Current Issue : October - December Volume : 2014 Issue Number : 4 Articles : 7 Articles
In this work a crack identification method is proposed for bridge type structures carryingmoving vehicle.The bridge ismodeled as\nan Euler-Bernoulli beam, and open cracks exist on several points of the beam. Half-car model is adopted for the vehicle. Coupled\nequations of the beam-vehicle systemare solved usingNewmark-Betamethod, and the dynamic responses of the beamare obtained.\nUsing these and the reference displacements, an objective function is derived. Crack locations and depths are determined by solving\nthe optimization problem. To this end, a robust evolutionary algorithm, that is, the particle swarmoptimization (PSO), is employed.\nTo enhance the performance of the method, themeasured displacements are denoised usingmultiresolution property of the discrete\nwavelet transform (DWT). It is observed that by the proposed method it is possible to determine small cracks with depth ratio 0.1\nin spite of 5% noise interference....
This paper describes a special mathematical images model to determine the sound level inside a close-fitting sound enclosure. Such\nan enclosure is defined as the internal air volume defined by a machine vibration noise source at one wall and a parallel reflecting\nwall located very close to it and acts as the outside radiating wall of the enclosure. Four smaller surfaces define a parallelepiped\nfor the volume.The main reverberation group is between the two large parallel planes. Viewed as a discrete line-type source, the\nmain group is extended as additional discrete line-type source image groups due to reflections from the four smaller surfaces. The\nimages group approach provides a convergent solution for the case where hard reflective surfaces are modeled with absorption\ncoefficients equal to zero. Numerical examples are used to calculate the sound pressure level incident on the outside wall and the\neffect of adding high absorption to the front wall. This is compared to the result from the general large room diffuse reverberant\nfield enclosure formula for several hard wall absorption coefficients and distances between machine and front wall. The images\ngroup method is shown to have low sensitivity to hard wall absorption coefficient value and presents a method where zero sound\nabsorption for hard surfaces can be used rather than an initial hard surface sound absorption estimate or measurement to predict\nthe internal sound levels the effect of adding absorption....
The S1 mode of the Lamb spectrum of an isotropic plate exhibits negative group velocity in a narrow frequency domain. This\nanomalous behavior is explained analytically by examining the slope of each mode first in its initial state and then near its turning\npoints....
As a basic study into 3-D audio display systems, this paper reports the conditions of moving sound image velocity and time-step\nwhere a discrete moving sound image is perceived as continuous motion. In this study, the discrete moving sound image was\npresented through headphones and ran along the ear-axis. The experiments tested the continuity of a discrete moving sound image\nusing various conditions of velocity (0.25, 0.5, 0.75, 1, 2, 3, and 4 m/s) and time-step (0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, and 0.14 s).\nAs a result, the following were required in order to present the discrete moving sound image as continuous movement. (1) The\n3-D audio display system was required to complete the sound image presentation process, including head tracking and HRTF\nsimulation, in a time shorter than 0.02 s, in order to present sound image movement at all velocities. (2) A processing time longer\nthan 0.1 s was not acceptable. (3) If the 3-D audio display system only presented very slow movement (less than about 0.5 m/s),\nprocessing times ranging from 0.04 s to 0.06 s were still acceptable....
This paper studies the sound transmission loss of perforated panels and investigates the effect of the hole diameter on the\nsound insulation performance under normal incidence of acoustic loading. The hole diameters are distinguished into micro\n(submillimeter) and macro (millimeter) sizes. In general, the transmission loss reduces as the perforation ratio is increased.\nHowever, by retaining the perforation ratio, it is found that the transmission loss increases as the hole diameter is reduced for\na perforate with micro holes due to the effect of resistive part in the hole impedance, which is contrary to the results for those with\nthe macro holes. Both show similar trend at high frequency where the fluid behavior inside the hole is inertial. Simple analytical\nformulae for engineering purpose are provided. Validation of the models with measurement data also gives good agreement....
A double-leaf partition in engineering structures has been widely applied for its advantages, that is, in terms of its mechanical\nstrength as well as its lightweight property. In noise control, the double-leaf also serves as an effective noise barrier. Unfortunately\nat lowfrequency, the sound transmission loss reduces significantly due to the coupling between the panels and the air between them.\nThis paper studies the effect of a microperforated panel (MPP) inserted inside a double-leaf partition on the sound transmission\nloss performance of the system. The MPP insertion is proposed to provide a hygienic double-leaf noise insulator replacing the\nclassical abrasive porous materials between the panels. It is found that the transmission loss improves at the troublesome mass-airmass\nresonant frequency if the MPP is located closer to the solid panel. The mathematical model is derived for normal incidence\nof acoustic loading....
This paper presents a two-stage soft thresholding algorithm based on discrete cosine transform (DCT) and empirical mode\ndecomposition (EMD). In the first stage, noisy speech is decomposed into eight frequency bands and a specific noise variance\nis calculated for each one. Based on this variance, each band is denoised using soft thresholding in DCT domain. The remaining\nnoise is eliminated in the second stage through a time domain soft thresholding strategy adapted to the intrinsic mode functions\n(IMFs) derived by applying EMD on the signal obtained from the first stage processing. Significantly better SNR improvement and\nperceptual speech quality results for different noise types prove the superiority of the proposed algorithm over recently reported\ntechniques....
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