Near-field Acoustic Holography Research Articles

A Fast Iterative Shrinkage/Thresholding Algorithm via Laplace Norm for Sound Source Identification

As a powerful tool, near-field acoustical holography (NAH) recognizes the sound source effectively. The traditional equivalent source method (ESM) calculated by the Tikhonov regularization method could be available in the low-frequency band. To improve the resolution quality of traditional ESM in the middle-frequency band and the high-frequency band, we introduced a fast iterative shrinkage/thresholding algorithm via Laplace norm for sound source identification based on the equivalent source method (LFISTA). In this paper, four methods including the Tikhonov regularization method, the monotonic two-step iterative shrinkage/thresholding (MTwIST), the fast iterative shrinkage/thresholding algorithm (FISTA), and the proposed method were compared for evaluation of performance. Both of the simulated and experimental results indicated that the proposed method identified the targeted sound sources in the entire frequency range more precisely than the Tikhonov regularization method did; the proposed method fixed the problem that MTwIST had the unqualified resolution and unstableness in the low-frequency band.

Source localization in reverberation environment based on improved equivalent sound source near-field acoustic holography algorithm

The compressive-equivalent source method near-field acoustic holography (C-ESM) is disturbed by reverberation in the enclosed space such as room and cabin, which leads to large reconstruction error and disturbs the judgment of sound source position. In order to solve this problem, an improved C-ESM algorithm based on room impulse response (RIR) is proposed to filter out reverberation interference in this paper. Different from the original equivalent source method, the improved algorithm constructs the transfer function through the room impulse response to establish the relationship between the equivalent source and the sound pressure on any plane in space, and the sparse signal reconstruction method of the compressive sensing technology is used to obtain the strength of the equivalent source. Then the transfer function to any plane of space is established according to the free field Green’s function, to eliminate the interference of reverberation and improve the effect of sound source location. The accuracy and effectiveness of the improved method are verified by preliminary numerical simulation. And the results show that compared with the original algorithm, this method has obvious advantages in sound source localization in a reverberant field.

Near-field acoustic holography for high-frequency weak sound sources under low signal-to-noise ratio

The mechanical noise in the cabin of the ship is so large that the leakage of high-pressure fluid is not easily noticed. In view of this situation, a near-field acoustic holography for high-frequency weak sound source under low signal-to-noise ratio is proposed. The method uses the empirical mode decomposition method to add weights to the time-domain sampling signals of each array element, and then uses the plane equivalent source near-field acoustic holography combined with compressive sensing to find the holographic surface acoustic pressure distribution. The simulation and experiment show that this method has certain feasibility under low signal-to-noise ratio, and the results are better than the method based on Fourier transform and the traditional boundary element method. It is of positive significance to apply it to engineering practice.

A generalized inverse cascade method to identify and optimize vehicle interior noise sources

The noise, vibration and harshness (NVH) emitted by a vehicle are very important to a customer's perception of the vehicle quality. A vehicle's NVH can be improved by considering the three following facets: the noise source, transfer path, and receiver. The identification and optimization of vehicle interior noise sources is crucial when attempting to reduce noise levels and improve sound quality. Although traditional methods, such as those utilizing sound pressure levels, nearfield acoustic holography, and transfer path analysis, can provide the magnitudes and contributions of noise sources, they cannot present specific methods for optimizing those noise sources. This study proposes a new method, the generalized inverse cascade method (GICM), to solve this problem. The GICM combines systems engineering with the interval optimization technique to identify and optimize vehicle noise sources. Applying the GICM to a decision problem involves the following three steps: (1) constructing the decision problem as a cascade tree; (2) developing a numerical model to quantify the cascade tree; and (3) solving the numerical model using the interval optimization method. A Volkswagen sedan is used in this study as an example, and a vehicular road test and subjective evaluation are implemented to record and evaluate the interior noise. The GICM, identifies potential abnormal interior noise sources, and a modified method is presented to optimize the abnormal noise sources by calculating the feasible intervals of design variables. A verification experiment shows that the vehicle interior noise is successfully optimized, thereby validating the proposed GICM.

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.

