Narrow Results

An efficient method to eliminate redundant frequencies present in one of the existing multi-frequency methods for analog fault diagnosis is proposed in this paper. First the two-dimensional fault dictionary is constructed where entries are gain signatures of all faults and frequencies. The faults belonging to the same quantization levels are numbered sequentially and a frequency that has an ambiguity set with the highest number faults is eliminated after verifying that there are no repetitions after the deletion of this frequency. In this manner, all frequencies are examined for deletion. Finally the test frequencies, which cannot be deleted, remain resulting in a minimal set of test frequencies of a network to isolate a given set of faults.

Another method proposes a technique, which generates more number of frequencies to isolate all the faults, if the test frequencies generated using the existing methods are not sufficient.

Multi-frequency calculation is usually very time-consuming due to the repeated numerical integration for numerous frequencies in acoustic scattering or radiation problems. A series expansion method has been proposed to speed up this process just by taking the frequency-dependent terms out of the integral sign. However, this method, constrained by the number of truncation terms, is only applicable to low and medium frequencies and/or small-size structures. This paper develops an improved series expansion method that can be employed in a wider frequency band and larger-scale problems but with less computing expense. In the present method, the frequency-dependent term kr in the integral kernel is firstly transformed into the range from -π to π due to the periodicity of sine and cosine functions. Afterwards, truncation error would be kept reasonably small while the number of expansion terms would not increase with kr. Test cases of acoustic radiation from a pulsating sphere and a cat's eye structure are conducted and numerical results show significant reduction of computational time but suffering little accuracy loss for multi-frequency problems with this approach.

The implicit frequency dependence of linear systems arising from the acoustic boundary element method necessitates an efficient treatment for problems in a frequency range. Instead of solving the linear systems independently at each frequency point, this paper is concerned with solving them simultaneously at multiple frequency points within a single iteration scheme. The proposed concept is based on truncation of the frequency range solution and is incorporated into two well-known iterative solvers - BiCGstab and GMRes. The proposed method is applied to two acoustic interior problems as well as to an exterior problem in order to assess the underlying approximations and to study the convergence behavior. While this paper provides the proof of concept, its application to large-scale acoustic problems necessitates efficient preconditioning for multi-frequency systems, which are yet to be developed.

Cognitive Radio is a promising technology, which has been applied to the underwater acoustics to optimize the utilization of spectrum and the reliability of communication. This paper reports a design proposal of a multi-frequency modem for underwater cognitive acoustic communication. The structure and function for the physical layer of the modem is described. An engineering application scheme is also proposed to provide a beneficial reference for the field of crack detection of underwater structure.