Acoustic Holography Technique Research Articles

An equivalent source method for holographic visualization of acoustic sources in an apertured enclosure based on exterior measurements

Most acoustical holography techniques are developed to visualize sound sources or reconstruct sound field in a free-space environment, where no boundary reflected or scattered sound are present. However, in some engineering applications, a sound scatter is an inseparable part of source mechanism. For example, for a centrifugal fan, the aerodynamic noise sources are the turbulent pressure inside the fan shell, but the source distribution is significantly altered if the fan shell is removed. Thus, in order to visualize the sources inside the fan shell by holography methods based on exterior sound measurements, the measurements need to be performed with the presence of the shell (which is a sound scatter). This obviously violates the free-space assumption for most acoustical holography methods. In this work, traditional free-space equivalent source methods are extended to visualize acoustic sources situated in an enclosure with openings based on sound measurements exterior to the enclosure. The key step is to replace the free-space Green’s function used in traditional equivalent source methods by a Green’s function that satisfies the boundary condition of the apertured enclosure. Simulations are performed to prove the concept of the proposed acoustical holography method.

Frequency domain method for holographic localization of rotating sound sources

Most of the previous studies employed beamforming techniques for the localization of rotating sources. These methods usually rely on the use of a virtual rotating array (VRA), in which the Doppler effects caused by source rotation can be eliminated. The VRA signal generation can be achieved in both frequency and time domains. In this presentation, compressive sensing (CS) near-field acoustic holography (NAH) has been extended to the reconstruction of rotating sources using the frequency domain (FD) VRA method. This combination can provide an improved resolution of spatial reconstructions at low frequencies compared to beamforming methods. In addition to the direct combination of the FD-VRA method and CS acoustical holography techniques, a more efficient algorithm for FD-VRA was developed to address the shortcomings of the traditional approach, such as higher computational cost and the requirement of a circular array with equally spaced microphones. The proposed FD-VRA method only involves a direct summation of the rotating sidebands of signals measured on a stationary array, which effectively reduces the computational load. Furthermore, an arbitrary array of microphones can be used. It will be shown that the proposed algorithm works for narrowband sources without overlapping frequency components in the rotating sidebands.

Efficient frequency domain method for holographic localization of rotating sound sources with arbitrary array configurations

Owing to the widespread use of rotating machinery, the localization of rotating sources and the prediction of associated sound fields were extensively studied in the past decades. Most of the previous studies employed beamforming techniques for the localization of rotating sources. These methods usually rely on the use of a virtual rotating array (VRA), in which the Doppler effects caused by source rotation can be eliminated. In this study, compressive sensing (CS) near-field acoustic holography (NAH), which has been extended to the reconstruction of rotating sources using the frequency domain (FD) VRA method, can provide reconstruction capabilities that beamforming techniques cannot offer, such as predictions of the source amplitude and phase information. Furthermore, an improved resolution of spatial reconstructions at low frequencies was achieved. In addition to the direct combination of the FD-VRA method and recently developed stationary source CS acoustical holography techniques, a more efficient algorithm for FD-VRA was developed to address the shortcomings of the traditional approach, such as higher computational cost and the requirement of a circular array with equally spaced microphones. The proposed FD-VRA method only involves a direct summation of the rotating sidebands of signals measured on a stationary array, which effectively reduces the computational load. More importantly, an arbitrary array of optimized microphones can be used. The proposed algorithm works accurately for narrowband sources without overlapping frequency components in the rotating sidebands.

Sequential structured volumetric ultrasound holography for self-positioning using monaural recording.

In this article, a structured acoustic holography technique in the self-positioning method of a single microphone from the monaurally recorded signals is proposed. A series of three-dimensional ultrasonic holograms, designed for positioning in a workspace, are sequentially projected. As a result, the microphone receives a position-dependent sequence of amplitude signals encoded with information on the observation position. Subsequently, the microphone position is determined by obtaining the peak position of the cross-correlation function between the received signal and the reference signal. Experiments were conducted using a custom-made phased array of 40-kHz ultrasound transducers to evaluate the positioning accuracy. It is demonstrated that when applied to a 100×100×50 mm3 workspace, the measurement error was less than 1 mm at all observation points in the numerical experiment, which was maintained for more than 96% of the points in the real-environment experiments. The proposed method is advantageous in that it does not use the phase information of the recorded signals, thus requiring no multiple synchronized recordings as the microphone-array-based methods. In addition, this scheme does not directly use the absolute value of the received amplitude as a positioning clue, which means that no amplitude-to-voltage calibration is required.

