Current Issue : October - December Volume : 2017 Issue Number : 4 Articles : 6 Articles
Automated surveillance of remote locations in a wireless sensor network is dominated by\nthe detection algorithm because actual intrusions in such locations are a rare event. Therefore, a detection\nmethod with low power consumption is crucial for persistent surveillance to ensure longevity of\nthe sensor networks. A simple and effective two-stage algorithm composed of energy detector (ED)\nand delay detector (DD) with all its operations in time-domain using small-aperture microphone\narray (SAMA) is proposed. The algorithm analyzes the quite different velocities between wind noise\nand sound waves to improve the detection capability of ED in the surveillance area. Experiments in\nfour different fields with three types of vehicles show that the algorithm is robust to wind noise and\nthe probability of detection and false alarm are 96.67% and 2.857%, respectively...
This paper presents a high-sensitivity hydrophone fabricated with a Microelectromechanical Systems (MEMS) process using\nepitaxial thin films grown on silicon wafers. The evaluated resonant frequency was calculated through finite-element analysis\n(FEA). The hydrophone was designed, fabricated, and characterized by different measurements performed in a water tank, by\nusing a pulsed sound technique with a sensitivity of âË?â??190 dB Ã?± 2 dB for frequencies in the range 50ââ?¬â??500Hz. These results indicate\nthe high-performance miniaturized acoustic devices, which can impact a variety of technological applications....
One performance measure of in-air ultrasonic radiators, such as wireless\npower transmission, is the power efficiency of the transducers. The efficiency\nof most in-air acoustic radiators is low, even at ultrasonic frequencies; however,\na large radiating plate with steps introduced by Gallego-Juarez et al ., can\nprovide efficient radiation. Their in-air acoustic radiator consists of a Langevin\ntransducer for wave excitation, a mechanical amplifier, and a stepped plate\nwith a large radiating area. This study describes a design processing technique\nfor a stepped-plate radiator developed for optimum energy transmission at\nthe target point in air. The total efficiency required to transfer the acoustic\nenergy was divided into three categories, and the design parameters of each\ncategory were calculated to maximize the efficiency. This design technique allows\noptimum acoustic radiation efficiency and maximum acoustic energy\ntransmission depending on various acoustic energy transfer conditions....
Cracked media are a common geophysical phenomena. It is important to study the propagation\ncharacteristics in boreholes for sonic logging theory, as this can provide the basis for the sonic\nlog interpretation. This paper derives velocityââ?¬â??stress staggered finite difference equations of\nelastic wave propagation in cylindrical coordinates for cracked media. The sound field in the\nborehole is numerically simulated using the finite-difference technique with second order in time\nand tenth order in space. It gives the relationship curves between the P-wave, S-wave velocity,\nanisotropy factor and crack density, and aspect ratio. Furthermore, it gives snapshots of the\nborehole acoustic wave field in cracked media with different crack densities and aspect ratios.\nThe calculated results show that in dry conditions the P-wave velocity in both the axial and radial\ndirections decreases, and more rapidly in the axial direction while the crack density increases.\nThe S-wave velocity decreases slowlyïâ?? with increasing crack density. The attenuation of the wave\nenergy increases with the increase in crack density. In fluid-saturated cracked media, both the\nP-wave and S-wave velocity increases with the aspect ratio of the cracks. The anisotropy of the\nP-wave decreases with the aspect ratio of the cracks. The aspect ratio of the crack does not\nobviously affect the energy attenuation....
Spherical microphone arrays have been paid increasing attention for their ability to locate\na sound source with arbitrary incident angle in three-dimensional space. Low-frequency sound\nsources are usually located by using spherical near-field acoustic holography. The reconstruction\nsurface and holography surface are conformal surfaces in the conventional sound field transformation\nbased on generalized Fourier transform. When the sound source is on the cylindrical surface, it is\ndifficult to locate by using spherical surface conformal transform. The non-conformal sound field\ntransformation by making a transfer matrix based on spherical harmonic wave decomposition is\nproposed in this paper, which can achieve the transformation of a spherical surface into a cylindrical\nsurface by using spherical array data. The theoretical expressions of the proposed method are\ndeduced, and the performance of the method is simulated. Moreover, the experiment of sound source\nlocalization by using a spherical array with randomly and uniformly distributed elements is carried\nout. Results show that the non-conformal surface sound field transformation from a spherical surface\nto a cylindrical surface is realized by using the proposed method. The localization deviation is around\n0.01 m, and the resolution is around 0.3 m. The application of the spherical array is extended, and the\nlocalization ability of the spherical array is improved....
The acoustic emission (AE) signals of metal materials have been widely used to identify the\ndeformation stage of a pressure vessel. In this work, Q235 steel samples with different propagation\ndistances and geometrical structures are stretched to get the corresponding acoustic emission signals.\nThen the obtained acoustic emission signals are de-noised by empirical mode decomposition (EMD),\nand then decomposed into two different frequency ranges, i.e., one mainly corresponding to metal\ndeformation and the other mainly corresponding to friction signals. The ratio of signal energy between\ntwo frequency ranges is defined as a new acoustic emission characteristic parameter. Differences\ncan be observed at different deformation stages in both magnitude and data distribution range.\nCompared with other acoustic emission parameters, the proposed parameter is valid in different\nsetups of the propagation medium and the coupled stiffness....
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