Noncontact inspection using identifying each resonance frequency of defect and laser Doppler vibrometer by spatial spectral entropy

In our noncontact acoustic inspection method, we have studied a noncontact non-destructive method to detect and visualize internal defects (depth to 10 cm) of composite material (especially concrete) from a long distance (5 – 33 m). The target surface is excited by aerial plane sound waves by a long-range acoustic device (LRAD) and the vibration velocity distribution on the two-dimensional lattice points is measured with a scanning laser Doppler vibrometer (SLDV). Conventionally, after signal processing, internal defects were detected by combining acoustical feature quantities (vibrational energy ratio and spectral entropy) [1,2]. This time, we propose spatial spectral entropy (SSE). SSE can automatically detect resonance frequencies of internal defects, and at the same time, can detect resonance frequencies of a high sensitivity SLDV. Using SSE analysis, internal defect detection and clear defect imaging were achieved for an actual concrete structure.