Distribution Of Acoustic Sources Research Articles

Gridless three-dimensional acoustic imaging based on the concept of sonons: Reconstruction of source directivity and equivalent spatial distribution

Three-dimensional acoustic imaging with microphone arrays is a challenging task due to the numerous degrees of freedom of the physical processes at stake. Instead of meshing a region of space to identify acoustic source distributions, the aim of this work is to propose a novel gridless method to reconstruct the sound field by an equivalent set of point sources with unrestricted coordinates, named “acoustic sonons”. Their radiation is expected to reproduce the same acoustic properties as the measured acoustic field, such as its pressure level, directivity and spatial coherence by recovering phase information. It is found that the proposed concept of sonons, while being very flexible, effectively provides equivalent representations for a variety of source distributions. The simulation results show that the method can be adapted to diverse acoustic field situations, whether they are composed of elementary sources or intricate volumetric source densities. The problem is formulated within a probabilistic framework, through a hierarchical Bayesian model inferred by a dedicated Markov chain Monte Carlo algorithm. The performance of the method is evaluated on analytical radiation problems and its ability to reconstruct the directivity is tested for two far-field radiation cases.

Numerical study of acoustic source localization of rotor using a novel discrete noise analysis strategy

In order to obtain the influence mechanism of the rotor noise and the local aerodynamic variation under different operating conditions, a novel discrete noise analysis strategy for the acoustic source localization is established. The analysis strategy has two aspects including the blade division method and the noise contribution calculation method. Firstly, the body-fitted rotor blade grids are preprocessed and refined at the division position before the flowfield simulation. Then, based on the flowfield result, the blade grids are divided into several blocks in the chordwise and spanwise directions, and the acoustic pressure of each block is predicted. Finally, a discriminant function for the contribution of the block to the rotor noise is proposed, and the acoustic source localization is carried out. The URANS equations and FW-H equations are used to capture high-fidelity flowfield and rotor acoustic pressure. Parameters such as the block position in different direction on the rotor blade and the collective pitches are quantified, and the relationship between air flow and aeroacoustics is discussed in detail. Some conclusions are obtained by analyzing the BO105 model rotor in hover. The acoustic pressure produced by the leading edge of the upper surface could cancel about 50% acoustic pressure of the remainder of these blocks. Increasing the force at this position will be benefit to the noise reduction. Acoustic source distribution is closely linked to the flow separation near the blade tip: the main acoustic source moves toward around 0.9 R section of the blade in the spanwise direction, and it moves from the leading edge towards the trailing edge in the chordwise direction.

Numerical Study on Aerodynamic Noise Reduction of Pantograph

A hybrid method incorporating the simulations of noise sources with delayed detached eddy simulation (DDES) and calculations of far-field noise with the Ffowcs Williams–Hawkings (FW-H) equation is used to study the suppression technique for the aerodynamic noise of a Faiveley CX-PG pantograph. Considering that China’s Fuxing bullet trains operate at 350 km/h, the inflow velocity of 350 km/h is applied in this paper. The noise radiated from the panhead area, middle area, and bottom area at an inflow velocity of 350 km/h is distinguished. The noise intensities at the standard observer show that the noise radiated from the panhead area is the strongest, and the sound pressure level spectrum value is larger than the other two in the range above 500 Hz. The influence of applying the wavy rods and modifying the contact strip shape on the aerodynamic noise is discussed in detail. By comparing the acoustic source distribution and the far-field noise intensity, it is found that applying the wavy rods can effectively reduce the panhead noise, especially around the peak frequency. Modifying the shape of the contact strip to a hexagon can suppress the vortex shedding, leading to a lower surface pressure level. Combining the strip modification and wavy rods, the total noise intensity can be diminished by about 3.0 dB.

Reconstruction of thermoacoustic emission sources from proton irradiation using numerical time reversal

Dose delivery in proton beam therapy for cancer treatment can be mapped by analyzing thermoacoustic emissions measured by ultrasound arrays. Here, a method is presented for spatial mapping of thermoacoustic sources using numerical time reversal, simulating physical re-transmission of measured emissions into the medium. The spatial distribution of acoustic sources is shown to be approximated by the amplitude envelope of the time-reversed field, evaluated at the time of emission. Given calibration of the array sensitivity and knowledge of tissue properties, this approach approximately reconstructs the induced acoustic pressure, equal to the product of radiation dose, density, and Grueneisen parameter. Numerical time reversal is implemented using two models for array elements, as either ideal line sources or diffracting rectangular radiators. Demonstrated reconstructions employ previously reported measurements of thermoacoustic emissions from proton energy deposition in tissue-mimicking phantoms. For a phantom incorporating a bone layer, reconstructions account for the higher sound speed in bone. Spatial resolution of reconstructions, assessed by widths of reconstructed Bragg peaks, is improved in the array direction by incorporation of diffraction effects. In comparisons with corresponding Monte Carlo simulations, source distributions correspond well with simulated proton dose, while source localization with respect to room coordinates is improved by incorporating sound speed inhomogeneities.

