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Tsuru H.,Nittobo Acoustic Engineering Co. | Iwatsu R.,Tokyo Denki University
International Journal of Adaptive Control and Signal Processing

In the marine engineering field, a sound wave is often utilized to visualize objects. In such a sensing method, an accurate numerical prediction of sound propagation is an important issue for theoretical considerations. Recently, a finite difference method in time domain (FDTD) is often applied to wave propagation. However, an existing FDTD sometimes fails to match the accuracy to be required. In the present paper, strategies to improve conventional methods are presented: the application of the compact finite difference on staggered grid with adjusted coefficients and the usage of optimized multistep time integration. It is shown that through these tactics, a highly accurate simulation is attainable. Copyright © 2009 John Wiley & Sons, Ltd. Source

Hirosawa K.,Nittobo Acoustic Engineering Co.
40th International Congress and Exposition on Noise Control Engineering 2011, INTER-NOISE 2011

Many multi-layered structures or materials have been developed to expand its effective frequency ranges and to improve its acoustical performances. The multi-layered materials are also useful in vehicles, because those should be thin and light-weighted to achieve both low fuel consumption and interior quietness. The porous materials can be used for the sound absorption, but they are usually effective only in high frequency. In order to overcome poor acoustical performance in low frequency, the perforated panels can be combined with the porous materials. This study is focused on the absorption characteristic of the multi-layered structure included the perforated panel. The statistical absorption coefficient of the structure is estimated by the transfer matrix method which the finite area effect is considered as an application of the radiation impedance. Source

Williams E.G.,U.S. Navy | Takashima K.,Nittobo Acoustic Engineering Co.
Journal of the Acoustical Society of America

A technique is described to image the vector intensity in the near field of a spherical array of microphones flush mounted in a rigid sphere. The spatially measured pressure is decomposed into Fourier harmonics in order to reconstruct the volumetric vector intensity outside the sphere. The theory for this reconstruction is developed in this paper. The resulting intensity images are very successful at locating and quantifying unknown exterior acoustic sources, ideal for application in noise control problems in interior spaces such as automobiles and airplanes. Arrays of varying numbers of microphones and radii are considered and compared and errors are computed for both theory and experiment. It is demonstrated that this is an ill-posed problem below a cutoff frequency depending on array design, requiring Tikhonov regularization below cutoff. There is no low frequency limit on operation, although the signal-to-noise ratio is the determining factor for high-spatial resolution at low frequencies. It is shown that the upper frequency limit is set by the number of microphones in the array and is independent of noise. The accuracy of the approach is assessed by considering the exact solution for the scattering of a point source by a rigid sphere. Several field experiments are presented to demonstrate the utility of the technique. In these experiments, the partial field decomposition technique is used and holograms of multiple exterior sources are separated and their individual volumetric intensity fields imaged. In this manner, the intensity fields of two uncorrelated tube sources in an anechoic chamber are isolated from one another and separated intensity maps are obtained from over a broad frequency range. In a practical application, the vector intensity field in the interior of an automobile cabin is mapped at the fundamental of the engine vibration using the rigid sphere positioned at the driver's head. The source regions contributing to the interior cabin noise are identified. © 2010 Acoustical Society of America. Source

Iwatsu R.,Tokyo Denki University | Tsuru H.,Nittobo Acoustic Engineering Co.
Theoretical and Applied Mechanics Japan

We consider an application of a group of fourth order compact finite difference schemes on staggered mesh and third order symplectic integration methods to the linear wave equation. The objective is to improve resolution efficiency of the conventional method for acoustic problems. In the present study, we report the results of stability analysis and phase error analysis. Four methods are analyzed and compared for a benchmark test problem. It is shown by the analysis and by the benchmark problem, that one method among the four methods tested perform substantially better than the rest of the methods. Source

Iwatsu R.,Tokyo Denki University | Tsuru H.,Nittobo Acoustic Engineering Co.
Theoretical and Applied Mechanics Japan

A group of new methods is proposed for numerical simulation of acoustic problems. Symplectic methods are known to be effective for long time integration of particle motions. They are also applicable to the partial differential equations. For the wave-like equations, it has been noted that methods with smaller phase lag are preferable. On the other hand, trigonometric fitting (TF) methods have been proposed for the integration of harmonic oscillators and other oscillatory problems. TF methods are capable of integrating with practically no phase error. However, their application is limited to the cases where frequency of the system is known beforehand. In order to utilize the advantage of these methods, symplectic spectral TF method is proposed in the present study. By combining the advantage of small phase error and the spectral representation of the waves, the method is capable of integrating the wave equations with extremely high accuracy. Performance of the method is tested for benchmark problems. © 2011 by National Committee for IUTAM. Source

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