Acoustic Holography Method Research Articles

Research on patch near-field acoustic holography based on two-level iteration

In order to solve the “window effect” and “aperture repetition effect” caused by limited measurement aperture, a patch near-field acoustic holography method based on two-level iteration is proposed. After analyzing the principle and process of patch near-field acoustical holography based on orthogonal spherical wave, a patch near-field acoustical holography method based on two-level iteration is proposed. The validity of the method is verified by simulation and experiment. Compared with the reconstruction results of patch near-field acoustical holography based on orthogonal spherical wave, it shows the superiority of patch near-field acoustical holography based on two-level iteration.

Experimental research on the sound recognition of the electric vehicle motor

By synthetically applying order tracking method and acoustic holography method, the noise source of the electric vehicle motor was discerned in the bench test. According to profiling of the test data, the motor noise order tracking diagrams and sound holography contour maps were employed to draw the noise map of the electric vehicle motor. It was revealed that the low and medium frequency components of the motor’s noise are mainly related to the rotational passing frequency of the bearing rolling element and cage, as well as the doubling frequencies of the rotational frequencies of the rotor. The medium and high frequency components of motor’s noise are mainly related to motor’s loads and speeds. In the 1000-2500 r/min speed stage of the motor with load, the main order of motor’s noise is the 72nd order and in the speed stage of 3000-5000 r/min, motor’s noise is mainly the 15th, 18th and 26th orders. When the motor speed increases from 5000r/min to 8000r/min, the main order of motor’s noise is the fourth order. When the motor was accelerated in no-load operation, its noise characteristics are different from those of the motor with load. When the motor run at no load and low speed, the 16th and 72nd order are the main order of motor noise, while at high speed, the 2nd order of motor noise and the orders of motor switching frequency noise caused by frequency converter are the main order of motor noise. The above conclusions will give the assistance to further research on the acoustic characteristic of the electric vehicle motor.

Nearfield acoustic holography-based methods for far field prediction

The paper is intended to explore the possibility of using near-field acoustic holography methods for determining the far field radiation profile of a sound source. An experiment was carried out in an anechoic chamber for this purpose. First, the acoustic pressures on a plane in the near-field of a source were recorded as a profile. The measured signals were then used to reconstruct the far field profile using the said algorithms, respectively. Second, the acoustic pressures in the far field were directly measured and the results were compared with the predicted ones. Of the four methods used in the study, the one that yields the best result was the so-called “Far plane series” method, yielding 12% error upon comparison with the directly measured values. In addition to that a research on possible reduction of the microphone array size was carried out. A significant reduction of the measurement facility was realized with the use of a linear array instead of the planar array that was made possible using the studied far-field prediction algorithms.

On the viability of the complex-intensity-based near-field acoustical holography method

Because of the instrumentation difficulties of Fourier acoustical holography, it is desirable to find a holography method that does not require reference signals during array scanning. Among the methods that have been investigated to remove the need for references are those based on acoustic intensity measurements, including the complex-intensity-based near-field acoustical holography (CIBNAH) method [A. Nejade, J. Sound Vib. 333(16), 3598–3608 (2014); Appl. Acoust. 116, 348–356 (2017)]. The CIBNAH method has previously been applied to simple contrived sources and real-world machinery but has not been verified using analytical source models. This work shows the application of CIBNAH to an analytical model of a simply-supported plate, revealing key shortcomings of the method. The theory behind CIBNAH is discussed in light of these shortcomings. It is shown that while CIBNAH may be useful for finding radiation hot spots, it is not an adequate method to overcome the need for references in scan-based acoustical holography. [Work supported by NSF.]