Improvement of orthogonal matching pursuit deconvolution beamforming method for acoustic source identification

The deconvolution approach for the mapping of acoustic sources (DAMAS) based on orthogonal matching pursuit (OMP-DAMAS) has attracted much attention due to its advantages of high spatial resolution and excellent capability to suppress spurious sources. In this paper, we propose an improved version of OMP-DAMAS based on fast Fourier transformation (FFT), abbreviated as FFT-OMP-DAMAS. This method assumes that the array point spread functions (PSFs) are spatially shift-invariant. The sum of the product of the acoustic source distribution and PSFs is converted into a convolution form, which is further converted into a product in the wave number domain. With these conversions, FFT can be used to improve the solving speed. Both simulations and experiments show that the proposed method not only inherits the advantages of OMP-DAMAS in terms of spatial resolution and spurious source suppression but also significantly improves the computational efficiency compared with OMP-DAMAS.

Discrete Approximations of Acoustic Source Distributions

The modeling of source a distributions of finite spatial extent in ultrasound and medical imaging applications is a problem of longstanding interest. In time-domain methods, such as the finite-difference time-domain or pseudospectral approaches, one requirement is the representation of such distributions over a grid, normally Cartesian. Various artifacts, including staircasing errors, can arise. In this short contribution, the problem of the representation of distribution over a grid is framed as an optimization problem in the Fourier domain over a preselected set of grid points, thus maintaining control over computational cost and allowing the fine-tuning of the optimization to the wavenumber range of interest for a particular numerical method. Numerical results are presented in the important special case of the spherical cap or bowl source.

Acoustic source distribution on aerofoils and its reconstruction from far-field array measurements

This paper is concerned with the acoustic source distribution on aerofoils interacting with a turbulent flow. The source distribution is usually taken to be equal to the pressure jump across the aerofoil, whose behaviour is well understood from classical unsteady aerofoil theory. However, much of this source does not radiate to the far-field. This paper deals with the application of Fourier methods to extract the equivalent source distribution on the aerofoil that radiates perfectly to the far-field. This paper shows that, whilst the actual pressure jump across a flat plate tends to infinity at the leading edge while dropping to zero at the trailing edge (due to the Kutta condition), the equivalent source distribution is more uniform across the aerofoil chord, which explains why the radiation spectrum and single-frequency directivity both exhibit strong oscillations due to interference from coherent sources across the chord. A method for extracting this far-field equivalent source distribution from an array of far-field measurements is proposed in this paper.

Holographic reconstructions of large-eddy simulations of a highly heated laboratory-scale jet

Near-field acoustical holography allows for the reconstruction of a sound field from a limited measurement. For aeroacoustic noise generated by military aircraft, reconstructions of the pressure field within the jet plume have provided insight into equivalent acoustic noise source distributions, though the relationship between these equivalent sources and actual flow pressures and velocities within the plume remain unknown. This study reconstructs the sound field of a highly heated laboratory-scale jet simulated by the Naval Research Lab's Jet Noise Reduction (JENRE) large eddy simulation solver. The simulated data allow for comparisons of the holographic reconstructions of the pressure and particle velocity fields to points along the nozzle lipline, to the Ffowcs Williams and Hawkings integration surface (FWHS), and to the far field. Pressure field reconstructions outside and inside of the FWHS match well, while particle velocity reconstructions follow similar trends as the velocities generated by the LES but severly underestimate the time harmonic complex amplitdues.

