Detection Effect of Resonance Frequency of Both Laser Doppler Vibrometer and Internal Defect of Concrete Structure by Spatial Spectral Entropy

In our noncontact acoustic inspection method, in order to detect internal defects (to a depth of about 10 cm from the surface)of concrete at the distance (5-30m)away, we have investigated the noncontact and non-destructive method. A flexural vibration, under the same principle as a hammering test, is induced by strong aerial sound waves excitation, and the vibration velocity distribution of concrete surface is measured using a laser Doppler vibrometer (LDV). Detection and visualization of internal defects became possible. However, in closed spaces such as underground large cavities, the resonance peak of LDV increases and the frequency band widens due to reverberation from surrounding concrete. It affects detection accuracy and visualization of internal defects of concrete. In order to except LDV resonance frequency band and perform data analysis, we propose “Spatial Spectral Entropy (SSE)”, that applied spectral entropy to real space of the measurement surface. As a result, not only the LDV resonance frequency and its frequency band but also the resonance frequency of an internal defect of concrete can be automatically detected. Further, both resonance frequencies can be distinguished.