Current Issue : July - September Volume : 2018 Issue Number : 3 Articles : 6 Articles
Tikhonov regularization method is effective in stabilizing reconstruction process of the near-field acoustic holography (NAH) based\non the equivalent sourcemethod (ESM), and the selection of the optimal regularization parameter is a key problem that determines\nthe regularization effect. In this work, a new method for determining the optimal regularization parameter is proposed.The transfer\nmatrix relating the source strengths of the equivalent sources to the measured pressures on the hologram surface is augmented by\nadding a fictitious point source with zero strength. The minimization of the norm of this fictitious point source strength is as\nthe criterion for choosing the optimal regularization parameter since the reconstructed value should tend to zero. The original\ninverse problem in calculating the source strengths is converted into a univariate optimization problem which is solved by a onedimensional\nsearch technique. Two numerical simulations with a point driven simply supported plate and a pulsating sphere are\ninvestigated to validate the performance of the proposedmethod by comparison with the L-curvemethod. The results demonstrate\nthat the proposed method can determine the regularization parameter correctly and effectively for the reconstruction in NAH....
The acoustic vector sensor (AVS) can measure the acoustic pressure fieldââ?¬â?¢s spatial gradient, so it has directionality. But its channels\nmay have nonideal gain/phase responses, which will severely degrade its performance in finding source direction. To solve this\nproblem, in this study, a self-calibration algorithm based on all-phase FFT spectrum analysis is proposed. This method is ââ?¬Å?selfcalibratedââ?¬Â\nbecause prior knowledge of the training signalââ?¬â?¢s arrival angle is not required. By measuring signals from different\ndirections, the initial phase can be achieved by taking the all-phase FFT transform to each channel. We use the amplitude of the\nmain spectrum peak of every channel in different direction to formulate an equation; the amplitude gain estimates can be achieved\nby solving this equation. In order to get better estimation accuracy, bearing difference of different training signals should be larger\nthan a threshold, which is related to SNR. Finally, the reference signalââ?¬â?¢s direction of arrival can be estimated. This method is easy to\nimplement and has advantage in accuracy and antinoise. The efficacy of this proposed scheme is verified with simulation results....
An acoustic metasurface made of a composite structure of cavity and membrane is proposed and numerically investigated. The\ntarget frequency is in the low frequency regime (570Hz). The unit cells, which provide precise local phase modulation, are\nrather thin with thickness in the order around 1/5 of the working wavelength. The numerical simulations show that the designed\nmetasurface can steer the reflected waves at will. By taking the advantage of this metasurface, an ultrathin planar acoustic axicon,\nacoustic lens, and acoustic nondiffracting Airy beam generator are realized. Our design method provides a new approach for the\nrevolution of future acoustic devices...
The acoustic characteristics of 2D single-oscillator, dual-oscillator, and triple-oscillator acoustic metamaterials were investigated\nbased on concentric ring structures using the finite element method. For the single-oscillator, dual-oscillator, and triple-oscillator\nmodels investigated here, the dipolar resonances of the scatterer always induce negative effective mass density, preventing waves\nfrom propagating in the structure, thus forming the band gap. As the number of oscillators increases, relative movements between\nthe oscillators generate coupling effect; this increases the number of dipolar resonance modes, causes negative effective mass density\nin more frequency ranges, and increases the number of band gaps. It can be seen that the number of oscillators in the cell is closely\nrelated to the number of band gaps due to the coupling effect, when the filling rate is of a certain value....
Soldiers and supporting engineers are frequently exposed to high low-frequency (<500Hz) cabin noise in military vehicles. Despite\nthe use of commercial hearing protection devices, the risk of auditory damage is still imminent because the devices may not be\noptimally customised for such applications. This study considers flax fibre-reinforced polypropylene (Flax-PP) as an alternative to\nthe material selection for the ear cups of commercial earmuffs, which are typically made of acrylonitrile butadiene styrene (ABS).\nDifferent weaving configurations (woven and nonwoven) and various noise environments (pink noise, cabin booming noise, and\nfiring noise) were considered to investigate the feasibility of the proposed composite earmuffs for low-frequency noise reduction.\nThe remaining assembly components of the earmuff were kept consistent with those of a commercial earmuff, which served as\na benchmark for results comparison. In contrast to the commercial earmuff, the composite earmuffs were shown to be better\nin mitigating low-frequency noise by up to 16.6 dB, while compromising midfrequency acoustical performance. Consequently,\nthe proposed composite earmuffs may be an alternative for low-frequency noise reduction in vehicle cabins, at airports, and at\nconstruction sites involving heavy machineries....
Acoustic emission has a direct correspondence to the internal damage of a material. To determine the effects of the loading rate on\nthe mechanical properties of rock, the initial damage was characterized using the acoustic emission technique when a uniaxial\npreloading was imposed on a cylindrical rock sample. On this basis, the uniaxial compression test was conducted on sandstone\nthat contains initial damage induced under a range of loading rates. The effects of the initial damage and loading rate on the\nmechanical properties of rock were analyzed. The uniaxial preloading generated randomly distributed microcracks in the natural\nrock. The results showed that the acoustic emission and positioning technique can characterize accurately the damage and its\nposition due to preloading. The development of microcracks was found to be strongly dependent on the loading rate. Moreover,\nthe loading rate accelerated the degradation of the rock strength. The effects of the loading rate and initial damage on the\nmechanical properties of rock are a complicated coupled process. From the experimental test result, a constitutive equation was\nconstructed based on the damage mechanics....
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