Obtaining acoustic intensity from multisource statistically optimized near-field acoustical holography

The acoustic holography (AH) method with single measuring plane has been well known as the conventional method and can be implemented by far field measurement with simple instruments. However, the noise source resolution of the AH is not sufficient. In order to improve the resolution in the noise source identification, several kinds of the acoustic holography methods have been so far proposed. For example, the near field acoustic holography (NAH) can provide high and accurate resolution of the holography by the nearfield measurement. However, the nearfield measurement within one wave length is sometimes impossible in the actual circumstances. The Acoustic Double Holography (A D H) proposed in this paper is a simplified approach with higher resolution of the noise source locations than that of the conventional AH methods. The ADH method basically uses dual measuring planes and does not require nearfield measurement. The sound pressure data detected on the rear plane are transformed into the virtual pressure data on the front plane taking into account of the distance between the plane and the object. Comparing the virtual pressure data with the actual data measured on the front plane, resolution on holography can be improved significantly. Computer simulation and an experiment with two loud speakers were executed in order to confirm the fundamental feature of the proposed method. Several advantages on the method with respect to resolution over the conventional AH method were discussed. Furthermore, the ADH measurement was carried out on running engine under the full load operation. Through these results, the highly noise radiating areas on the engine surface were detected and reduced with noise shielding material. The overall engine noise level was reduced by 1.5dBA as the first stage in this noise control work.

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.

In this paper, the basic principle of planar near field acoustic holography is expatiated and the reconstruction formula of sound source localization is given. We take theoretical results of sound pressure radiating from a point source to the holographic plane as the experimental data, and then substitute it in the reconstruction formula to reconstruct the sound source information. By changing holographic plane size, spacing and holographic measurement distance, we compared the sound source reconstruction results with the known sound source to see how these measured conditions parameters affect the reconstruction accuracy. Test results show that precise result can be got when the sound bandwidth exceed 5Khz, the hologram plane size is 6 times greater than the sound wavelength, the spacing of measurement point on holographic plane is less than 1/3 of the wavelength and the holographic measurement distance is less than 1/34 of the wavelength. We applied the planar near field acoustical holography in plant disease stress monitoring, and choose the signal with frequency of 2KHz to carry out the reconstruction experiment. The holographic plane was 0.6m side length square, measuring point spacing was 0.06m and the distance between hologram plane and reconstruction plane was 0.015m. Three sound sources were finally identified. By modifying the parameter setting and comparative analysis, we seek the best selection for simulation and use it as a reference in the ultrasonic signal tests of the diseases stressed plant. Having collected acoustic emission signals, we identified the sound source by near field acoustic holography and located plant diseases stress. Consequently, we can conduct a new crop disease forecast and precision spraying of pesticides.

Source localization problem in near-field acoustical signals on solids surfaces is one of the computationally challenging tasks. This paper concentrates on a solution to this problem. The modeling of the presence of noises while measuring the acoustic pressure field produced due to the sound sources in the vicinity is done using acoustical holography method. The pressure field produced is modeled mathematically using Rayleigh-Sommerfield Algorithm and Helmholtz integral. Near-field acoustic holography (NAH) is performed using the conventional method termed as Fourier Transform based Acoustical Holography. Mathematically modeled pressure field is super positioned and NAH is used to synthesize the original pressure field. In this paper, we have done a simulation model for localizing acoustical sources on the solid surfaces. A pipelining method is described for super-positioning the truncated pressure field to match the actual measured pressure field. Windowing technique is adopted to address spectral leakage effects and the abnormality is caused to the proximity of the boundaries. The results reveal that the proposed methodology can apply to localize sources on diverse kind of surfaces.

Near-field acoustical holography (NAH) microphone array measurement technologies are currently being investigated by NASA, the Naval Research Lab (NRL) and Boeing. One type of array geometry under investigation is a spherical arrangement of microphones that enables the computation of the intensity field surrounding the sphere using spherical NAH theory. Due to the simplicity of the required computation and the ability to pre-compute several of the required quantities, the intensity field in a volume surrounding the sphere can be reconstructed and rendered in real time as a measurement is being made. Thus, sound sources can be identified in real time by roving the spherical array around in a sound field near suspected sources and tracking the sound intensity back to the source. This presentation demonstrates the capabilities of a real time spherical NAH system that has been developed by NASA, the NRL and Boeing. The topology of the software will be discussed and the capabilities of such a system will be demonstrated.