Boundary element methods based nearfield acoustic holography for fluids with constant flow

In this work, we present integral formulations to recover an arbitrarily shaped structure's vibrations from nearfield pressure measurements when the medium contains constant flow. This problem is central to the application of the Nearfield Acoustic Holography (NAH) technique. The classical application of the NAH for arbitrarily shaped geometries utilizes an integral formulation that not includes flow information. In many instances, we can argue that the classical application of NAH will be appropriate for a small subsonic flow instead of the more complicated integral formulations that contain the flow information. The main question that we should ask is how large the flow should be to impact the classical NAH reconstruction. Similarly, we want to quantify the advantage of using the integral formulations with flow information for these conditions. We intend to address these questions using numerically simulated data from point sources. [This work has been supported by the Office of Naval Research.]

Krylov Subspace iterative methods for time domain boundary element method based nearfield acoustical holography

In this paper we study the reconstruction of the acoustic field on an arbitrarily shaped vibrating surface using the Time domain Boundary Element Method (TBEM) based Near-field Acoustic Holography technique, or Inverse TBEM (ITBEM). The main focus of our study is the indirect formulation known as the single layer representation and the impact of the dicretization using higher order polynomial basis for both space and time. The solution of the resultant matrix system from this formulation contains a special sparse structure that can be efficiently implemented using Krylov subspace iterative methods like Least Squares QR (LSQR). We show, using numerical data, that the semi-convergent behavior of the LSQR iterations allows the effective mitigation of the ill-posed nature of the numerical solution to this matrix system. Similarly, we provide a numerical study of the ITBEM reconstruction when the signals contain problematic frequencies information. The numerical data is generated over a spherical geometry using Gaussian pulses.

Acoustofluidic Holography for Micro- to Nanoscale Particle Manipulation.

Acoustic-based techniques can manipulate particles in a label-free, contact-free, and biocompatible manner. However, most previous work in acoustic manipulation has been constrained by axisymmetric patterns of pressure nodes and antinodes. Acoustic holography is an emerging technique that offers the potential to generate arbitrary pressure distributions which can be applied to particle manipulation with higher degrees of freedom. However, since current acoustic holography techniques rely on acoustic radiation forces, which decrease dramatically when the target particle size decreases, they have difficulty manipulating particles in the micro/nanoscale. Here, we introduce a holography technique that leverages both an arbitrary acoustic field and controllable fluid motion to offer an effective approach for manipulating micro/nano particles. Our approach, termed acoustofluidic holography (AFH), can manipulate a variety of materials, including cells, polymers, and metals, across sizes ranging from hundreds of micrometers to tens of nanometers.

Partial field decomposition of the simulated noise from a highly heated laboratory-scale jet

The decomposition of jet noise fields into self-coherent and mutually incoherent parts is an important step preliminary to the application of acoustical holography techniques. Cross-spectral matrices of point arrays sampled from large eddy simulations of a laboratory-scale jet operating at a high temperature ratio are decomposed using a singular value decomposition to obtain partial fields. The shape and relative strengths of the individual partial fields highlight the difference between the coherent radiation in the aft direction and the incoherent portion of the radiation towards the sideline. Azimuthally, levels are mostly constant but require multiple partial fields to accurately represent the low coherence at high frequency.

Sound field separation method with single holographic surface based on particle velocity measurement

All the current sound field separation methods based on particle velocity are apply measurement surfaces or a single measurement surface with pressure-velocity. In order to acquire less measurement data and efficient calculation, a sound field separation method with single holographic surface based on particle velocity is proposed. According to the principle of equivalent sources near-field acoustical holography technique, this method can separate directly the sound filed information radiated by target source from coherent sound fields. Numerical simulation analyzed the results with different frequencies and signal-to-noise ratio (SNR). The results show that this method can separate the coherent sources accurately and efficiently.