Characterizing distinct components of tactical aircraft noise sources

Noise from a tactical aircraft can impact operations due to concerns regarding military personnel noise exposure and community annoyance and disturbance. The efficacy of mission planning can increase when the distinct, complex acoustic source mechanisms creating the noise are better understood. For each type of noise, equivalent acoustic source distributions are obtained from a tied-down F-35B operating at various engine conditions using the hybrid method for acoustic source imaging of Padois, Gauthier, and Berry [J. Sound Vib. 333, 6858-6868 (2014)]. The source distributions for the distinct noise types are obtained using different sections of a 71 element, ground-based linear array. Using a subarray close to the nozzle exit plane, source distributions are obtained for fine-scale turbulent mixing noise and broadband shock-associated noise, although grating lobes complicate interpretations at higher frequencies. Results for a subarray spanning the maximum sound region show that the multiple frequency peaks in tactical aircraft noise appear to originate from overlapping source regions. The observation of overlapping spatial extent of competing noise sources is supported by the coherence properties of the source distributions for the different subarrays.

Analysis of the Singular Values for Conductivity Reconstruction in Magneto-Acoustic Tomography With Magnetic Induction

Due to the acoustic source distribution and limited bandwidth ultrasound measurement, the dominant ultrasound signals always come from the boundary of the electrical conductivity in magneto-acoustic tomography with magnetic induction (MAT-MI). To make full use of the strong boundary ultrasound signals, a system matrix is built which shows the relationship between the electrical conductivity and the ultrasound signals. By analyzing the singular values of the system matrix, the necessary signal to noise ratio(SNR) level is estimated in this study. An inverse procedure based on the truncated singular value decomposition(TSVD) method is presented to improve the quality of the reconstructed MAT-MI image. Simulation results show that the reconstructed conductivity images by using the new algorithm match better than that of the back-projection algorithm. Both the simulation results and the experiment results prove the reconstructed image is close to the original conductivity distribution when the more singular values are used in the inverse procedure. Meanwhile, as the number of singular values increases, the effect of noise will be enhanced in the reconstructed image. The proposed reconstruction algorithm can improve the quality of the reconstruction image for a low SNR system. Moreover, the system matrix based reconstruction algorithm proposed in this work will help to analyze the physical process and to obtain accurate high-resolution reconstructions for MAT-MI.

Data-educed broadband equivalent acoustic source model for supersonic jet noise.

A broadband equivalent acoustic source distribution can be used to model the sound field near a high-speed jet. Such models must account for the spatiospectral variation of the sound levels. This work presents a technique for obtaining such a model using a spectral decomposition method associated with large and fine-scale turbulent mixing noise to create broadband equivalent source distributions for each noise type. The large-scale turbulent mixing noise is represented by frequency-dependent wavepackets, while the fine-scale turbulent mixing noise is modeled as a frequency-dependent incoherent, extended source distribution. This technique is applied to acoustical measurements from an ideally expanded, unheated Mach 1.8 jet. The wavepackets model the sound field levels in the maximum radiation region, but the second incoherent source distribution is required to obtain the levels at the other locations. The combination of the incoherent source distribution and the wavepacket provides a broadband, equivalent acoustic source representation that adequately models the sound field for Strouhal numbers between 0.04 and 0.25. At higher Strouhal numbers, better agreement is obtained when accounting for a frequency-dependent shift in the apparent acoustic source region. This frequency-dependent source region is more important closer to the jet than in the far field.

The effect of blade outlet angle on the acoustic field distribution characteristics of a centrifugal pump based on Powell vortex sound theory

Numerical simulation method based on Powell vortex sound theory is adopted to explore the mechanisms of acoustic field distribution characteristics of a centrifugal pump with different blade outlet angles. Results reveal that with the increasing of the angle, the fluctuation range of acoustic source at the tongue in time domain extends, which causes the same fluctuation characteristics of acoustic pressure. As the angle increases from 15° to 35°, the total acoustic source intensity (TASI) and total acoustic pressure level (TAPL) at the tongue increase by 1.68% and 2.54% on average under various rotational speeds, respectively. Additionally, the TASI and TAPL show the similar directivity characteristics under different angles, and the change of the angle has slight effect on the extremum distribution characteristics. Besides, the A-weighted average acoustic pressure level (Lp) and A-weighted average acoustic source intensity level (La) are adopted to describe the overall level of TAPL and TASI. When the angle increases from 15° to 35°, the La and Lp increase gradually and increase by 1.59% and 2.21% on average under various rotational speeds, respectively. The study reveals the acoustic source and acoustic pressure distribution characteristics in time and frequency domain qualitatively and quantitatively, and manifests that the distribution and variation patterns of acoustic source lead to those of acoustic pressure. The conclusions could lay a foundation for the empirical formula proposition between acoustic pressure and acoustic source.

