Near-field Acoustic Holography Method Research Articles

Application of Near-Field Acoustic Holography to Identify the Mechanical Noise Sources on the Submarine

In order to localize and identify the mechanical noise sources on the submarine, a kind of double-layer submarine model with ring-stiffened was established, and the broadband actuators were put on the inner structure to simulate the mechanical equipment. The radiated acoustic field of the submarine model was reconstructed by the near-field acoustic holography method, which was also compared with the theoretical acoustic field. The simulation results showed that the reconstruction pressure amplitude agreed well with the theoretical calculation results, indicating that the acoustic field of submarine model could be reconstructed accurately by the method of near-field acoustic holography, and the mechanical noise sources would be localized and identified accordingly.

Non-intrusive planar velocity-based nearfield acoustic holography in moving fluid medium

Nearfield Acoustic Holography (NAH) is a powerful acoustic imaging method, but its application in aeronautics can be limited by intrusive measurements of acoustic field. In this paper, a moving fluid medium NAH procedure using non-intrusive velocity measurements is proposed. This method is based on convective Kirchhoff-Helmholtz integral formula. Convective equations and convective Green's function are used to derive convective real-space propagators including airflow effects. Discrete Fourier transforms of these propagators allow the assessment of acoustic fields from acoustic pressure or normal acoustic velocity measurements. As the aim is to derive an in-flow velocity-based NAH method, this study is especially focused on real convective velocity-to-pressure propagator. In order to validate this procedure, simulations in the case of monopole sources radiating in various uniform subsonic flows have been performed. NAH provides very favorable results when compared to the simulated fields. A comparison of results obtained by the real propagator and those obtained by the wave number-frequency-domain one developed by Kwon et al. [J. Acoust. Soc. Am. 128(4), 1823-1832 (2010)] shows the interest of using the real-form in the case of pressure backward propagation from velocity measurements. The efficiency of the developed procedure is confirmed by a wind tunnel campaign with a flush-mounted loudspeaker and non-intrusive Laser Doppler Velocimetry velocity measurements.

The Noise Source Localization of Industrial Sewing Machine by NAH Method

According to the Near-Field Acoustic Holography (NAH) theory, we designed a measurement system, which was used to localize the noise source on five hologram surfaces of a industrial sewing machine. Moreover, we compared the localization results of the noise source on the front surface of the industrial sewing machine by NAH and sound intensity method respectively. The measurement results can provide a theoretical basis for controlling the noise of industrial sewing machine.

Temperature Distribution Estimated by Optimization and Near-Field Acoustical Holography

We propose a method to estimate temperature distribution non-destructively using combination of near-field acoustical holography (NAH) method and optimization method. NAH method is valid to calculate forward and back propagations of ultrasonic wave in a uniform medium. To estimate of temperature distribution, we proposed a modified method of NAH, called sectional NAH (SNAH), for using NAH in an inhomogeneous medium. In SNAH, a calculation space is discretized into a number of sections, so that the temperature in each section becomes nearly uniform, and NAH calculation is performed in each section. Calculation results using SNAH in the inhomogeneous medium well agreed with the calculation result using finite element method (FEM). By using SNAH, the temperature distribution is estimated as bellows. Firstly, sound fields, which are complex amplitude of harmonic oscillation, in three planes are measured. Next, sound field at one of measured plane is calculated from the other measured sound fields, which are calculated from other measurement planes using SNAH, with an initial sound velocity distribution. Then, difference between calculated and measured sound fields is minimized by optimizing the sound velocity distribution using multi-start downhill simplex method. In this method, a temperature distribution is obtained as the sound velocity distribution. In simulations, temperature distributions given as linear and Gaussian distributions were estimated. Validity of our proposal methods is confirmed by simulations.

Aperture extension for near-field acoustical holography applied to jet noise

Near-field acoustical holography (NAH) techniques are used to investigate noise source characteristics of high-power jets on military aircraft. Sound field reconstruction of large sources, measured with an aperture of limited size, may generally be performed with the use of patch NAH methods. Patch methods, such as statistically optimized near-field acoustical holography (SONAH), help to mitigate the effects of a truncated measurement aperture by avoiding the use of the spatial discrete Fourier transform operation. However, the lack of information outside the measurement aperture may lead to other errors, particularly when large propagation distances are required. Some missing data must be recovered to propagate beyond the immediate measurement region. Numerical aperture extension methods, in conjunction with SONAH, are employed to characterize high-power jet noise sound fields. These methods include complex interpolation and extension, analytic continuation, and in-plane holographic projection. [Work supported by Air Force SBIR.]

