Time‐based energy analysis of acoustic radiation and structural vibration using generalized near‐field acoustical holography measurements

Abstract

A time‐based energy analysis of structural vibration and radiation is carried out with near‐field acoustical holography measurements. By using the force driving the structure as a reference for the measured pressure, the impulse response can be determined over the frequency range of the force signal. Assuming that the signals are linear and shift invariant in time, the structural vibration and radiated acoustic field due to a synthetic force are determined. This is done by multiplying the frequency domain descriptions of the measured impulse response and the synthetic force. The signal processing, which will be outlined, occurs in the frequency domain. The results are then transformed into the time domain via discrete Fourier transforms where the analysis is done. Thus, from a single measurement, the response to force time histories other than the original measurement force can be studied, assuming that the new force signal can be described with the frequency range of the original measurement. Results for a fluid‐loaded, point driven finite cylindrical shell will be shown, with emphasis placed on examining the energy radiated to the acoustic far field, the energy in the acoustic near field, the energy delivered by the driver to the structure, and the energy absorbed by damping in the structure. For finite time length forces, a large amount of acoustic energy enters the structure rather than radiating to the far field. This time‐based analysis provides many insights into the mechanisms of fluid/structure interactions that are not observable by a purely frequency based analysis.