Axisymmetric propagation in a liquid-filled elastic tube waveguide is considered, with emphasis on the two modes existing down to zero frequency. Previous work by Del Grosso is used as the basis of the theoretical description of modal phase velocities and particle displacement profiles in such waveguides. It is shown that certain combinations of material properties can produce a mode which, in the zero frequency limit, has plane-wave motion in the liquid. Two examples of waveguides with very different wall compliance, aluminum/water and PVC/water, are studied numerically and experimentally. Numerical calculations are used to show the frequency dependence of phase velocity in all waveguide modes and the radial dependence of complex particle displacement amplitude in the two low-frequency modes (ET0 and ET1). Contrasting behavior in the two waveguides is seen—approximate plane-wave motion in the liquid occurs in the ET0 mode of the aluminum/water waveguide, but in the ET1 mode of the PVC/water waveguide. Experimental measurements of the frequency dependence of phase velocity in the ET0 and ET1 modes of these waveguides are also presented. Good agreement with numerical predictions is obtained in both cases, although experimental difficulties more severely limit the frequency range of measurement in the PVC/water waveguide.

The use of single and double layers of piezoelectric material to form acoustically active surfaces for the elimination of reflected and transmitted waves is first discussed from a theoretical standpoint. Experiments are then described in which the piezoelectric composite piezorubber is used as the transducing material in single and double layer active surfaces. It is demonstrated that with the single layer, either reflection or transmission can be eliminated, while both reflection and transmission can practically be eliminated with the double layer. The electrical driving signals applied to the active surfaces in order to accomplish this are shown to agree well with theoretical models using material parameters for piezorubber determined from impedance measurements.

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Mössbauer spectra of several iron meteorites have been measured by a resonant scattering technique rather than by the conventional transmission method, thereby eliminating the necessity for the preparation of thin samples. No significant differences were observed in the spectra of specimens of mechanically deformed, shocked, and unshocked iron meteorites, nor in the absorption spectra of artificially shocked and unshocked fayalite.

The use of such materials as PVDF and piezorubber in making smart acoustically active surfaces is discussed and demonstrated. By using one layer as a sensor and applying the signal detected in this layer through an amplifier and filter to a second driving layer it is possible to control either the reflection or transmission of plane waves striking the surface normally. By using two driving layers, both transmission and reflection can be simultaneously controlled. When transmissions are eliminated the control of reflection is independent of changes in backing impedance. By adding a second sensing layer along with two driving layers, reflections and radiation from a vibrating surface can be eliminated simultaneously. Stability problems involved in using pressure and/or velocity sensors in various control algorithms are discussed.