The biwa, a family of Japanese lutes played with plectrum, produces a tone of peculiar timbre called the sawari tone caused by dynamic contact of the vibrating string with a small instrumental device called the sawari. This paper presents a physical model consisting of a linear string-resonator system to which the sawari is added so as to work as a unilateral constraint to the vibrating string. Included also is a stick-slip mechanism of plucking to simulate the so-called touch noise that exists as a precursor in the biwa tones (and in the tones of some other string instruments played with plectrum.) A numerical simulation was conducted in search for the condition that produces sawari-like tones as characterized by a unique temporal profile of the total amplitude and its constituting partials. The result showed that the system produces simulated tones that resemble a genuine sawari tone, called the reference tone, when the size of sawari is about the same as that of the instrument on which the reference tone was produced, under the condition that the model string is plucked at a point belonging to a specific portion off the bridge.
A reciprocal sound transmission experiment was carried out during June 10–13, 2015 along a cross-strait line in the Bali Strait with strong tidal currents to measure the vertical section structures of the range-averaged current and temperature at a 3 min interval. The five-layer structures of the range-averaged current and temperature in the vertical section were reconstructed by regularized inversion of the travel time data for two rays. The hourly-mean current showed the generation of nonlinear internal tides with amplitudes of 1.0–1.5 ms-1 and periods of 6 h superimposed on semi-diurnal internal tides with amplitudes decreasing from the upper to lower layer. The hourly-mean temperature was characterized by variations with amplitudes of 1.0–1.5°C and periods of 6 and 8 h. The current variations showed an out-of-phase relation between the upper and lower layers while the temperature data varied in-phase for all five layers. The two–day-mean current and temperature showed a stratified structure, varying from -0.6 to -0.1 ms-1 and from 23.8 to 28.0°C, respectively. The five-layer current and temperature were significantly above the inversion errors.
A time-dependent adjoint approach for obtaining sensitivity derivatives for shape and topology optimizations of two-dimensional acoustic metamaterials and phononic crystals is presented. The acoustic wave propagation problem is solved in the time-domain using a Streamline Upwind/Petrov Galerkin formulation. Topology parameterization is accomplished using the homogenization method, and shape optimization is subsequently used afterwards to fine tune the geometries. The combined strategy is compared with penalty-based topology optimization. The gradient-based design procedure is suitable for large numbers of design variables, and results are shown for achieving effective material properties and noise reductions, respectively.
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