Time-space domain nearfield acoustical holography for visualizing normal velocity of sources

In this paper, the time-space domain Green’s function relating the sound pressure to the normal velocity of particles is used to adapt time-space domain nearfield acoustical holography (TSD-NAH) to recover the transient normal velocity of a planar source from acoustic measurements. This method provides a simple and effective visualization of the time-evolving vibration field of plate-like structures. Numerical results on a baffled membrane model show that this method avoids some of the signal processing errors introduced by other state-of-the-art methods. A case study on a baffled plate shows that the velocity reconstruction remains satisfactory (with less than 20% error) for back-propagation distances up to 10 cm and typical signal-to-noise ratios (⩾20 dB). Finally, TSD-NAH is applied on an experimental set of data obtained from a plexiglass plate impacted with a steel rod; the computed velocity field is visualized in time and space domains. It is then showed that the computed velocity field can be used to recover unknown properties of the source. In particular, the Young’s modulus of the tested plate has been estimated with a precision better than 5%.

Experimental characterization of basic violin structural acoustics

Considering its simple construction the complex material-shape traditional violin, fashioned from nominally orthotropic materials to form a ported, compliant-wall cavity, enjoys a notable scientific complexity, e.g., four separate radiation mechanisms, coupling between the two lowest cavity modes A0 (Helmholtz resonance, ∼275 Hz) and A1 (1st longitudinal “slosh” mode, ∼470 Hz), and between A0 and two 1st corpus bending (B1) modes near 500 Hz. Comprehensive EMA (velocity/force), radiativity (pressure/force) and near-field acoustical holography ( f -hole volume flows/force) measurements plus bridge tuning systematics were combined to establish a dual-region structural acoustics model of the violin. In the “deterministic” region the large-volume-change B1 modes drive large, in-phase f-hole volume flows accounting for >50% of B1 radiation and excite A0; oppositely directed upper-bout/lower-bout cavity volume flows excite A1. A wall-driven, dual-Helmholtz resonator network successfully modeled A0 and B1 radiation. In the “statistical” region where the major bridge and plate tuning effects occur, radiation efficiency (averaged-over-sphere radiativity divided by averaged-over-corpus mobility) led to radiation damping and critical frequency. With total damping from mobility fits the radiation/total damping ratio (fraction of vibrational energy radiated) was combined with a parametric model of mobility. Splicing the regions created the “dynamic filter” model of violin radiation.Considering its simple construction the complex material-shape traditional violin, fashioned from nominally orthotropic materials to form a ported, compliant-wall cavity, enjoys a notable scientific complexity, e.g., four separate radiation mechanisms, coupling between the two lowest cavity modes A0 (Helmholtz resonance, ∼275 Hz) and A1 (1st longitudinal “slosh” mode, ∼470 Hz), and between A0 and two 1st corpus bending (B1) modes near 500 Hz. Comprehensive EMA (velocity/force), radiativity (pressure/force) and near-field acoustical holography ( f -hole volume flows/force) measurements plus bridge tuning systematics were combined to establish a dual-region structural acoustics model of the violin. In the “deterministic” region the large-volume-change B1 modes drive large, in-phase f-hole volume flows accounting for >50% of B1 radiation and excite A0; oppositely directed upper-bout/lower-bout cavity volume flows excite A1. A wall-driven, dual-Helmholtz resonator network successfully modeled A0 and B1 radiat...

Comprehensive experimental characterization of violin vibration and radiation dynamics

Measuring only violin radiation cannot remove ambiguities in resonance peak identification arising from various minor substructure couplings, or whether the radiation arose from the f-holes or surfaces. Measuring only vibrations gives no clue as to how well each mode radiates or even how the vibrational energy was dissipated. To put these into a structural acoustics milieu, comprehensive measurements of all the violin’s energy loss paths were essential. To accomplish this a wide range of calibrated transducers (accelerometers, multiple microphones, near- and far-field microphone arrays, scanning laser vibrometers) were used in a wide range of experiments: cavity mode analysis using interior microphones; interior gas-exchange; EMA on a violin with and without a soundpost; waterfill experiments on a rigid violin-shaped cavity; automated zero-mass-loading, surface-normal (and 3-D) scanning laser EMA on violins supported in a near-zero-damping support fixture; automated far-field radiation measurements over a sphere in an anechoic chamber; bridge tuning effects; near-field acoustical holography over the f -holes. Each technique had limitations, some severe. Some experiments were “table top”; some required expensive associated facilities like an anechoic chamber. Costs of transducers and power-amplifier-control electronics, computer-based acquisition hardware/software varied dramatically; most operational problems occurred in the computer-based data acquisition/analysis systems.

