This paper presents experimental investigation on acoustic characteristics of a contraction of area in a pipe with mean flow. Parameters describing the characteristics of the contraction are reflection coefficients of the inlet and outlet of the contraction and the transmission loss. These values are estimated from the experimental results comparing with calculated results. At first, fundamental parameters on the sound transmission in the pipe with mean flow, for instance, the viscothermal attenuation coefficient and the reflection coefficient at the open end of the pipe, are determined by experimental results of the standing waves in the pipe having the constant cross-section. And then, the acoustic parameters of the contraction are estimated by comparing the experimental results of the standing waves with the calculated results of these by numerical simulation of the reflection and the transmission coefficients at the inlet and the outlet of the contraction.
This paper presents the experimental results on a propagation characteristics of elastic waves and vibration isolation effects of an open trench. First, elastic wave propagation in the gelatine gel and the reflection and diffraction phenomena of the waves by trench have been observed. Next, the surface wave propagation characteristics and vibration isolation effects of the trench have been measured using the elastic half-space model made of the chemical grout gel. The experimental results have been compared with the theoretical values of the stresses radiated from surface point source in the half-space. Some of the results obtained are as follows. Vibration isolation effect of the trench is affected by the values of the stress just below the bottom of the trench at the arrival of the waves. It is effective for vibration isolation to arrange a trench as its bottom exists on the direction that the stresses of the waves are smallest and also effective to set a trench as far as possible from the vibration source.
In this paper an analytical solution as a separable function of azimuth angle and axial dimension is presented in order to calculate the acoustic radiation patterns from a finite source of uniform amplitude, vibrating sinusoidally, on a rigid elliptic cylinder of infinite length. The approximate solution at far field is obtained by the saddle-point method. It is shown that the pattern in a plane orthogonal to the axis of the elliptic cylinder is independent of the axial distribution of the source, and is identical to the pattern for a source of infinite axial extent. Acoustic radiation patterns in the planes, orthogonal to and containing the axis of the elliptic cylinder, and acoustic radiation impedance are computed numerically for the particular cases in which a finite source located on the major and/or minor axis of the ellipse. Acoustic radiation characteristics of azimuth and axial extent of the source is discussed in comparison with point source approximation.
Normal incidence absorption coefficients α of samples of metal-fibrous porous absorbing materials, whose thickness (d) is comparatively thin and specific flow resistance (R) is different, are measured in the atmosphere of high temperature between room temperature and 400℃. The change of absorption characteristics with temperature is investigated also theoretically. Because all of the characteristics of such a sample with back-air space show peaked forms, in this paper, aiming at the maximum absorption coefficients α_<max>, the resonant frequencies f_0, and the bandwidth in the neighborhood of f_0, these changes with temperature are inquired into closely. The experimental results indicate that f_0 and bandwidth always considerably increase as the temperature rises for every sample, on the other hand, that the change of α_<max> differs with each sample and shows the behavior of decrease or increase according to the value of unit-area specific flow resistance R_f (=R×d) at room temperature. It is found that these characteristics changes are explained quantitatively by using the Beranek eqation of α calculated from specific flow resistance (R) and considering temperature changes of sound velocity in the air (c), density of air (ρ), and coefficient of viscosity of air (η) included in the equation.
A new concept based on K=UH, a product of total mechanical energy U and entropy H as employed in our recent papers, has been introduced in this paper. This is done because the entropy of vibration by our definition is unsuitable for demonstrating complexity when comparing vibrations with remarkably different total energies. To consider general properties associated with vibrational complexity, including sound fields, the authors discuss two damped vibrating systems using H and K calculated from normal modes. One system has reactions directly proportional to its velocity, the other energy dissipation due to internal friction. Especially in the latter, the basic assumptions in the S. E. A. method are examined for rectangular plates. Subsequnetly, both H and K are shown to have certain properties for excitations in which driving frequencies are outside the frequency regions of normal modes yeilding dominant contributions. K is generally sensitive compared to H, particularly with any dynamically changing total energy, and able to express the megnitude of U and information on modal distribution of energy. Application of H and K to the S. E. A. method would supplement its theoretical foundations.