Development of double nearfield acoustic holography method (Basic experimental results)

Development of double nearfield acoustic holography method (Basic experimental results)

The Development of Double Nearfield Acoustic Holography method

The Development of Double Nearfield Acoustic Holography method

Near-field acoustic holography with three-dimensional scanning measurements

Data acquisition is one of the key steps in near-field acoustic holography. Most of the existing measurement methods become time-consuming or even impractical when the total number of measurement points is excessively large, for example for the sound field reconstruction of a large-scale source or for a mid-high frequency problem. Alternatively, a three-dimensional scanning near-field acoustic holography (3DSNAH) method is hereby proposed to overcome such problems providing the sound field is time stationary. Data measurement at a large number of points can be accomplished in a matter of minutes by using scanning measurements to acquire the holographic data. The data is then used in an inverse boundary element method to obtain a detailed reconstruction of the sound field radiated by the sound source. Comparison with the traditional point-by-point measurement method demonstrates that the proposed method can acquire holographic data of good quality and reconstruct the sound field with a reasonable accuracy, which verifies the feasibility of the 3DSNAH.

The coherent sound field separation method combining compressive sensing

When multiple coherent sound sources are distributed on the same side of the holographic surface, the conventional method must use known information such as sound source distribution and geometry. In view of this situation, a method of coherent sound field separation combined with compressive sensing is proposed. The method is based on the plane equivalent source near-field acoustic holography method and the orthogonal matching pursuit algorithm of compressive sensing technology. Get the distribution of the virtual sound source on the plane, and then the virtual sound sources are grouped and selected. Finally, the distribution of sound pressure in the near-field plane after separation of the sound field is calculated using the conduction matrix. The method does not require other prior knowledge. The simulation results show that this method can be used as an effective complementary method for coherent sound field separation based on single-sided measurement.

Quantitative measurement of ultrasonic pressure field using combination of optical phase contrast and nonlinear acoustic holography methods

Ultrasound has been widely used in medicine for both diagnosis and therapeutics. To assure its safety and efficacy, it is necessary to establish a fast and accurate method of measuring an ultrasonic field. Hydrophone scanning, which is a standard method for pressure measurement, requires a long measurement time and may disturb the field. In this study, an optical phase contrast method is combined with an acoustic holography method with nonlinear propagation simulation. The nonaxisymmetric pressure field of pulse ultrasound upstream of the focus was measured by the optical phase contrast method combined with a computed tomography (CT) algorithm, and the entire three-dimensional pressure field was reconstructed by acoustic holography. The result obtained by the proposed method agreed well with that by the hydrophone measurement without absolute pressure calibration and confirmed the markedly reduced required measurement time.

The Development of Double Nearfield Acoustic Holography method

The Development of Double Nearfield Acoustic Holography method

Acoustic holography combined with radiation force measurements to characterize broadband ultrasound transducers and calibrate hydrophone sensitivity

Accurate characterization of broadband ultrasound (US) transducers operating either in pulsed or CW modes is necessary for their use in biomedical applications. The acoustic holography method allows reconstruction of the pattern of vibrations at the transducer surface for use as a model boundary condition to simulate ultrasound fields in water or tissue. A calibrated hydrophone is often not available to perform such measurements. Here, it is proposed to combine transient acoustic holography with radiation force balance (RFB) measurements at multiple frequencies to quantitatively determine 3D distributions of the acoustic field at different source frequencies, the corresponding acoustic power of the source, and the hydrophone sensitivity within the frequency bandwidth of the transducer. First, a transient acoustic hologram is measured using a short-pulse excitation of the source by raster scanning the hydrophone along a surface in front of the source and recording the waveform at a large number of points (typically 10-40 thousand). Then, RFB measurements are conducted for different frequencies within the pulse bandwidth to determine axial component of the acoustic radiation force. These data are related to the corresponding values calculated from the holograms at each frequency component to determine the corresponding hydrophone sensitivities. [Work supported by RSF-14-12-00974.]

Use of acoustic holography to quantify and correct geometrical errors in ultrasound field characterization

The development of medical ultrasound devices requires characterization of the ultrasound fields they radiate. Recently, acoustic holography methods have gained increased acceptance for field characterization. Because such an approach allows reconstruction of the full 3D field, the beam axis, its orientation relative to the positioner, and any field quantities of interest can be readily determined. Inherently, the accuracy of such projection methods can be sensitive to uncertainties in the positions at which field measurements are recorded. Although commonly used industrial positioning systems have linear axes with repeatability specifications much less than an ultrasound wavelength, the assembled orthogonality of three linear axes is not guaranteed. Here we analyze a typical raster scanning holography experiment by considering two distinct coordinate systems: ideal rectilinear coordinates aligned with the transducer (1.2 MHz, aperture 124 mm, F-number 1) and non-orthogonal coordinates aligned with the positioner (Velmex Unislides). By locating a distinct field feature in both projection calculations and independent hydrophone measurements, a misalignment of positioner axes of about 0.50 was identified. The impact of positioner non-orthogonality on field characterization metrics is discussed along with the potential use of this approach for a priori positioner calibration. [Work supported by NIH EB007643, NIH EB016118, and RSF-14-15-00665.]