Localization of Sound Sources during Full Scale Fatigue Test of the Vertical Stabilizer with the Acoustic Holography Technique

Abstract An acoustic holography and its practical applications in engineering began to develop at the end of the 20th century. Currently, this technique is being commonly used to locate sound sources. This paper presents the use of an acoustic camera to locate sound sources during the Full Scale Fatigue Test of the MiG-29 stabilizer. During fatigue tests, the tested structure issues a series of sounds in the form of glitches, creaks or beats. These sounds are typical for a structure subjected to dynamic loading, but they can also be a source of diagnostic information about places of fatigue failures. The paper presents the results of measurements made during the fatigue test. Thanks to the analysis of the measurement results, it was possible to identify areas that are the basic source of sounds.

The nearfield acoustic holography using a concentric rigid microphone array to identify cylindrical sources disturbed by environment noise

Many nearfield acoustic holography (NAH) techniques were proposed to identify acoustical noise sources, which were usually available in the free sound field. To extend NAH to more applications in noisy environments such as factories, an NAH based on a rigid rectangle microphone array was proposed to reconstruct normal velocities of vibrating plates in the environment with disturbing noise sources recently. In this presentation, an NAH using rigid microphone array with the profiles matching the vibrating surface is proposed to identify the cylindrical surface sources. The microphone array is flush-mounted at the bottom plane, and two concentric side surfaces and two rectangular side surfaces are designed to satisfy the Neumann's boundary condition. The transfer matrix between the normal velocities of sources and measured pressure is expressed as the summation of acoustic modes of the volume within the rigid array. The disturbing waves from environments can be decomposed by the inverse patch transfer functi...

Military jet noise source imaging using multisource statistically optimized near-field acoustical holography

The identification of acoustic sources is critical to targeted noise reduction efforts for jets on high-performance tactical aircraft. This paper describes the imaging of acoustic sources from a tactical jet using near-field acoustical holography techniques. The measurement consists of a series of scans over the hologram with a dense microphone array. Partial field decomposition methods are performed to generate coherent holograms. Numerical extrapolation of data beyond the measurement aperture mitigates artifacts near the aperture edges. A multisource equivalent wave model is used that includes the effects of the ground reflection on the measurement. Multisource statistically optimized near-field acoustical holography (M-SONAH) is used to reconstruct apparent source distributions between 20 and 1250 Hz at four engine powers. It is shown that M-SONAH produces accurate field reconstructions for both inward and outward propagation in the region spanned by the physical hologram measurement. Reconstructions across the set of engine powers and frequencies suggests that directivity depends mainly on estimated source location; sources farther downstream radiate at a higher angle relative to the inlet axis. At some frequencies and engine powers, reconstructed fields exhibit multiple radiation lobes originating from overlapped source regions, which is a phenomenon relatively recently reported for full-scale jets.

Design of Active Noise Control Systems Using Ultrasonic Transducers and Acoustic Holography Techniques

Design of Active Noise Control Systems Using Ultrasonic Transducers and Acoustic Holography Techniques

Building leakage detection and quantification using statistically optimized nearfield acoustic holography technique

This paper presents a building infiltration detection and quantification technique using statistically optimized nearfield acoustic holography (SONAH) technique. A model building with known cracks on its wall was investigated in this study. The model building housed a synthetic acoustic source. A hologram measurement was performed outside the model building and the sound pressure levels on the walls were reconstructed. The correlation between the reconstructed pressure levels and the area of the crack was obtained. It was found that the acoustic technique can successfully detect and quantify the leakages from the building model. Two different frequencies of reconstructions are compared and it was found that the lower frequency reconstructions were more accurate. Effects of various regularization methods, phase matching, and quantization error are discussed. Various methods to suppress the wrap-around error in the reconstructions will be addressed.

