A new method for measuring acoustic velocities of thin materials is proposed in a VHF range. The principle is to measure the phase changes, as interference fringe-shifts, caused by inserting a specimen in a liquid whose acoustic velocity is known. In this method, a specimen immersed in a liquid cell is illuminated obliquely by plane waves which are launched from a plane ultrasonic transducer. Ultrasonic waves transmitted obliquely through the specimen are detected and changed into an electrical signal by a focussing ultrasonic transducer, which is scanned mechanically in a raster pattern to produce interference fringe patterns on the CRT display by combining an electrical reference signal with the received electrical signal. Acoustic velocities of some polymer films and a biological tissue, for example, are measured in a frequency range between 160 and 190 MHz. Measurement errors are discussed for this experimental system.
In this paper, a new evaluation method of sound transmission loss of general N-fold walls is proposed by use of the improved statistical energy analysis (S. E. A. ) method as reported in our previous paper. In that paper, the sound transmission loss of a general double-wall was clearly explained in the low frequency region by taking additionally the proper non-resonant power flows into consideration to the primary S. E. A. method as developed by Crocker and Price. This improved S. E. A. method is exactly appropriate for the evaluation of wall transmission losses in both the low and high frequency regions under an actual situation with finite dimensions of transmission room, reception room and panels. On the other hand, this situation is unable to be properly evaluated utilizing the normal methods, i. e. , by the usual wave equation theory, or the equivalent circuit theory. From the above viewpoint, the effectiveness of our general evaluation method of N-fold wall transmission loss by this improved S. E. A. method has been theoretically and experimentally confirmed for many kinds of N-fold wall structures, such as the ordinary (parallel) triple-wall, the absorbent (parallel) triple-wall, the ordinary (parallel) double-wall and the non-parallel double-wall.
A conical shell has been utilized as a sound radiator of direct radiator type loudspeakers and its vibration has a main role in their frequency characteristics. It is important to understand the vibratory behabiour and to find the natural frequencies of conical shells. They are the primary information which designer requires. The aim of the present work is to provide these fundamental knowledge or a perspective of axisymmetric vibrational modes and the associated eigenvalues of conical shells through the numerical calculation based on the finite difference approach. Modal shapes for the shells with fixed-free ends are presented for some apical angles. The effect of the apical angle, the meridional length and the thickness of the conical shells on their eigenvalues, coupling of the bending vibrations to the extensional and the driving-point admittance at their inner edge are discussed.
Experiments of the degenerate parametric amplification were done in a plane progressive wave tube. Due to the parametric interaction of a powerful pump wave and a weak signal wave, SPL of the weak signal wave was about 2dB higher than that of the pump-free wave in postshock region. By the use of an amplitude-modulated signal instead of the added signal at the source, a 2dB improvement in gain was obtained in the parametric sound pressure amplitude. It was also observed that the initial phase of the weak signal wave governs its propagating behavior remarkably and that the difference of amplitudes between the pump and the weak signal waves plays an important role in the parametric amplification.
The soundboard without strings in the low frequency region vibrates nearly equal to the isotropic plate for correcting the anisotropy of the wood plate by ribs. In the higher freuency region the local character of the soundboard appears. The boundary condition in the low frequency region lies between fixing and support. The higher the frequency is, the more the boundary condition closes to fixing. The influence of stretching strings is that the resonance frequency of each mode becomes high and Q becomes low. The character of the soundboard is shown in quantity by measuring the driving point character at the positions of string support on the bridge. Moreover it is important factor for finding the influence on the strings. In the low frequency region the fundamental mode is put down and many partials are built up. In the middle frequency region the higher mode vibration than the fundamental vibration of the soundboard is used, so that the fundamental mode is no matter what it may be. In the high frequency region the admittance becomes high by taking the vibration between ribs. The acoustical character when the soundboard is driven with a constant current is studied. The directivity of sound pressure is considered.