Thermo-acoustic imaging based on heat absorption distribution

Thermo-acoustic imaging with the current injection is a novel coupling imaging technology with electromagnetic field and sound field. It combines the advantages of high contrast of the electrical impedance tomography and the high spatial resolution of sonography, and then it has the potential of early diagnosis. In order to verify the feasibility of thermo-acoustic imaging with the current injection for complicated biological tissues, the principle of thermo-acoustic imaging with the current injection was analyzed. By injecting pulse current to phantom through a pair of electrodes, the joule heating is produced inside the objects and an acoustic signal is excited due to heat expansion. Heat absorption is considered thermo-acoustic source, the complex geometric model with low electrical conductivity was established, and the acoustic source and acoustic pressure distribution were obtained by coupling analysis of electromagnetic field and acoustic field. Meanwhile experimental system was established, and the experiments were carried out aiming at animal bone and fresh pork liver which are combined with low conductivity gel phantom (1, 0.5 S/m). The acoustic signal was detected by ultrasonic transducer with the center frequency of 1 MHz, and then the acoustic pressure and B-scan image were obtained. The results indicate that the B-scan image of thinner bone experiment can reflect the distribution of object accurately. Although the hard bone has a certain attenuation effect on the acoustic signal, a clear image can still be obtained, and the images can reflect the change of the conductivity of the gel. The experiment results of fresh pork liver show that this method can response the conductivity changes of low electrical conductivity biological tissue. This study confirms the feasibility of thermo-acoustic imaging with the current injection for the imaging of complex biological tissue. The novel method provides further evidence that it can be used for the diagnosis of human diseases.

Localizing bioacoustic signals with long-baseline hydrophone arrays

Real-time localization and tracking of vocalizing marine mammals is a powerful tool to a) mitigate the risk of ship strikes and disturbance of the animals as well as b) to understand the impact of anthropogenic noise on habitat use of cetaceans. A long baseline hydrophone array has been installed in Squally Channel, a culturally, ecologically, and economically important marine environment in northern British Columbia, Canada. The array consists of four synchronized bottom-mounted hydrophones that permanently record and radio-transmit data to a land-based laboratory. Automated analysis tools have been developed to detect and localize transient bioacoustic signals from three or more hydrophones in real-time. They comprise the correlation of hydrophone signals, the construction of a replica surface using acoustic propagation models, and signal localization from a spatial likelihood surface. An overview of the localization method is presented. We assess spatial localization precision and discuss random and systematic sources of error stemming from the signal type and quality, bathymetry, sound speed profile, as well as the spatial distribution of hydrophones and acoustic sources. Simulation results are compared to field measurements. Based on our findings, challenges and opportunities associated with localization using long baseline arrays in coastal environments are discussed.

Sparse Representation Using Multidimensional Mixed-Norm Penalty With Application to Sound Field Decomposition

A sparse representation method for multidimensional signals is proposed. In generally used group-sparse representation algorithms, the sparsity is imposed only on a single dimension and the signals in the other dimensions are solved in the least-square-error sense. However, multidimensional signals can be sparse in multiple dimensions. For example, in acoustic array processing, in addition to the sparsity of the spatial distribution of acoustic sources, acoustic source signals will also be sparse in the time-frequency domain. We define a multidimensional mixed-norm penalty, which enables the sparsity to be controlled in each dimension. The majorization-minimization approach is applied to derive the optimization algorithm. The proposed algorithm has the advantages of a wide range for the sparsity-controlling parameters, a small cost of adjusting the balancing parameters, and a low computational cost compared with current methods. Numerical experiments indicate that the proposed method is also effective for application to sound field decomposition.

Resolution enhancement of two-dimensional grid-free compressive beamforming.

Reconstructing the acoustic source distribution via imposing a sparsity constraint on a continuum, the atomic norm minimization (ANM) based grid-free compressive beamforming can eliminate the basis mismatch of conventional grid-based compressive beamforming. However, it works well only for sufficiently separated sources, which prohibits high resolution. The drawback arises because it uses an atomic norm to measure the source sparsity, while the atomic norm is not a direct sparse metric and its minimization is equivalent to the sparsity constraint only when the sources are sufficiently separated. This paper devotes itself to overcoming the drawback for the two-dimensional ANM based grid-free compressive beamforming. First, a sparse metric that can promote sparsity to a greater extent than the atomic norm is proposed. Then, using this metric a minimization problem is formulated and the majorization-minimization (MM) solving algorithm is introduced. MM iteratively conducts atomic norm minimization with a sound reweighting strategy, and therefore the developed method can be termed as iterative reweighted atomic norm minimization (IRANM). Both simulations and experiments demonstrate that whether a standard uniform rectangular array or a non-uniform array constituted by a small number of microphones is utilized, IRANM can overcome the drawback and thus enhance the resolution.

