The convergence characteristics of the time-domain filtered-x LMS algorithm are subject to the distribution of eigenvalues of the autocorrelation matrix of the filtered reference signal. However, evaluation in the time domain does not lead to easy understanding of the physical meaning. This paper presents that the behavior of the filtered-x LMS algorithm in the time domain can be evaluated in the frequency domain if the adaptation process of the filter is slow. The evaluation method presented here facilitates a better understanding of the physical meaning, that is, the convergence characteristics of the algorithm can be evaluated at each frequency (FFT) bin separately, and the effect of the step-size parameter μ, the power gain of the secondary path, and the power spectrum of the reference signal can be evaluated individually at every frequency bin. Furthermore, the evaluation method of the behavior of the adaptive filter in the frequency domain requires fewer computations as compared with those in the time domain. Results of computer simulations of the convergence characteristics of the time-domain filtered-x LMS algorithm estimated in the frequency domain agreed well with the theoretical results.

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Sound speed that is the response of media to sound propagation provides important information to evaluate elastic characteristics quantitatively. It is desired to develop precise measurement technique of sound speed. Sound speed of media can be determined by measuring travel time of the sound traversing media when the distance between transmitter and receiver is known. The zero-crossing method is popular to measure the travel time using sound pulse. This method is useful in measuring sound speed of homogeneous medium. In homogeneous medium, the wave form of received signal is almost independent of the travel distance of the sound pulse. In inhomogeneous medium, the wave form of received signal is depend on the characteristics of media. The measured sound speed by the zero-crossing method means the speed of early received sound. In this paper, an extended zero-crossing method is proposed. In order to evaluate the time interval, some time range within pulse signal is considered for processing. It increases the reliability of measured sound speed, and is helpful to consider on the physical meaning of measured sound speed of inhomogeneous media.

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An evaluation dealing with the measurement of flow rate in duct by means of the frequency shift of the transmitted wave through an intensively generated Karman vortex street is presented. Using a simplified model of a vortex crossing the ultrasonic beam of a disk source, the propagation of ultrasonic wave in water with a vortex is examined and the effect of the vortex on the phase delay is investigated as well. The frequency shift of the transmitted wave is therefore obtained. The frequency shift has been found to become larger in magnitude while the vortex is passing the axis of the disk source. To explain and analyze numerically the mechanism of the measurement, the frequency shift is evaluated versus several kinetic parameters of the vortex separately, such as the rotational direction, the moving speed and rotational speed of the vortex, which are assumed to be determined by the flow rate under a certain measurement system. Consequently it is anticipated that the flow rate can be predicted from these kinetic parameters with the help of the fluid mechanics. Finally, the frequency shift affected by the distance and dimension of the vortex with respect to the disk source is discussed for the design and analysis of the measurement.

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This paper describes a new design method of dynamic range controllers, such as compressor, limiter, expander and noise gate, on a DSP for digital audio systems. For the compressor, we have previously proposed an efficient implementing method on DSP by calculating the polynomial expression to approximate the power transformation. Based on the approximation algorithm, we extend the positive compression ratio to the negative compression ratio for the compressor. Then, we design an expander based on the facts that the ideal compression characteristic could be obtained by application of a polynomial expression. By defining an expansion (downward expansion) characteristic with two threshold levels, the gain for getting the expansion may be decomposed into a gain of an upward expansion with an integer expansion ratio, a gain of a compression and gain shift. By this method, the ideal input-output characteristic can be achieved by using seven-order polynomial expression for dynamic range control of 50dB. The digital signal processing on DSP is about 130 steps for the compressor, and 120 steps for the expander with a given expansion ratio equal to or smaller than 15.

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When a high speed train enters into a long tunnel, a tunnel pressure wave occurs. When the tunnel pressure wave propagates and approaches the tunnel exit, low frequency noise radiates from the exit and causes vibration of residential dwellings. Based on a model experimental system, this paper proposes an original way to attenuate positive pressure gradient of tunnel pressure wave with a negative pressure gradient generated by positive pressure and discusses how to realize it. Computational simulation indicated that attenuation of the positive gradient of tunnel pressure wave due to the negative pressure gradient of positive pressure is effective.

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An aim of this paper is to compare a proposed simple model for the target strength (TS) of swimbladder fish with existing models. Through the comparison we verify the validity of the simple model. Particularly, two more sophisticated models are compared with: namely, the exactly solvable soft spheroid model and the ray-Kirchhoff approximate model. The comparisons show that at high frequencies or intermediate frequencies the present model can yield results in reasonable agreement with that from the other two models respectively. Indirect comparison with experimental data is also briefly made. It is shown that the model may facilitate general calculation of the target strength of swimbladder fish.

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