CML flap model main airfoil reduced the peak but increased other noise somewhat. Directivity measurements show that the CML flap can be more directional than the blunt flap.