Remote acoustic detection of mechanical changes in a vibrating plate in an unknown reverberant environment

Acoustic radiation from a vibrating mechanical structure subject to broadband forcing is inherently dependent on the structure's material, geometry, and boundary conditions. Remote measurements of radiated sound can be used to detect mechanical changes (i.e., defects) when compared to known baseline measurements from the same structure. However, proper determination of a structure's acoustic signature may not be possible in highly reverberant environments due to reverberation contamination. Herein, experimental results are presented for the remote acoustic detection of clamped-boundary defects in a nominally 30 × 30 × 0.3 cm aluminum plate in a reverberant environment. Synthetic Time Reversal (STR) is used to estimate the free-field acoustic signature of the plate from recordings made in a reverberant environment with a 15-element microphone array at signal-to-reverberation ratio levels of -7 to -13 dB in a 100 Hz to 2.0 kHz bandwidth. These reconstructed time domain signals are then cross-correlated with baseline measurements of a known fully-clamped plate and classified as either changed or unchanged. Using common classifier statistics, this approach to remote acoustic damage detection using STR is found to be superior to equivalent waveform correlation approaches based on unprocessed signals and conventional time-domain beamforming outputs.