Sound generation by turbulent premixed flames

This paper presents a numerical study of the sound generated by turbulent, premixed flames. Direct numerical simulations (DNS) of two round jet flames with equivalence ratios of 0.7 and 1.0 are first carried out. Single-step chemistry is employed to reduce the computational cost, and care is taken to resolve both the near and far fields and to avoid noise reflections at the outflow boundaries. Several significant features of these two flames are noted. These include the monopolar nature of the sound from both flames, the stoichiometric flame being significantly louder than the lean flame, the observed frequency of peak acoustic spectral amplitude being consistent with prior experimental studies and the importance of so-called ‘flame annihilation’ events as acoustic sources. A simple model that relates these observed annihilation events to the far-field sound is then proposed, demonstrating a surprisingly high degree of correlation with the far-field sound from the DNS. This model is consistent with earlier works that view a premixed turbulent flame as a distribution of acoustic sources, and provides a physical explanation for the well-known monopolar content of the sound radiated by premixed turbulent flames.

A Marchenko equation for acoustic inverse source problems.

From acoustics to medical imaging and seismology, one strives to make inferences about the structure of complex media from acoustic wave observations. This study proposes a solution that is derived from the multidimensional Marchenko equation, to learn about the acoustic source distribution inside a volume, given a set of observations outside the volume. Traditionally, this problem has been solved by backpropagation of the recorded signals. However, to achieve accurate results through backpropagation, a detailed model of the medium should be known and observations should be collected along a boundary that completely encloses the volume of excitation. In practice, these requirements are often not fulfilled and artifacts can emerge, especially in the presence of strong contrasts in the medium. On the contrary, the proposed methodology can be applied with a single observation boundary only, without the need of a detailed model. In order to achieve this, additional multi-offset ultrasound reflection data must be acquired at the observation boundary. The methodology is illustrated with one-dimensional synthetics of a photoacoustic imaging experiment. A distribution of simultaneously acting sources is recovered in the presence of sharp density perturbations both below and above the embedded sources, which result in significant scattering that complicates the use of conventional methods.

Improving the efficiency of deconvolution algorithms for sound source localization.

The localization of sound sources with delay-and-sum (DAS) beamforming is limited by a poor spatial resolution-particularly at low frequencies. Various methods based on deconvolution are examined to improve the resolution of the beamforming map, which can be modeled by a convolution of the unknown acoustic source distribution and the beamformer's response to a point source, i.e., point-spread function. A significant limitation of deconvolution is, however, an additional computational effort compared to beamforming. In this paper, computationally efficient deconvolution algorithms are examined with computer simulations and experimental data. Specifically, the deconvolution problem is solved with a fast gradient projection method called Fast Iterative Shrikage-Thresholding Algorithm (FISTA), and compared with a Fourier-based non-negative least squares algorithm. The results indicate that FISTA tends to provide an improved spatial resolution and is up to 30% faster and more robust to noise. In the spirit of reproducible research, the source code is available online.

Aeroacoustic Source Distribution in an Inline Tube Array With a Pitch Ratio of 3

The flow induced acoustics in an inline tube bank (P/d = 3) subject to cross flow, indicative of a generic heat exchanger geometry, are examined over a range of flow velocities using particle image velocimetry (PIV) coupled with acoustic modal analysis using finite element analysis (FEA). The objective is twofold: to determine if the method originally developed for tandem cylinders is applicable to more geometrically complex configurations, with more restricted optical access; and hence to investigate the spatial distribution of acoustic sources within the tube array. The spatial and temporal aeroacoustic source distribution has been successfully obtained experimentally for the case of Strouhal acoustic coincidence (i.e., fa = fv). It is found that the acoustic sources are most intense behind the first row due to the spatial compactness of the vortices. However, a strong negative source (i.e., a sink) is also present in this location, so that the net contribution of the first row wake is small. In subsequent rows, the sources are weaker and more dispersed, but the sink is reduced dramatically. The result is that after the first row the remaining rows of the array contributes energy to the acoustic field. It is noted that, for the coincidence case in the tube bundle studied here, the spatial distribution of sources in the region around the first and second row is similar to the precoincidence regime found for tandem cylinders. This apparent contradiction requires further investigation. Nonetheless, it is concluded that the method of combining PIV with FEA to determine the source distribution can be applied to more complex geometries than previously reported.