High speed non-contact acoustic inspection method using long distance acoustic irradiation-induced vibration

Abstract

The non-contact acoustic inspection method using air-borne sound can detect the cavity defect and crack of inside concrete near the surface. As a feature of this technique, focusing on the physical phenomenon of flexural vibration of defects, we succeed in improving energy efficiency to vibrate the defect effectively. It is possible to measure all day using a small dynamo. Moreover, using a low-output laser Doppler vibrometer (LDV), high signal to noise (S/N) ratio is achieved by our proposed broadband “Tone Burst Wave” and “Time & Frequency-gate (T-F gate).” Therefore, although a possible distance from a sound source and a LDV to target is dependent on output power of acoustic radiation, it was already confirmed that a measurement of long distance over 30 m away is possible under the condition that a target is excited efficiently. In order to shorten measurement time, we propose the “Multi Tone Burst wave” that fully utilized the principle of T-F gate. After many experiments using a concrete test objects and some experiments of real concrete structures (bridge, etc.), we confirmed the validity of high-speed measurement technique of the “Multi-Tone Burst wave.”