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Published Articles

The Volume 13, No 4, December 2008

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Research of on-line noise source identification based on the Grey neural network

Yang Yang and Shuangwei Wang


The accurate identification of an acoustic source is a fundamental problem in noise control. In real measurements, if a contribution of separate sources to the whole is identified properly, it is possible to apply appropriate mod-ifications to the sound source that contributes most significantly to the whole signal. This paper presents a new approach to acoustic noise identification by using modern spectrum estimation, Grey Relational Analysis (GRA), and Back-Propagation neural networks. The modern spectrum estimation method was used to recognize the main noise source, and GRA was used to estimate the similarity among different curves of the power spectrum. Finally, the Artificial Neural Network (ANN) was used to do curve fitting, which give the results more wide applications.
The ranking of the noise sources was determined on the basis of their individual contributions to the overall noise. The results of simulations confirmed the feasibility and validity of the method proposed in this paper, and it will play an important role in noise control, signal source identification, and other fields.

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Influences and simplifications of diffraction functions in nonlinear acoustical parameter measurement systems

Djilali Kourtiche, Laurent Alli`es, Mustapha Nadi and Ahmed Chitnalah


This paper presents an analytical formulation for correcting the diffraction associated with the second harmonic of an acoustic wave, that is more compact than that usually used. This new formulation resulting from an approximation of the correction applied to the fundamentals, which makes it possible to obtain simple solutions for the average second harmonic acoustic pressure but is sufficiently precise when measuring the parameter of nonlinearity B/A in the finite amplitude method. Comparisons with other expressions requiring numerical integration show that the solutions are precise in the near field. Furthermore, the effect of diffraction in the B/A parameter measurement system is discussed.

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Experimental application of high precision k-space filters and stopping rules for fully automated near-field acoustical holography

Rick Scholte, Ines Lopez, N.Bert Roozen, Henk Nijmeijer


In general, inverse acoustics problems are ill-posed. Without proper regularization action taken, noisy measurements result in an increasingly disturbed solution of the inverse acoustics wave equation as the distance from the measurement plane to the desired source grows. Two distinctive steps take place in the regularization process for planar near-field acoustical holography (PNAH): first, a low-pass filter function is defined and secondly a stopping rule is applied to determine the parameter settings of the filter. In acoustical imaging practice, it turns out to be very hard to determine the right filter for a certain case, ideally by means of an automatic search for the (near-) optimal parameters. This paper presents the practical application of a novel automated method that combines fitted filters for a broad number of possible experimental sources combined with highly efficient stopping rules by taking advantage of k-space. Also, a number of well-known and newly developed filter functions and stopping rules are discussed and compared with one another. Results based on actual measurements demonstrate the effectiveness, applicability, and precision of the fully implemented and automated regularization process for PNAH. Practical results even show acoustic source visualization below one millimeter primarily by successful application of k-space regularization. Implementations include modifications of Tikhonov, exponential and truncation low-pass filters, L-curve, Generalised Cross-Validation (GCV) and the novel Cut-Off and Slope (COS) parameter selection methods for PNAH. COS iteration in combination with either a modified exponential or Tikhonov low-pass filter results in an automated selection of the regularization parameters and eventually a fully automated PNAH system.

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Response analysis of multi-storey structures on flexible foundation due to seismic excitation

Swapan K. Chakraborty, Swapan K. Sarkar


In this article, the seismic response of multi-storey buildings with soft ground floors is investigated. The floors of the buildings are assumed to rest on the flexible elastic foundation media underlain by heterogeneous elastic halfspace, excited by input wave motion is studied. Analytical solutions are derived for a two-dimensional continuous building model and for the monochromatic anti-plane excitation. It has been shown that the vibration experienced by the flexible foundation of a structure during an earthquake may be substantially different from the motion that the ground perceives in its absence. It has also been observed that the presence of the flexible foundation influences the propagation of polarised shear motion to such an extent that the motion causing disturbance in the ground floor cannot make headway to the upper part of the building. One of the important consequences of this analysis is that all the above-mentioned effects may be possible in other ways as a result of the contribution of an antisymmetric mode of vibration to the overall response.

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