Jeong-Guon Ih

On the reconstruction of the vibro‐acoustic field over the surface enclosing an interior space using the boundary element method

The vibrational velocity, sound pressure, and acoustic power on the vibrating boundary comprising an enclosed space are reconstructed by the boundary element method based on the measured field pressures. The singular value decomposition is used to obtain the inverse solution in the least‐square sense and to express the acoustic modal expansion between the measurement and source fields. In general, such an inverse operation has been considered an ill‐posed problem having a divergence phenomenon involved with extremely small measurement errors. The ill‐conditioned nature of the acoustic inverse problem is caused by the singularity of the transfer matrix which produces nonradiating wave components. In order to minimize the singularity and to also reduce the number of measurement points, optimal measurement positions are determined by the effective independence method. Regularization methods are used to stabilize the reconstructed field by suppressing nonradiating components resulting in the singular transfer...

On the multiple microphone method for measuring in-duct acoustic properties in the presence of mean flow

Nowadays, the two-microphone method is accepted as the standard as specified in ASTM E1050-90 for measuring in-duct acoustic properties. However, research results on using the least square method with multiple measurement points and broadband excitation have been reported for enhancing the frequency response of the two-microphone method. In this paper, the effects of varying the relative measurement positions on errors in the estimation of the acoustic quantities is studied for the multiple microphone method. Both the theoretical and experimental results show that, among possible sensor positioning configurations, the equidistant positioning of sensors yields the smallest error within the effective measurement frequency range. In addition, it is noted that the measurement accuracy can be increased and the effective frequency range can be widened by increasing the number of equidistant sensors. Measurement examples are shown and the results support the findings.

Analysis of higher‐order mode effects in the circular expansion chamber with mean flow

The effects of higher‐order acoustic modes produced by the areal discontinuities of the simple expansion chamber with mean flow on the acoustic performance are studied. The chamber is modeled as a piston‐driven circular rigid tube with no losses and, by using the Fourier–Bessel expansion, a general expression of the output pressure to the given input uniform volume velocity is obtained for a whole chamber. Quantitative estimation for the transmission loss can be performed using the derived four‐pole parameters, whereas the characteristics of the chamber, as a result of the interactions between the plane wave and the transverse waves, can be easily investigated with respect to the relative locations of the inlet/outlet, the chamber length, and the mean flow. The expression as a form of C in four‐pole parameters is adopted to describe the chamber characteristics for computational convenience. All the parameters involved in the experimental works by Eriksson [L. J. Eriksson, J. Acoust. Soc. Am. 72, 1208–1211...

Dispersion of elastic waves in random particulate composites

Elastic wave propagation in a discrete random medium is studied to predict dynamic effective properties of composite materials containing spherical inclusions. A self‐consistent method is proposed which is analogous to the well‐known coherent potential approximation in alloy physics. Three conditions are derived that should be satisfied by two effective elastic moduli and effective density. The derived self‐consistency conditions have the physical meaning that the scattering of a coherent wave by the constituents in the effective medium vanishes, on the average. The frequency‐dependent effective wave speed and coherent attenuation can be obtained by solving the self‐consistency conditions numerically. At the lowest resonance frequency, the phase speed increases rapidly and the attenuation reaches the maximum in the composites having a large density mismatch. The lowest resonance is caused mainly by the density mismatch between matrix and particles and higher resonances by the stiffness mismatch. The dispe...

Empirical model of the acoustic impedance of a circular orifice in grazing mean flow

Although there are many analytical and empirical models for orifice impedance, the predicted acoustical performance when adopting any one of them sometimes shows a large discrepancy with the measured result in some cases. In order to obtain a new practical and precise empirical impedance model under grazing flow conditions, the acoustic impedance of circular orifices has been measured with a variation of the involved parameters under very carefully tested and controlled measurement conditions. The parameters involved in determining the acoustic impedance of an orifice are comprised of the orifice diameter, orifice thickness, perforation ratio, mean flow velocity, and frequency. The range of involved parameters is chosen to cover the practical data span of perforates in typical exhaust systems of internal combustion engines. The empirical impedance model is obtained by using nonlinear regression analysis of the various results of the parametric tests. The proposed empirical model of orifice impedance, with a very high correlation coefficient, is applied to the prediction of the transmission loss of concentric resonators, which have geometric configurations typical of acoustically short and long through-flow resonators. By comparing the measured and predicted results, in which the predictions are made by employing many previous orifice impedance models as well as the present model, it is confirmed that the proposed orifice impedance model yields the most accurate prediction among all other existing impedance models.