Detecting building leakages using nearfield acoustic holography technique: A numerical simulation

A crack on a building wall and sound generated inside the building using an artificial sound source were simulated using Matlab. Various noise types affecting the pure sound signal, such as background noise in the room, were simulated. The forward problem of sound propagation from the crack surface to a virtual microphone array was simulated using the free-space green’s function. Nearfield acoustic holography (NAH) was applied to perform the inverse problem to calculate the sound pressure at the crack surface and determine the crack location on the building wall. Effects of various errors and noise on the sound pressure reconstruction using NAH were studied. The reconstructed sound pressure levels on the wall surface were compared with the originally simulated sound pressure data at the crack surface. Various crack sizes and shapes were simulated to determine the correlation between the size of the crack and the reconstructed pressure field.

Vibration and noise source identification methods for a diesel engine

In this study, we used a four-cylinder inline diesel engine as an example to explain how to use test methods and multi-body dynamic techniques to identify the exact source of noise and vibration. A 1 meter noise test was conducted to indicate that the engine front end is the main source of noise. The near-field sound intensity method and the near-field acoustic holography technique were used to identify the chain drive system as the source of noise and vibration. A 3D dynamic analysis model of the timing chain drive system was established using RecurDyn software. Results of the analysis show that the hydraulic tensioner is incompatible with the timing drive system of the bush roller chain. Hence, the timing chain system causes a loud noise and strong vibration. Some strategies on how to optimize the timing chain drive system are presented at the end of this paper.

Identification and localization of the sources of cyclostationary sound fields

A cyclostationary sound field is approximately regarded as a stationary sound field when analyzed by the traditional planar near-field acoustic holography (PNAH), which ignores the periodical time-variant property and makes it hard to reflect accurately the radiation characteristics based on the hologram results. In this article, a cyclostationary planar near-field acoustic holography (CPNAH) technique is proposed in which the cyclic spectral density (CSD) instead of the complex sound pressure is adopted as reconstructing physical quantity and the physical properties of CSD are utilized. And to avoid plenty of calculation and improve accuracy of extracting the cyclic property, pre-processing of holographic data, the method known as the gathering slices of CSD is also proposed. The experiment results demonstrate that the cyclic properties of cyclostationary sound fields can be extracted and the sources of cyclostationary sound fields can be exactly identified and localized by means of CPNAH.

Sound Source Identification and Acoustic Contribution Analysis Using Nearfield Acoustic Holography

The main goal of the present paper is to provide a method of source identification. Firstly, statistically optimal near-field acoustical holography (SONAH) techniques are applied to locate sound sources with the reflected sound field. In the presence of reflection plane parallel and perpendicular to the source plane, the incoming wave and reflected waves are separated based on the acoustic superposition principle and acoustic mirror image principle to satisfy the condition of the sound sources reconstruction using SONAH. Secondly, contribution of noise source to the special field point is analyzed and noise source ranking of interior panel groups are evaluated based the proposed three step acoustic contribution method. Finally, this method is verified experimentally.

Reference-less acoustic holography techniques based on sound intensity

Abstract This paper describes a detailed implementation and application of the intensity-based holography method called BAHIM, and it investigates its possibilities and its limitations. Although this method is attractive in the sense that it bypasses the reference signal requirement, i.e., one of the main difficulties of the classical near-field acoustic holography, its degree of accuracy does not appear to be adequate. This constraint has consequently led to the development of another method called CIBNAH, based on complex intensity, as an alternative. This novel approach provides additional accuracy in detecting and localizing sources and good compatibility, in form and amplitude, with NAH results.

Distribution and propagation of sound sources in the vocal tract

In the current study, an array of 128 microphones is strategically positioned outside a vocal tract model that is placed above an excised canine larynx. Acoustic holography technique is then used to identify the distribution of sound in the vocal tract by using simultaneous measurements from the microphone array. The results show that with no downstream constriction (i.e., open mouth) the energy coming from the higher harmonics is concentrated near the vibrating folds. When downstream constriction is added (i.e., partially closed mouth), the energy in the higher harmonics is shifted further downstream toward the mouth. These results suggest that acoustic holography can be used as a new tool to assess how sound sources propagate in the vocal tract during normal speech and consequently determine the severity of certain speech disorders such as hypernasality and nasal emission.