Applied Mechanics and Materials Vols. 130-134

Abstract: The exhaust noise, which falls into low-frequency noise, is the dominant noise source of a diesel engines and tractors. The traditional exhaust silencers, which are normally constructed by combination of expansion chamber, and perforated pipe or perforated board, are with high exhaust resistance, but poor noise reduction especially for the low-frequency band noise. For this reason, a new theory of exhaust muffler of diesel engine based on counter-phase counteracts has been proposed. The mathematical model and the corresponding experimental validation for the new exhaust muffler based on this theory were performed.

Abstract: A robust Capon adaptive beamforming imaging method was proposed for stepped-frequency through-the-wall radar. According to the ideal of robust Capon beamforming (RCB) under the steering vector uncertainty set, Capon optimal weight vector in frequcency domain is obtained by accurately estimating Compensation function vector. Then signal energy is estimated by freqency weighting, and coherence factor (CF) weighting is applied to suppress sidelobe level. Therefore, this method not only achieves higher resolution and much better interference suppression capabilities, but also improves imaging contrast.

Abstract: Surface texturing of silicon can reduce the reflectance of incident light and increase the conversion efficiency of solar cells. Many approaches have been present in texturing silicon solar cell. In this paper, ultrasound is introduced into acid solution to texture the surface of multicrystalline silicon (mc-Si) solar cell. The morphology images are obtained by scanning electron microscope (SEM), more holes and regular distributed morphology for the presented method can be observed. Antireflectance of the mc-Si surface is measured by spectrophotometer, and the reflectance of 10.2% is obtained for the presented method, which is much less than that of acid etching.

Abstract: In this paper, we combine the characteristics of acoustic emission detection with the present condition of truck rolling bearing’s fault detection ,adopting the acoustic emission technique to examine railway freight car ’s journal bearing inner ring looseness .We put forward the method which wavelet packet analyze and withdraw bearing acoustic emission signal’s feature vector. Through the energy distribution comparison of inner ring’s non-looseness and looseness, we extract the ratio between high frequency’s internal energy and total energy as the characteristic parameter , which can recognize the inner ring’s loose fault effectively. Acoustic emission technique is a dynamic detection method, which can get materials’ important information,fatigue crack propagation,under the dynamic loading conditions. In comparison with the traditional vibration detection, it has the characteristics of the early prediction of bearing fault.

Abstract: This paper dwelled on the principle of Computational Aero-Acoustics (CAA), and utilized the Boundary Element Method (BEM) combined with the Computational Fluid Dynamics (CFD) based on Lighthill’s analogy to the automobile flow model, and converted the fluctuating flow pressure near the vehicle’s surface into the dipole source boundary condition in acoustics grid, and eventually succeeded in simulating the external aerodynamic noise field of automobile by introducing the dipole source boundary condition into the automobile’s BEM model. The distribution of vehicle’s external aero-acoustics field and the directivity of vehicle’s surface aerodynamic acoustic dipole source were also discussed carefully in this paper. The results show that: The head and tail of car are the main aerodynamic noise source radiation areas, and most of the dipole source’s SPL value is more than 70dB; the variation in car speed greatly impacts on the directivity of aerodynamic noise field near the car’s tail surface (θ=165°~195°).

Abstract: In the article, the method of experimental model test is used to optimize the shape of the interception structure. The hydraulic characteristics including flow pattern, diversion ratio and water grade ling along the tunnel is studied minutely. Base on the initial design, the interception structure is revised, and the recommended scheme has been got, which properly meet the demands of hydraulic characteristics mentioned above.

Abstract: This paper gives a new algorithm that based on normalized autocorrelation robustness selection. Aiming at the situation that the voice was vitiated by noise and the difference of pollution extent between different parts of the voice, we recognized that preserving the polluted parts of voice will bring in negative affection to the confirmation system of speaker. Normalized autocorrelation not only shows the correlation of short-time voice frame, but also shows the polluted extent of voice. As a result, we can wipe off the deeply polluted voice through the normalized autocorrelation. Experiments have shown that through the procedure of normalized autocorrelation we can not only wipe off the polluted voice, but also improve confirmation ability of speaker in high noise environment.

Abstract: Considering the inflexibility and limitations of the traditional frequency-domain OCDs reliability screening method using noise parameters at fixed frequency-points, we put forward a new method using the normalization variance of a large frequency-band noise spectrum in this paper. The possible sources of excess noise in OCDs and the relationship between low-frequency noise and reliability are studied at first, and then we describe the detailed process of the method for reliability screening, including the normalization method of noise spectrum, the calculations of variance and related characteristic quantities. At last, compare the screening results obtained by both of the previous method and this new method, and it demonstrated that this method is much more accurate and reliable.

Abstract: The determination of the sensitivity of the acoustical characteristics of vibrating systems with respect to the variation of the design parameters can provide a method to low-noise design of mechanical structure objectively and quantitatively. Using the Distributed source energy boundary point method, the expressions of the change of the acoustical energy density with respect to design variable is presented in this paper. The Distributed source energy boundary point method is a speedy and precise method which can avoid the complex computing of the singularity integral in EBEM. The correctness and availability is validated by the numerical simulation.

Abstract: In order to reduce computational complexity and memory requirements using conventional boundary element method (CBEM) for large scale acoustical analysis, a fast solving algorithm called the Fast Multipole BEM (FMBEM) based on the fast multipole algorithm and GMRES iterative solver is developed without composing the dense influence coefficient matrices. The multipole level structure is introduced to accelerate the solution of large-scale acoustical problems, by employing a concept of cells clustering boundary elements and hierarchical cell structure. To further improve the efficiency of the FMBEM with iterative solvers, a block diagonal matrix method is used in the system of equations as the left pre-conditioner. Numerical examples are presented to further demonstrate the efficiency, accuracy and potentials of the fast multipole BEM for solving large-scale acoustical problems.