Non-destructive and Non-contact Test Method Utilizing Sound Intensity

A new method for detecting the flexural vibration in a test object is proposed. A loudspeaker is set in front of the test object and driven to apply sound waves to the object surface. An intensity probe is placed close to the surface front to measure the sound intensity in the direction from the loudspeaker to the object. When there exists no flaw and the object surface is rigid enough, almost all the acoustic power incident from the loudspeaker is reflected. The measured sound intensity is then zero. In contrast, if there is a flaw inside the object and the specific frequency of the flexural vibration of the surface coincides with the applied sound frequency, a flexural vibration is excited in the object; part of the acoustic power incident upon the surface is dissipated in this internal flexural vibration and the measured sound intensity becomes a non-zero value. Therefore, a flaw in an object can be detected by measuring the sound intensity at the front of the object surface while varying the frequency of the sound radiated from the loudspeaker. An experimental apparatus was built which employed two condenser microphones as the intensity probe. Several model experiments were performed and the results showed the success of the proposed method.