Reconstruction of nonlinear ultrasound field of an annular therapeutic array from acoustic holograms of its individual elements.

Acoustic holography method has been shown to provide accurate reconstruction of 3D ultrasound fields generated by various medical transducers including multi-element arrays as well as to set a boundary condition for nonlinear field modeling at high pressure levels. Here an approach of measuring holograms of individual array elements for modeling of an entire array field is proposed and tested for a 3 MHz 16-element annular array (48 mm diameter and 35 mm radius of curvature). The array is a part of a high intensity focused ultrasound system with magnetic resonance guidance used for developing thermal and mechanical methods of tissue ablation in mouse tumors. The holograms measured separately for each array element were combined together to obtain a boundary condition for the array with all operating elements. Modeling results were compared to low-amplitude beam scans and good agreement was demonstrated. Then, nonlinear field simulations were performed at increasing power outputs based on the 3D Westervelt equation. It was shown that the transducer is capable to produce focal waveforms with 140 MPa shock amplitude at 110 W acoustic power and thus is well suited for evaluating shock-based ablation therapies in small animals.

A sparse equivalent source method for near-field acoustic holography

This study examines a near-field acoustic holography method consisting of a sparse formulation of the equivalent source method, based on the compressive sensing (CS) framework. The method, denoted Compressive-Equivalent Source Method (C-ESM), encourages spatially sparse solutions (based on the superposition of few waves) that are accurate when the acoustic sources are spatially localized. The importance of obtaining a non-redundant representation, i.e., a sensing matrix with low column coherence, and the inherent ill-conditioning of near-field reconstruction problems is addressed. Numerical and experimental results on a classical guitar and on a highly reactive dipole-like source are presented. C-ESM is valid beyond the conventional sampling limits, making wide-band reconstruction possible. Spatially extended sources can also be addressed with C-ESM, although in this case the obtained solution does not recover the spatial extent of the source.

Noise source identification by using near field acoustic holograpy and focused beamforming based on spherical microphone array with random unifrom distribution of elements

With the development of techlology, noise controlling has received wide attention in recent years. Noise source identification is the key step for noise controlling. Spherical microphone array, which can locate the noise source of arbitrary direction in three-dimensional space, has been widely used for noise source identification in recent years. Conventional methods of locating noise source include spherical near field acoustic holography and spherical focused beamforming. The acoustic quantities are reconstructed by using spherical near field acoustic holography method to realize the noise source identification, while the noise source can also be located by using focused beamforming based on spherical harmonic wave decomposition. However, both these methods have their own limitations when they are used in identifying the noise source. Spherical near field acoustic holography has low resolution at high frequency with a far distance from noise source to measurement array for noise source identification, whereas the spherically focused beamforming has low localization resolution at low frequency. Noise source identification is discussed here, and a 64-element microphone spherical array with randomly uniform distribution of elements is designed. The combination methods of noise source identification by using spherical near field acoustic holography and mode decomposition focused beamforming are investigated. The performance of the proposed combination method is simulated, and an experiment on noise source identification is carried out based on the designed spherical microphone array to test the validity of proposed method. Research results show that the high-resolution noise source identification can be achieved by using near field acoustic holography when reconstruction frequency is 100-1000 Hz with a distance 0.3-0.45 m from noise source to the center of spherical array, while high resolution of noise source localization can be achieved by using spherical wave decomposition beamforming when signal frequency is 1000-5000 Hz with a distance 0.5-3 m from noise source to the center of spherical array. Spherical array with random uniform distribution of elements maintains stable identification ability in all bearings. The spherical near field acoustic holography has high-resolution distinguishing ability in near field and at low frequency, while the focused beamforming method has high-resolution distinguishing ability in far field and at high frequency. Therefore the noise source can be efficiently identified by using the proposed combination method of near field holography and focused beamforming with less elements and small aperture spherical microphone array.

