From the measurement of the acoustic pressure on a cylindrical, two-dimensional contour located close to the surface of an underwater, vibrating cylinder, the complete three-dimensional sound field can be reproduced (reconstructed) with the aid of a computer. This reconstruction technique, called GENAH (generalized nearfield acoustical holography), is unlike conventional holography because it provides a super resolution image of the sound-pressure field from the surface of the cylinder to the farfield. At the same time, GENAH reconstructs, from this two-dimensional measurement, the vector velocity and the vector intensity fields (energy flow) in the nearfield of the source, and identifies modes of surface vibration of the cylinder. Experimental results are provided and the accuracy of GENAH is demonstrated by comparison with the two-hydrophone technique.

The vibrational characteristics of a point-driven ‘‘double shell’’ (two concentric submerged cylindrical shells coupled by the entrained fluid) are investigated theoretically and experimentally. Of particular interest are the shielding effects, if any, of the outer shell upon the inner shell. The theory on the double shell is based on Flügge’s infinite-shell equations, the Helmholtz wave equation, and boundary conditions at the fluid–structure interfaces. This theory is used to model a finite double-shell structure in wave number-frequency space. Experiments are carried out in which generalized near-field acoustical holography (GENAH) is employed to provide the experimental vibration characteristics in wave number-frequency space of the finite double shell. It is confirmed theoretically and experimentally that the outer shell of the double shell exhibits two separate dispersion curves: A higher-frequency dispersion curve exhibits in-phase vibrations with respect to the inner shell, and a second lower-frequency curve, out-of-phase vibration. The higher-frequency dispersion curve of the double shell is very similar to the dispersion curve of the single shell (the inner shell without the outer shell), and thus is identified as a forced wave number response. The lower-frequency curve seems to be dependent on the free wave number response of the outer shell alone of the double shell. A double-shell structure can usually reduce its vibrational amplitudes by splitting the response of single-shell’s forced vibration into the responses of inner-shell’s forced vibration and outer-shell’s induced vibration. However, it radiates low-frequency underwater sounds inevitably according to the lower-frequency dispersion curve. Furthermore, the appearance of the inner shell’s dispersion curve on the outer shell seems to indicate that the shielding influence of the outer shell is not completely effective.

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The vibrational energy in a structure and the radiated acoustic energy are analyzed in the time domain using data from near-field acoustical holography measurements. A signal processing method is described that uses data from a single broadband acoustical holography measurement to determine the structural and acoustic responses for synthetic forces different from the original measurement force. The processing is based on the assumption that the measured acoustic pressure and particle velocity are related to the drive force by a linear shift invariant transfer function. Finite time length synthetic forces are examined for measurements from a fluid-loaded, point-driven, finite cylindrical shell. Comparisons of the amount of energy input to the structure by the driver, the energy injected into the fluid near field, and the energy radiated to the fluid far field show large amounts of energy that enter the fluid near field while the drive force is on, but then re-enter the structure where it is damped once the drive force is turned off. We are also able to see the rate that energy radiates to the acoustic far field and the areas of the structure which contribute most significantly to the far-field radiation. Results show that approximately 50% of the far-field energy is radiated during the active time of a transient driving force.