Optimization of linear and planar array geometries for unmanned underwater vehicle acoustic imaging techniques

Unmanned underwater vehicles (UUVs) are becoming more prevalent in industrial and scientific applications. A wide range of designs is available and many platforms are highly configurable, resulting in an ever-changing acoustic profile. Changes in the UUV’s noise may affect onboard sensors and potentially disturb the environment. An acoustic source imaging process is under development to provide a detailed analysis of the UUV noise in a rapid-turnaround environment. Measurements from a hydrophone array in the geometric near field of a UUV may be input to an acoustic inverse method [e.g., generalized inverse beamforming (GINV) or statistically optimized near-field acoustical holography (SONAH)] to spatially separate noise components and ascertain their levels. However, source resolution and accuracy are highly dependent on array design. Results of a recent numerical case study are presented towards the optimization of linear and planar array geometries. The array geometry is iterated and tuned based on the resultant beamwidth, sidelobe levels, and source localization accuracy for a range of octave frequency bands and source locations. The optimization results provide insight into the critical design factors for acoustical imaging techniques, as well as a suitable array geometry for improved UUV acoustic imaging.

Interior near-field acoustic holography based on finite elements.

This paper deals with the mathematical framework of near-field acoustic holography based on finite elements in application to the acoustic response of a fluid within a closed cavity to the enclosure boundary conditions. The finite element method is an effective implementation of the modal approach for arbitrary geometries and provides advantages for certain wavenumber intervals in rooms. An inverse implementation of the direct problem can benefit from using generalized coordinates with modally reduced system matrices. A solution can be obtained via singular value decomposition together with Tikhonov regularization. This paper investigates acoustic mode spectrums of acoustic transfer functions, which has a major effect on the reconstruction of particle velocities from given sound pressures in a simple cavity model. It is found that the largest considered modal wavenumber in the acoustic transfer matrix should be twice the maximum excitation wavenumber. Furthermore, the relation between reconstruction errors and the detectability of evanescent waves depending on the wavenumber of excitation is considered. The proposed method is validated experimentally by reconstructing particle velocities on the inner boundaries of an Airbus A400M fuselage based on measurements of the inner pressure field. Results are compared with structural velocities measured with a laser Doppler vibrometer.

Spatially sparse sound source localization in an under-determined system by using a hybrid compressive sensing method.

Near-field Acoustical Holography is a powerful tool for sound source identification and sound field reconstruction. Generally, many microphone measurements are required to construct a source model that can span the whole sound source region while simultaneously avoiding measurement errors and ensuring a high spatial sampling rate. That type of measurement is economically costly and hard to perform in industrial environments. Motivated by the desire to be able to use a relatively small number of microphone measurements to reconstruct a sound field and to accurately identify sound source locations, an Equivalent Source Method (ESM) is considered here. In particular, the focus is on the combination of a monopole-distribution ESM and a relatively small number of microphone measurements, thus creating an under-determined system. Wideband Acoustical Holography and l1-norm Convex Optimization are introduced to solve this under-determined inverse problem. Based on the attributes of these two methods, a hybrid method combining the best features of each is proposed to identify sound source locations.

Laser-Assisted Reconstruction of Vibro-Acoustic Behaviors of an Arbitrarily Shaped Vibrating Structure

This paper presents a new method known as laser-assisted, modified Helmholtz Equation Least Squares method-based reconstruction to determine vibro-acoustic quantities of an arbitrarily shaped vibrating structure subject to a noncontact acoustic excitation. Unlike the traditional near-field acoustical holography (NAH) that relies on the acoustic pressures measured in the near-field, this new methodology enables one to collect the input data at a remote distance by using a laser vibrometer, making measurement setup much easier than all previous NAH approaches. Most importantly, by measuring the normal components of the surface velocities at the accessible areas, one can acquire near-field information of a vibrating structure directly, rather than indirectly through reconstruction based on the acoustic pressures measured in the near field. To ensure the accuracy in reconstruction of all the vibro-acoustic characteristics of a structure, the acoustic pressures at a few points in space are also measured. By combining these partial input data, one can acquire a complete picture of the vibro-acoustic field of an arbitrarily shaped vibrating structure, which includes operation deflection shapes, distributions of the acoustic pressures and the normal component of the acoustic intensities on a source surface, and the time-averaged acoustic power of the structure. When the input power is specified, one may determine the frequency response functions, the natural modes, the structural damping ratios of the structure, sound transmission loss, sound transmission path, etc. Examples of using this technology to acquire an in-depth understanding of the vibro-acoustic characteristics of an automobile front dash panel are presented.