Development of Double Nearfield Acoustic Holography Method

Development of Double Nearfield Acoustic Holography Method

Sound source identification on automotive muffler surface using single explosion excitation and near-field acoustic holography method

Recent automobiles tend to have larger interior space, resulting in the smaller space for arranging the muffler. Flat shape mufflers are required for this reason but cause relatively large radiation noise from the surface. In order to improve understanding of radiation noise generation mechanism of flat shape muffler, we tried to identify the noise sources using the near-field acoustic holography (NAH) method on muffler surface excited by a single explosion pulse, which simulates a single pulsation in the exhaust system,. Two types of mufflers were tested with different shapes. Results show that some areas had large vibration velocity at specific frequencies depending on the muffler shape. The flat muffler tends to generate large radiation noise and vibration velocity as compared with a conventional one. In the front and rear outer plates of the muffler, large vibration velocity exists within a certain range of frequency. Within the same frequency range, the flat muffler has more number of the peak of radiation noise as compared with the conventional one.

Complete de-Dopplerization and acoustic holography for external noise of a high-speed train.

Identification and measurement of moving sound sources are the bases for vehicle noise control. Acoustic holography has been applied in successfully identifying the moving sound source since the 1990s. However, due to the high demand for the accuracy of holographic data, currently the maximum velocity achieved by acoustic holography is just above 100 km/h. The objective of this study was to establish a method based on the complete Morse acoustic model to restore the measured signal in high-speed situations, and to propose a far-field acoustic holography method applicable for high-speed moving sound sources. Simulated comparisons of the proposed far-field acoustic holography with complete Morse model, the acoustic holography with simplified Morse model and traditional delay-and-sum beamforming were conducted. Experiments with a high-speed train running at the speed of 278 km/h validated the proposed far-field acoustic holography. This study extended the applications of acoustic holography to high-speed situations and established the basis for quantitative measurements of far-field acoustic holography.

Three‐dimensional application of improved equivalent source method for noise source identification

Near-field acoustical holography methods have bad resolution at medium-to-high frequencies. This is especially a problem for three-dimensional source identification, when the microphone array is not arranged around the source. To overcome this issue, an improved equivalent source method (IESM) with two mutually perpendicular sub-arrays is used. Similar to the equivalent source method, the equivalent source strengths of IESM can be evaluated by the steepest descend method on both sets of microphones. Using two perpendicular planar arrays surrounding the source, the spatial resolution of IESM can be improved in both directions. By introducing the steepest descend and threshold filtering method, IESM based on two mutually perpendicular planar arrays can suppress the ghost sources associated with the real sources. Hence, it can identify the main sources much more accurately at medium-to-high frequencies. The proposed method also is used on both synthetic and experimental data. The results also showed that IESM give source maps that are more interpretable than conventional near-field acoustical holography methods.

Fast wideband acoustical holography.

Patch near-field acoustical holography methods like statistically optimized near-field acoustical holography and equivalent source method are limited to relatively low frequencies, where the average array-element spacing is less than half of the acoustic wavelength, while beamforming provides useful resolution only at medium-to-high frequencies. With adequate array design, both methods can be used with the same array. But for holography to provide good low-frequency resolution, a small measurement distance is needed, whereas beamforming requires a larger distance to limit sidelobe issues. The wideband holography method of the present paper was developed to overcome that practical conflict. Only a single measurement is needed at a relatively short distance and a single result is obtained covering the full frequency range. The method uses the principles of compressed sensing: A sparse sound field representation is assumed with a chosen set of basis functions, a measurement is taken with an irregular array, and the inverse problem is solved with a method that enforces sparsity in the coefficient vector. Instead of using regularization based on the 1-norm of the coefficient vector, an iterative solution procedure is used that promotes sparsity. The iterative method is shown to provide very similar results in most cases and to be computationally much more efficient.