Acoustic Array Systems: Theory, Implementation, and Application

Presents a unified framework of far-field and near-field array techniques for noise source identification and sound field visualization, from theory to application.Acoustic Array Systems: Theory, Implementation, and Applicationprovides an overview of microphone array technology with applications in noise source identification and sound field visualization. In the comprehensive treatment of microphone arrays, the topics covered include an introduction to the theory, far-field and near-field array signal processing algorithms, practical implementations, and common applications: vehicles, computing and communications equipment, compressors, fans, and household appliances, and hands-free speech. The author concludes with other emerging techniques and innovative algorithms.Encompasses theoretical background, implementation considerations and application know-howShows how to tackle broader problems in signal processing, control, and transudcersCovers both farfield and nearfield techniques in a balanced wayIntroduces innovative algorithms including equivalent source imaging (NESI) and high-resolution nearfield arraysSelected code examples available for download for readers to practice on their ownPresentation slides available for instructor useA valuable resource for Postgraduates and researchers in acoustics, noise control engineering, audio engineering, and signal processing.

Prediction of sound transmission loss through multilayered panels by using Gaussian distribution of directional incident energy

In this study, a new prediction method is suggested for sound transmission loss (STL) of multilayered panels of infinite extent. Conventional methods such as random or field incidence approach often given significant discrepancies in predicting STL of multilayered panels when compared with the experiments. In this paper, appropriate directional distributions of incident energy to predict the STL of multilayered panels are proposed. In order to find a weighting function to represent the directional distribution of incident energy on the wall in a reverberation chamber, numerical simulations by using a ray-tracing technique are carried out. Simulation results reveal that the directional distribution can be approximately expressed by the Gaussian distribution function in terms of the angle of incidence. The Gaussian function is applied to predict the STL of various multilayered panel configurations as well as single panels. The compared results between the measurement and the prediction show good agreements, which validate the proposed Gaussian function approach.

Design of an optimal wave-vector filter for enhancing the resolution of reconstructed source field by near-field acoustical holography (NAH)

In near-field acoustical holography using the boundary element method, the reconstructed field often diverges due to the presence of small measurement errors. In order to handle this instability in the inverse problem, the reconstruction process should include some form of regularization for enhancing the resolution of source images. The usual method of regularization has been the truncation of wave vectors associated with small singular values, although the determination of an optimal truncation order is difficult. In this article, an iterative inverse solution technique is suggested in which the mean-square error prediction is used. A statistical estimation of the minimum mean-square error between measured pressures and the model solution is required for yielding the optimal number of iterations. The continuous curve of an optimal wave-vector filter is designed, for suppressing the high-order modes that can produce large reconstruction errors. Experimental results from a baffled radiator reveal that the reconstruction errors can be reduced by this form of regularization, by at least 48% compared to those without any regularization. In comparison to results using the optimal truncation method of regularization, the new scheme is shown to give further reductions of truncation error of between 7% and 39%, for the example in this article.

The reactive attenuation of rectangular plenum chambers

Abstract A mathematical formulation for evaluating the acoustic performance of plenum chambers is derived by using the eigenfunction expansion technique. The chamber is modelled as a piston-driven rectangular rigid tube with no losses and containing a medium otherwise at rest. The residual acoustic velocity potential in the chamber is obtained by superposing the three-dimensional velocity potentials due to each of the uniformly fluctuating pistons. All the geometrical parameters such as the size of the chamber and ports, the arbitrary locations of inlet/outlet ports and the shapes of the ports are taken into consideration in the formulation as well as the acoustic higher order modes generated at the area discontinuities of port-chamber interfaces. By using the derived 2 × 2 transfer matrices, any type of reactive rectangular plenum chamber including the throughflow, flow-reversal, Helmholtz resonator and end-in/side-out configurations can be characterized, and their insertion or transmission losses can be estimated very easily through the appropriate truncation of acoustic modal series. Computed values are compared with those predicted by the foregoing numerical methods and by the classical plane wave theory, and the former agree very well with the results of the present study. In addition, 90° and 180° duct bends are analyzed and the results are in good agreement with previous experimental ones. The derived transfer matrices can be easily incorporated as basic modules in any existing computer programs incorporating four-pole parameters for predicting the acoustic performance of silencing systems.

Use of nonsingular boundary integral formulation for reducing errors due to near-field measurements in the boundary element method based near-field acoustic holography

In the conformal near-field acoustic holography (NAH) using the boundary element method (BEM), the transfer matrix relating the vibro-acoustic properties of source and field depends solely on the geometrical condition of the problem. This kind of NAH is known to be very powerful in dealing with the sources having irregular shaped boundaries. When the vibro-acoustic source field is reconstructed by using this conformal NAH, one tends to position the sensors as close as possible to the source surface in order to get rich information on the nonpropagating wave components. The conventional acoustic BEM based on the Kirchhoff-Helmholtz integral equation has the singularity problem in the close near field of the source surface. This problem stems from the singular kernel of the Green function of the boundary integral equation (BIE) and the singularity can influence the reconstruction accuracy greatly. In this paper, the nonsingular BIE is introduced to the NAH calculation and the holographic BIE is reformulated. The effectiveness of nonsingular BEM has been investigated for the reduction of reconstruction error. Through interior and exterior examples, it is shown that the resolution of predicted field pressure could be improved in the close near field by employing the nonsingular BIE. Because the BEM-based NAH inevitably requires the field pressure measured in the close proximity to the source surface, the present approach is recommended for improving the resolution of the reconstructed source field.