Various structural measures against vibration and noise were taken in a training ship, the Oshima Maru. However, an unpleasant sound persisted in the mess hall, where crews take their breaks. To reduce the noise, active controllers were investigated to satisfy the causality constraint in their update. Some of the controllers were preconditioned using the inverse of the plant because their convergence rates are limited by the dynamics and coupling within the plant response. The overall response from the output of the control filter to the output of the error sensor is thus equal to the all-pass part of the plant response. Because this response has a flat magnitude, the convergence speed of the adaptive algorithm is not affected by resonances in the plant response, as it is for the normal filtered reference LMS algorithm. The algorithms were compared under the same conditions to investigate differences in their properties and also corrected to satisfy the causality of their update processes. A comparison of convergence properties shows that satisfying the causality constraint in their update results in an improvement, and consequently the effect was confirmed by the filtered reference - filtered error LMS algorithm. Simulations are presented for a control system that was introduced using plant responses measured from a loudspeaker to a microphone in the mess hall inside the Oshima Maru. After investigating the convergence speed in various gradient descent adaptation algorithms, the results were integrated with the actual plant response and applied to the active control of ship interior noise. It was also found that although the preconditioned LMS algorithm converges dramatically faster than the ordinary gradient descent adaptation algorithms with an accurate plant model, its convergence rate is still sensitive to the auto-correlation and cross-correlation properties of the reference signals.
It has been recently reported that the physical properties such as the porosity and the density in the surficial marine sediment vary largely with the depth. The characteristics of acoustic wave reflection from such transition layer of the surficial marine sediment seem to be very different from that from homogeneous sediment. In this study, the incident angle dependence of the reflection coefficient for the transition layer of the surficial marine sediment model is calculated using OASES (Biot-Stoll model). The effects of the transition layer and the frequency dependence of the reflection coefficient characteristics are investigated. Next, the characteristics of the incident angle for the reflection coefficient are measured in water tank and in situ. Beach sand (fine sand) are used for water tank measurements, and in situ measurements are done in a very fine sand bottom. The operating frequency is 150 kHz. The incident angles are 0, 20, 50, and 60 degrees. The measured results are compared with the calculated results using OASES (Biot-Stoll model), and the effects of the transition layer on the characteristics of the reflection coefficient are investigated.
A phonemic perception was assumed to be a process in which representation of speech signals is matched to phonemic representation in long-term memory. Since stimulation is supposed to activate relevant memory representation, priming effects were compared between speech sounds and their sinewave analogs, either in the speech mode or nonspeech mode of perception. An analog target imitating syllables /ba/ or /ga/ in Session 1 or their original speech target in Session 2 was preceded by one of 5 primers, which were these 4 target sounds, and a 1,000 Hz pure tone, with one of 6 interstimulus intervals (ISIs). In Session 1, half of the participants were asked to identify analogs as ill-synthesized syllables, “BA” or “GA,” and the other half were asked to label them as computer sounds, named “A” or “B.” In Session 2, all of them identified original syllables. Regardless of target types and ISIs, subjects given speech instructions showed faster identification of targets preceded by phonemically identical primers, while subjects given nonspeech instructions identified targets faster only when targets were preceded by physically identical primers. These results support that speech sounds and their analogs are accessible to common memory representation.
The systematized computer simulation of the short ultrasonic pulse system is performed using a transmission line model. Optimum condition of the acoustic impedance and thickness of the intermediate layer are considered first. The effects of electrical source impedance for the ultrasonic pulse waveform are also discussed. From the simulation results, the conventional quarter wave acoustic intermediate layer does not always realize optimum pulse performance. The importance of electrical source impedance in pulse performance is also emphasized.
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