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Published Articles

The Volume 9, No 1, March 2004

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Theory and Experiment on Sound Reduction by an Irregularly-Edged Screen over a Hard Ground

Chun Biao Gan, Kai Ming Li and Hon Yiu Wong


Diffraction by a thin barrier with a square-wave form, a triangle-wave form or a randomly jagged top edge is investigated. The top edge of the sound barrier is divided into many infinitesimal segments with different heights to act as a series of sound sources. Asymptotic approximations are developed for predicting the diffraction field on the hard ground in the shadow zone of these three types of barriers from the theory of Menounou?s.26 Based on the theory, the excess attenuation vs. the diffracted field by the straight barrier with the maximum height is simulated. Laboratory measurements are reported for comparison with the excess attenuation of triangle-wave form and randomly jagged-edged barriers by the present approximations. Differences between the present approximations, current existing models and the experimental data are very small. It is shown that the barrier with the triangle-wave form or randomly jagged edge can perform better to reduce the diffraction field over a wide frequency range even if the referenced sound field is taken as the diffracted field by the straight barrier with the maximum height. Moreover, by simple simulations, it is found that the randomly jagged barrier behaves just like a straight one when the source or the receiver is far away from the barrier, and almost the same excess attenuation can be observed as long as the horizontal width of each segment is made small enough for the barrier with a triangle-waveform edge or a randomly jagged edge. The formula presented here is very fast, simple and intuitive and this idea can be applied to the study of sound fields diffracted by many other types of irregular barriers.

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A New Generalised Approach for Feature Representation in Vibroacoustical Fault Diagnosis

L. Gelman, P. Jenkin, M. Sanderson,C. Thompson, M. J. Crocker


A new generalised approach for health monitoring has been formulated. The generalised approach consists of using two new monitoring features simultaneously, which are based on the Fourier transform and which take into account the covariance between features. The generalisation of the approach improves the effectiveness of health monitoring. The monitoring effectiveness between the generalised approach and the power spectral density approach was compared for vibroacoustical monitoring. It was found that the power spectral density approach is not optimal, and represents only a particular case of the generalised approach. The power spectral density approach is optimal only if both: the correlation coefficient between new features is equal to zero, and the standard deviations of the features are equal. Use of the generalised approach provides an improvement in monitoring effectiveness as compared with the power spectral density approach. Application of the generalised approach was considered for the vibroacoustical diagnostics of damping.

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Vibrating String with a Variable Length

Abdelkader Megueni, Lakhdar Gaffour


The propagation of waves in a string with stationary boundary conditions is well known. The transverse vibrations of a string where one extremity is moving with a prescribed motion is considered. We cannot apply the method of separation of variables. This method leads to very complex equations. A solution developed previously by L. Gaffour is introduced in this study.1 The solution for the case of a string with a time-varying length with appropriate boundary conditions is examined. This solution is exact and is expressed in terms of functional Fourier series. It is developed for the linear motion of the extremity of the string. The modes of vibrations and the energy ratio are formulated in a precise way. The curves of eigenmodes, energy ratio, and energy are presented.

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Aero-Acoustic Performance of a Tube with Forward-Slanted Oblique Perforations for Reducing the Noise of a Supersonic Jet

Md. Tawhidul Islam Khan, Kunisato Seto, Zhixiang Xu and Mori Takeshi


The aero-acoustic performance of a forward-slanted perforated tube attached to a convergent nozzle was evaluated experimentally. The sound pressure level and thrust of the tube were investigated and a good performance in noise suppression was found. Flow visualisation with the Schlieren system along with a high-speed video camera showed the disappearance of shock structure in the flow issued from the tube with forward-slanted perforations but not present in the flow from a solid tube without any perforations. The performance of the perforated tube with forward-slanted perforations was compared with that of different tubes 1) with backward-slanted perforations and 2) with perforations normal to the axis and 3) with a solid tube without any perforations. Among the tubes tested, the perforated tube with forward-slanted perforations showed the best performance. The tube suffered from minimum thrust loss compared with the backward- and the normally perforated tubes. The only a drawback of the tube with forward-slanted perforations was its generation of a tonal component at low pressure. The elimination of the inner sharp edge of the perforation or the reduction of tube thickness negated the drawback and improved the aero-acoustic performance of the tube with forward-slanted perforations.

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A New Method to Identify the Sixteen Coefficients of Hydrodynamic Bearings Used in Disk Drive Spindle Motor

J.H. Wang and W.L. Hwang


The spindle motor is an important component used in most disk media devices. Nowadays, the spindle is generally supported by two journal bearings with a herringbone-grooved structure. Because the two bearings generally are not identical, there are sixteen bearing coefficients to be identified. In this work, two new identification methods are proposed. The basic concept of the two methods is the same, but there is a slight difference between them. The proposed methods use the measured transfer functions to identify the parameters. Because measurement noise is unavoidable, the derivation of the proposed methods has taken the noise effect into special consideration. The simulation results show that the identification accuracy of the proposed methods is far better than that of an existing method, which also has been especially developed to identify the sixteen coefficients of two bearings.

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