Detection of resonance frequency of both the internal defects of concrete and the laser head of a laser Doppler vibrometer by spatial spectral entropy for noncontact acoustic inspection

Abstract

In order to detect internal defects of concrete, we have studied noncontact non-destructive acoustic inspection and have detected and visualized internal defects using acoustic vibration excitation on the target surface by aerial sound waves and two-dimensional vibration velocity distribution by a laser Doppler vibrometer. Because a high sensitive scanning laser Doppler vibrometer (SLDV) is used, as the sound pressure increases, the reverberation of sound waves from the surroundings may cause resonance in the galvano mirror system of the laser head of the SLDV. For real concrete structures, for example a tunnel inner wall (distance about 5-10 m), there was no problem because the sound pressure was not so large. In the case of a viaduct, it was measured remotely (about 30 m), but because it is surrounded released space, the influence was not so large. However, in a closed space surrounded by concrete, the sound pressure needs to be increased as the distance increases, and the influence of the reverberation from the surroundings on a laser head cannot be ignored. Therefore, we propose a method to solve them, and at the same time, detect the resonance frequency of internal defects and improve the visualization of the defect.