On the stability of transient nearfield acoustic holography based on the time domain equivalent source method.

Transient nearfield acoustic holography based on the time domain equivalent source method suffers from the instability that is caused by the use of the time marching scheme. In this paper, the time marching scheme is reformulated to a large iterative scheme. By conducting the eigenanalysis of this large iterative scheme, a necessary condition for stability, i.e., the maximum magnitude of eigenvalues should not be larger than one, can be obtained. Moreover, the causes of instability are analyzed according to the eigenvalues distribution. By virtue of the eigenanalysis, the mechanisms and drawbacks of three previous stabilization methods based on the Tikhonov regularization, the truncated singular value decomposition (TSVD), and the multistep approach are analyzed. To overcome their drawbacks, the classical golden section method is applied to search the regularization parameters and filter parameters based on the necessary condition for stability. Furthermore, the time averaging technique is introduced into the stabilization methods based on the Tikhonov regularization and the TSVD to eliminate the high-frequency oscillation and release the difficulty of searching the filter parameter, respectively. Numerical simulation results indicate that all the improved methods can realize the stabilization of solutions.

Data extension near-field acoustic holography based on improved regularization method for resolution enhancement

In recent years, patch near-field acoustic holography (NAH) has gradually emerged for the reconstruction of sound field radiated from large-scale sound source under the condition of small holographic aperture in engineering practice. However, this technique has not solved the problem that when the projection of large-scale sound source is located on the edge of the expanded hologram, high-precision reconstruction results are unavailable. In addition, the reconstruction of sound field is a typical inverse problem, so the existence of measurement errors makes the solution of the inverse problem be ill-posed and the accuracy of sound field reconstruction is greatly reduced. To overcome these difficulties, a new technique combining an improved method of orthogonal spherical wave superposition, data extension and modified regularization method is proposed to enhance the spatial resolution of reconstructed images. First, the sound pressure is reconstructed on an extended hologram using the improved method orthogonal spherical wave superposition and data extension. At the same time, the method modified Tikhonov regularization (MTR) is introduced to solve the edge error and other ill-posed reconstruction problems. Through iteration the accuracy of the regenerated data is improved. Finally, the extended data are used to reconstruct the sound field. The results of numerical simulation and experiments show that this technique can effectively improve the spatial resolution of the reconstructed image and the reconstruction is more robust.

Robust reconstruction of equivalent source method based near-field acoustic holography using an alternative regularization parameter determination approach.

The core of equivalent source method based near-field acoustic holography is to solve the equivalent source strength with the measured pressure and the transfer matrix, which is a typical ill-posed inverse problem. In this work, an explicit formula is proposed in which the regularization parameter is proportional to the product of the hologram distance and the maximum singular value of the transfer matrix. Utilizing the proposed approach, robust acoustic field reconstruction can be achieved even when the hologram distance is 5 times larger than the average microphone spacing.

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.

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.]

Bearing Fault Diagnosis Based on Spatial Features of 2.5 Dimensional Sound Field

The traditional acoustic‐based diagnosis (ABD) technique based on single‐channel testing has a significant engineering value. Since its diagnosis robustness is sensitive to sound signal acquisition location, it develops slowly. To solve this problem, the 2‐dimensional (2D) sound field variation near the machine is adopted for diagnosis by the near‐field acoustic holography (NAH)‐ based fault diagnosis method with array measurement. However, its performance is limited due to the neglect of the sound field normal change information. To dig the sound field fault information further, a 2.5‐dimensional (2.5D) acoustic field diagnosis method is presented in this paper and its performance compared with the 2D technology is verified by the bearing diagnostic test. Different from the 2D technique with only one source image, the 2.5D acoustic field model consists of source image, holographic sound image, and the differences between them, and its effective feature model is constructed by Gabor wavelet feature extraction and random forest feature reduction algorithm. The diagnostic effect of the 2.5D technique compared with the 2D technique increases more than 11% in the bearing diagnostic test. It provides new ideas for the development of the NAH‐based fault diagnosis method, and further improves the ABD technique‐based array measurement.

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