Complex pressure measurements have been made on multiple planes surrounding three violins, while bowed with a mechanical bowing apparatus: a Scherl & Roth student instrument, Hutchins’ SUS295, and a Hutchins’ mezzo-violin. The data are used with a multiplanar near-field acoustic holography (NAH) algorithm to reconstruct the surrounding radiated sound fields of each instrument, including pressure and intensity from the surface extending out to the far field. Areas of significant radiation on each violin are identified, compared, and related to structural differences between the instruments. [Work supported by NSF Graduate Research Fellowship, Lucent Technologies/Bell Laboratories GRPW Grant, and AAUW Selected Professions Dissertation Fellowship.]

Near‐field acoustical holography (NAH) techniques can be optimized if the method capitalizes on the geometry of the noise source under investigation. Helmholtz‐equation least squares method (HELS) uses the solutions of Helmholtz equation in spherical coordinates as basis functions for the pressure field. HELS is an efficient NAH technique if the source and the measurement surfaces are spherical in nature. For nonspherical cases, such as radiation from a plate or bar, it takes a large number of functions to represent the field. In these cases, there is also a question about where to place the origin of the wave functions. In search of a HELS‐type method that could be applied to nonspherical sources, a study into the features of conical coordinates has been conducted. Because the Helmholtz equation is separable in conical coordinates, the solutions can be used, in a manner similar to HELS, as basis functions to represent the pressure field. For conical coordinates, the basis functions are spherical Hankel functions and Lame functions. This HELS‐type formulation in conical coordinates could be a natural choice for NAH on conical sources and may be appropriate for NAH on jet noise. [Work supported by Blue Ridge Research and Consulting and Air Force Research Laboratory.]

In the conformal near-field acoustical holography (NAH), using the boundary element method (BEM), the vibroacoustic information on the source surface can be indirectly reconstructed by utilizing the measured field pressure and the inverse transfer matrix. The involved vibroacoustic transfer matrix is generally ill conditioned and the reconstruction process should include the singular value decomposition (SVD) in order to solve the problem in the inverse procedure. The accuracy of the reconstructed field is deteriorated substantially due to the ill conditioning of the transfer matrix and the inevitable measurement noise of field pressure related to the nonpropagating wave components. In this study, the computational processes of SVD and wave-vector filtering in the BEM-based NAH are discussed and their physical meanings are investigated through a simulation example. The vibroacoustic transfer matrix decomposed by SVD permits an effective regularization of the source field: This is possible because the involved information in the transfer matrix can be separated into the radiation efficiency and the wave modes at the source and surface fields. In particular, it is clearly shown that the restored source image can be improved dramatically by adopting the wave-vector filter that suppresses the nonpropagating wave components appropriately that cause the deterioration of the reconstruction accuracy.

Acoustic holography is a technique to visualize the sound field propagating from a sound source. It is used to analyse and locate sources of noise, and even allows to predict the propagation of sound in the environment. However, this technique involves simultaneous measurements of up to 256 microphones, arranged in an array. We propose an alternative approach with 4 or less microphones which are re-positioned using an articulated robot arm. Provided the measurement conditions are stable, this alternative approach with consecutive measurements allows to make use of acoustic holography to investigate various sources of noise. The case of fan noise is demonstrated where the focus is put on practical aspects conducting near-field acoustic holography experiments using a robot arm, as well as post-processing measurement data. As a result, 2 complimenting techniques, static and dynamic near-field acoustic holography, are introduced and their merits are discussed.

Near-field Acoustical Holography is a tool that is conventionally used to visualize sound fields through an inverse process in a three-dimensional space so that either sound field projections or sound source localization can be performed. The visualization is conducted by using sound pressure measurements taken in the near-field region close to the surface of the unknown sound source. Traditional Fourier-based Near-field Acoustical Holography requires a large number of measurement inputs to avoid spatial truncation effects. However, the use of a large number of measurements is usually not feasible since having a large number of microphones is costly, and usually the array is limited in size by the physical environment, thus limiting the practicality of this method. In the present work, because of the desire to reduce the number of microphones required to conduct acoustical holography, a method known as Statistically Optimized Near-field Acoustical Holography initially proposed by Steiner and Hald was analyzed. The main difference between the present work and the concept mentioned by Steiner and Hald is the cylindrical coordinate system employed here for the purpose of experimenting on a bladeless fan, which resembles a cylindrical structure and which could be assumed to be a cylindrical source. The algorithm was first verified via simulations and measurements, and was then applied to experimental data obtained via pressure measurements made with a cylindrical microphone array. Finally, suggestions for noise control strategies for the bladeless fan are described, based on the measurement results.

