The singly flagellated bacterium, Vibrio alginolyticus, moves forward and backward by alternating the rotational direction of its flagellum. The bacterium has been observed retracing a previous path almost exactly and swimming in a zigzag pattern. In the presence of a boundary, however, the motion changes significantly, to something closer to a circular trajectory. Additionally, when the cell swims close to a wall, the forward and backward speeds differ noticeably. This study details a boundary element model for the motion of a bacterium swimming near a rigid boundary and the results of numerical analyses conducted using this model. The results reveal that bacterium motion is apparently influenced by pitch angle, i.e., the angle between the boundary and the swimming direction, and that forward motion is more stable than backward motion with respect to pitching of the bacterium. From these results, a set of diagrammatic representations have been created that explain the observed asymmetry in trajectory and speed between the forward and backward motions. For forward motion, a cell moving parallel to the boundary will maintain this trajectory. However, for backward motion, the resulting trajectory depends upon whether the bacterium is approaching or departing the boundary. Fluid-dynamic interactions between the flagellum and the boundary vary with cell orientation and cause peculiarities in the resulting trajectories.

Active acoustic shielding (AAS) is the system that can attenuate a sound passing through an open window. AAS is constructed from a number of AAS cells set in an array with having an approximately collocated microphone and speaker. Each AAS cell is individually controlled by a single-channel feedforward method. The concept of AAS was proposed by the authors and its feasibility was demonstrated in the previous report by performing some simple simulations and experiments. In this study, an AAS window with four AAS cells in a small open window was manufactured and installed in the door of a test room. Its noise-reducing performances were measured for noise from outside. The effects of multiple noise sources, moving noise sources and reflected sound in the room were also examined. As the results, the AAS window is demonstrated to be able to attenuate not only normal incident sound but also oblique incident sound in the frequency range from 500Hz to 1.5 or 2kHz. Moreover, noise reduction is obtained over a wide area in the room. Additionally the AAS window is also effective for multiple sound sources and moving sound sources.

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