NAH기법을 이용한 공조용 컴프레서 소음저감 응용

The paper demonstrates how to deal with the Near-field Acoustic Holography method (NAH) for reducing noise source which is generated in air-conditioning compressor. Sound radiation was measured for both normal compressor and fault compressor. The acoustic noise holograms are predicted by measuring at discrete multiple points around the two compressors and are thereafter reconstructed on the surface of the compressors. 1344 measuring points are used by two microphones in which one is scanning microphone and the other is reference microphone. NAH is a good tool for the visualization of Sound so we can show the source of sound graphically. In this paper, the NAH method found the exact noise source position on the surface of the compressor and the noise-related sub part in the compressor. We found the NAH and is very useful as a noise reduction tool for home-appliance device.

Fan noise visualization and noise source identification by using nearfield acoustical holography.

A nearfield acoustical holography (NAH) method combined with a non-stationary source compensation procedure is applied to identify the noise sources of two open fans. The measured noise data of a low-speed, small fan indicate the highest peak levels at the blade passing frequencies (BPFs) of 102 and 204 Hz. It is observed that there is no noise component induced by vortex shedding at the blade tips, or leading or trailing edges of the small fan. It is also observed that the structural resonance frequencies of the small fan blades are not coincident with the BPFs: thus, the structural vibration responses of the blades contribute insignificantly to the fan noise. The NAH results of the small fan show acoustic pressure fluctuations on the blade surfaces at the BPFs. The measured noise data of a high-speed, large fan show the frequency components associated with BPFs as well as vortices shed at blade edges. Similar to the small fan case, the resonance frequencies of the fan blades cannot be observed in the measured acoustic data. The NAH results of the high-speed, large fan successfully exhibit acoustic pressure fluctuations at BPFs as well as noise patterns radiated from the vortex shedding.

Scan-based near-field acoustical holography on partially correlated fields: Theory and numerical experiments.

Conventional near-field acoustical holography (NAH) requires a coherent field. For a coherent source, a scan-based approach can be used with one reference microphone to stitch the phase. For a noncoherent field, scan-based NAH can be performed if the virtual coherence technique is used. This technique uses multiple reference microphones to decompose the partial field into mutually uncorrelated partial fields, which are then processed by some NAH method. In particular, the statistically optimized near-field acoustical holography (SONAH) method is used with modified Tikhonov regularization. Numerical experiments are performed on a series of point sources with source strengths chosen to mimic the source characteristics of high-powered jets. The experiments are designed to aid in determining the proper number and location of reference microphones for doing NAH work on high-powered jet noise. [Work supported by Blue Ridge Research and Consulting and the Air Force Research Laboratory.]

Basic theory and properties of statistically optimized near-field acoustical holography

To avoid the requirement set by standard near-field acoustical holography (NAH) to measure an area that fully covers the source, a set of so-called patch NAH methods has been introduced. One such method is the statistically optimized NAH (SONAH). In this method, the acoustic quantities on a mapping surface near the measurement surface are calculated by using a transfer matrix defined in such a way that all propagating waves and a weighted set of evanescent waves are projected with optimal average accuracy. The present paper gives an overview of the basic theory of SONAH, including a description of phenomena such as spatial aliasing and wave-number domain leakage. A revised and generalized mathematical formulation is given, covering the calculation of all three components of particle velocity and the use of up to six virtual source planes. A set of formulas for the inherent estimation error level of the method is derived and used to visualize the regions of validity of the SONAH predictions for some typical microphone array geometries. The sensitivity of the inherent error level distribution to changes in the parameters of the SONAH algorithm is also investigated.

Acoustic source property prediction based on near-field measurements in planar coordinate

Near-field acoustical holography (NAH) can be used to predict the acoustical properties of noise sources by starting from measurements of near-field pressures. In particular, the sound pressure, particle velocity, and intensity can all be estimated on a source plane. In the present work, an equation relating acoustical source properties and the pressure radiated from a source is derived from the Kirchoff–Helmholtz integral equation and is here referred to as the inverse Rayleigh method. The inverse Rayleigh method reduces to the Rayleigh integral when the source is located on an infinite rigid baffle. However, the same equation can also be used to predict the acoustic properties of a source in a more general environment: i.e., without assuming the presence of an infinite rigid baffle. In this article, the inverse Rayleigh method is described and then its relative accuracy is compared with other procedures that can be used to predict source particle velocity from near-field pressure measurements on a planar hologram. By comparing the accuracy of the present NAH method with other procedures in the case of a large hologram, it was found that the inverse Rayleigh method is accurate and computationally efficient for predicting the source particle velocity from near-field pressure measurements both with and without a rigid baffle.