In this study, the influence of the levels of pavement roughness on tire noise is examined using Nearfield Acoustical Holography (NAH) to measure noise from single tires mounted on a two-wheel trailer towed over different types of pavements. Contributions to the radiated noise from the leading edge, trailing edge, and sidewall of two tires are identified. Two experimental tires — a blank tire and a monopitch tire — are evaluated on three pavements — smooth asphalt, stud-damaged concrete, and Ellsworth — at 56 km/hr. From the measured complex pressure, acoustic intensity is reconstructed on side plane of the tires using NAH procedures. Additionally, sound power spectra levels are presented. Tire noise generating mechanisms associated with each pavement are inferred from measurements. The experimental results are compared with theories on pavement-induced tire noise available in the literature.

Near-field acoustic holography (NAH) is a well established method for the reconstruction of sources. In this paper, the inclusion of an underwater moving medium is introduced and a new set of equations for NAH is developed. For a stationary acoustic field, the method continuously sweeps the acoustic field by using a line array of hydrophones fixed in space. This method enables the visualization of noise generated by moving noise sources and removes Doppler effect. Theoretical formulation has been systematically addressed which demonstrates how the idea of utilizing this method constructs the desired hologram. At last, the validity and the correctness of method are assessed by underwater near-field acoustic holography experiments based on measurement of horizontal line array of hydrophones. The experimental results show the merits of this method in the reconstruction of acoustic field, identification and localization of underwater moving noise sources.

Near-field acoustical holography is a technique that has been widely used to visualize noise sources from pressure measurements in spaces that can be assumed to be anechoic or semi-anechoic. Previously, a dual surface acoustical holography procedure based on making measurements on two surfaces between the source and a reflecting surface was introduced to remove the effects of reverberation. Little work has been performed in which beamforming has been used to visualize sources based on dual surface, near-field measurements in a reverberant environment: such a procedure is described here. Because many practical measurement environments are not completely anechoic, the source resolution accuracy of dual surface acoustical holography and beamforming procedures in reverberant environments is compared here by using numerical simulations. It has been found that dual surface acoustical holography provides the clearest representation of the source location when sound waves radiating from the source and the reflected waves are propagating in the opposite directions and when the measurement surfaces are conformal with the source geometry. However, it has also been found that dual surface beamforming provides more consistent source resolution performance regardless of the relative direction of wave propagation of the source and reflected waves.

Smart phones have dramatically evolved on the functional and aesthetic fronts in recent years. However, as smart phones get slimmer, problems arise where the noise radiated from the smart phone, albeit small in SPL, interferes with the conversation and degrades the call quality. The noise radiation is primarily due to the vibration of the internal electronics components and its transmission through the circuit board and the casing. In this paper the nearfield acoustic holography (NAH) is used to characterize the 3-D sound field of the smart-phone radiated noise, which is tailored specifically to deal with the small size and the low SPL of the sound radiator. Quantities such as acoustic pressure, particle velocity, and intensity at the smart phone surface are obtained and utilized to identify parts that are responsible for noise generation. In this respect, the NAH proves to be an effective tool for the characterization and control of the smart phone acoustics.

Nearfield acoustic holography (NAH) is an indirect technique for identifying noise sources and visualizing acoustic field. Recently, several different methods, such as the spatial Fourier transform method, the boundary element method (BEM) and the Helmholtz equation-least squares (HELS) method, have been used to realize the NAH successfully. In the paper, a novel numerical method, the distributed source boundary point method (DSBPM), is proposed to realize the NAH. In the method, the transfer matrices from the reconstructed surface to the hologram surface are constructed indirectly by a set of particular solution sources located inside the vibrating structure, and their inverses are carried out by singular value decomposition (SVD) technique. Additionally, considering the high sensitivity of the reconstructed solution to measurement errors, the Tikhonov regularization method is implemented to stabilize the reconstruction procedure and the regularization parameter is determined by L-curve criterion. Compared with the BEM-based NAH, the variable interpolation, the numerical quadrature, and the treatments of singular integral and nonuniqueness of solution are all avoided in the proposed method. Two numerical examples and an experiment are investigated to validate the feasibility and correctness of the proposed method.