Theorical and experimental research on the HELS method in the cyclostationary acoustic field

Helmholtz equation least square method (HELS) is based on the assumption that the acoustic field can be approximated by the superposition of a series of spherical waves. According to the principle of least square error between the reconstruction and measurement, the expansion number of the spherical wave equation and the related coefficients can be determined by the least-square method. The HELS method is applied in the practice due to its efficient computing and less measurements comparing with other acoustical holography algorithms. HELS and other near-field acoustic holography methods are all based on the stationary acoustic field. As a special example of the non-stationary acoustic field, the cyclostationary acoustic field is usually analyzed by dealing with signals in a single channel and its nth order statistics are often utilized to detect mechanical faults. In the present paper, HELS method is modified to analyze the cyclostationary acoustic field with the variance of cyclic spectrum density. The reconstruction in the appropriate cyclic frequency will supply visible acoustic field information, especially that hidden under the cyclostationary condition. A numerical simulation is done to verify the validity of the modified HELS, in which the effects of the ways of measurement are discussed. At last, a spherical sound box driven by cyclostationary signal is set in a semi-anechoic chamber and the acoustic field is reconstructed by modified HELS method.

Research on the reconstruction of the vibro-acoustic field generated by the complicated sources using the modified wave superposition algorithm

The nearfield acoustic holography (NAH) method based on the wave superposition algorithm (WSA) assumes that the sound field is induced by a series of simple sources inside the vibrational object boundary. The sound field can be reconstructed after the strength of virtual sources is determined in an inverse procedure. The theory, numerical simulation, and application of WSA-based NAH is often concerned with the simple sources. In practice, the vibrational objects are more complicated and sometimes should even be treated as multiple separated objects. The reconstruction of a sound field by using conventional WSA-based NAH directly is not satisfactory. In the presented modified WSA-based NAH, complicated vibrational objects are separated into several parts and looked at as independent components. Whole sound fields are generated by all these components. The comparisons between conventional and modified WSA-based NAH are carried out through numerical simulations. It shows that the more accurate result is obtained by applying the modified WSA-based NAH to the complicated objects. An experiment of a compressor with complicated structures is conducted to illustrate the validity of modified WSA-based NAH.

Methods for reconstructing acoustic quantities based on acoustic pressure measurements

This paper presents an overview of the acoustic imaging methods developed over the past three decades that enable one to reconstruct all acoustic quantities based on the acoustic pressure measurements taken around a target source at close distances. One such method that has received the most attention is known as near-field acoustical holography (NAH). The original NAH relies on Fourier transforms that are suitable for a surface containing a level of constant coordinate in a source-free region. Other methods are developed to reconstruct the acoustic quantities in three-dimensional space and on an arbitrary three-dimensional source surface. Note that there is a fine difference between Fourier transform based NAH and other methods that is largely overlooked. The former can offer a wave number spectrum, thus enabling visualization of various structural waves of different wavelengths that travel on the surface of a structure; the latter cannot provide such information, which is critical to acquire an in-depth understanding of the interrelationships between structural vibrations and sound radiation. All these methods are discussed in this paper, their advantages and limitations are compared, and the need for further development to analyze the root causes of noise and vibration problems is discussed.

Numerical study of finite-amplitude source reconstruction in one-dimension.

Since its inception in the 1980s, near-field acoustic holography (NAH) has been widely used to determine structural source properties. More recently, it has also been used to determine aeroacoustic source behavior. Although several alternate forms of NAH have been developed, all are based on the (linear) Helmholtz equation and would presumably not function correctly for a finite-amplitude pressure wave propagating nonlinearly. However, when nonlinear effects or reconstruction distances are small, NAH may still be a useful tool for determining radiation characteristics of finite-amplitude sources such as jets and rockets. A one-dimensional numerical propagation scheme has been used to propagate broadband noise with various spectral shapes and amplitudes. A simple NAH-based magnitude reconstruction is used to determine frequency-, amplitude-, and propagation distance-dependent errors indicating in what regions linear NAH methods may produce large errors. The results indicate that the presence of shock coalescence causes relatively large errors in linear sound field reconstructions of broadband noise sources.

The Application of Wave Superposition Algorithm to Identify Cyclostationary Sound Source

The cyclostationary characteristics can not be extracted clearly in reconstruction of the acoustic field radiated from the cyclostationary sources by the conventional near-field acoustic holography procedures. The cyclostationary near-field acoustic holography method uses the cyclic spectrum density function as the reconstruction variance instead of the spectrum or the power spectrum density, which decomposes the carrier wave and modulating wave in the appropriate cyclic frequency. The reconstructed CSD shows the distribution of the modulating wave of cyclostationary sources and supplies more cyclostationary acoustical information than the conventional NAH. In the present work, the theory of the cyclostationary NAH combined with the wave superposition is introduced, which can be used for the arbitrarily shaped cyclostationary sources. A numerical simulation and a speaker experiment are employed to validate this WSA-based cyclostationay NAH method. The results of the simulation show great agreement between the reconstruction and the analytical value. The distribution of the CSD of the cyclostationary acoustical source on the surface coincides with the speaker box accurately.

A study on regularization parameter choice in Near-field Acoustical Holography

Regularization plays an important role in Near-field Acoustical Holography (NAH), and choosing the right amount of regularization is crucial in order to get a meaningful solution. An automated method such as the L-curve or Generalized Cross-Validation (GCV) is often used in NAH to choose a regularization parameter. These parameter choice methods (PCMs) are attractive, since they require no a priori knowledge about the noise. However, there seems to be no clear understanding of when one PCM is better than the other. This paper presents comparisons of three PCMs: GCV, L-curve and Normalized Cumulative Periodogram (NCP). The latter method is new within NAH and it is based on the Fourier transform of the residual vector. The methods are used in connection with three NAH methods: Statistically Optimized Near-field Acoustical Holography (SONAH), the Inverse Boundary Element Method (IBEM), and the Equivalent Source Method (ESM). All combinations of the PCMs and the NAH methods are investigated using simulated measurements with different types of noise added to the input. Finally, the comparisons are carried out for a practical experiment. This aim of this work is to create a better understanding of which mechanisms that affect the performance of the different PCMs.

Estimation for vibration mode of membrane by NAH method

Near Field Acoustic Holography (NAH) is the method of measuring sound information on the radiant surface in the near field as a hologram. By use the technique, we can visualize the vibration of the radiant surface. Thus, we can easily understand the object of the acoustic conditions in acoustic image processing. We made the measurement on one side of the drum using array microphone while the driven source is attached to the other side. In addition, the measurement is not touched on the vibrating membrane surface. This paper describes the estimated results of the vibration mode on the vibration membrane of a drum. And this is possible by apply it to the vibration controlling field for detecting the sound structure on the membrane.

Applications of Near‐field Acoustic Holography on Small Apertures

Small, finite measurement apertures are a major impediment for accurate and fast application of near‐field acoustic holography (NAH). In theory, FFT‐based NAH methods are very powerful and acquire extremely fast inverse calculations of the sound field. However, in practice the usually small amount of sensors in a measurement array limit the possibilities of classical implementations of NAH, especially near the edges of the array or measurement grid. An alternative method to, for example, spatial windowing, patch NAH, SONAH, etc. is the recently introduced border‐padding technique. This method makes it possible to apply FFT‐based NAH with a large increase of accuracy compared to the original methods, specifically at the measurement grid edges, while maintaining the high speed of the inverse process PNAH is known for. This work focuses on the application of border‐padding and the practical implementation in a newly developed PNAH system. It illustrates a large variety of extended possibilities for PNAH that border‐padding enables. Apart from the earlier introduced standard border‐padding, experimental results are shown for a highly improved border‐padding method. Measurements and calculations on industrial products illustrate the wide applicability of the presented method.

Identification of source velocities with Inverse Patch Transfer Functions method

The identification of source velocities remains an important problem in noise control. For this purpose, several methods were developed such as Near-field Acoustic Holography (NAH) or inverse Boundary Elements Method (iBEM). An alternative method, based on the double measurement of pressure and particle velocity fields surrounding the source is presented. This method has been developed in the SILENCE European project framework. In this method, called inverse Patch Transfer Functions method (iPTF), measurement and identification surfaces are divided into elementary areas called patches. Then, source velocities are computed from acoustic field and inversion of impedances matrices obtained by FEM. Theoretically, this method presents two main advantages: it can be applied to sources with complex 3D geometries and measurements can be carried out in a non-anechoic environment, even in the presence of other sources. In the present paper, theoretical background of iPTF is exposed and results are presented on a source with simple geometry (an L-shaped plate) and discussed.

Reconstruction and Separation in a Semi-Free Field by Using the Distributed Source Boundary Point Method-Based Nearfield Acoustic Holography

In a semi-free field, the acoustic field is composed of two components: the direct sound and the reflected sound. Because it is difficult to separate the direct sound from the acoustic field, conventional nearfield acoustic holography (NAH) methods cannot reconstruct an acoustic source and predict the acoustic field directly. Through utilization of the distributed source boundary point method (DSBPM)-based NAH, a treatment method for a semi-free field is proposed. In the method, the source in a semi-free field can be reconstructed correctly, and the acoustic field can be predicted and separated. An experiment on a speaker in a semi-anechoic chamber is carried out to verify the proposed method. By comparing the reconstructed and predicted results in DSBPM-based NAH with and without the proposed method, the proposed method is validated. The disadvantages of NAH without any treatment method in a semi-free field are demonstrated.