A technical approach by an acoustic impact method was discussed for non-destructive evaluation of defects on materials. Non-contacted detector is attractive in many areas of non-destructive evaluation, and empirically used in industries. A fast Fourier transform frequency analysis enables to evaluate the vibrational damping of the specimen, and to detect the change in frequency due to a defect such as a crack. The detective frequency is generally audible and depends on ability of auditory sense in many cases. In this paper a frequency spectrum analysis at relatively high frequency has been fundamentally conducted with a square plate of brass and stainless steel specimens. Artificial cracks and holes were put on a given portion of the specimens in order to investigate acoustic response. The change in intensity and frequency of the spectrum caused by a small defect was remarkably appeared in a range of high frequency, and proved the validity of this type of acoustic impact method. A theoretical approach was discussed for the frequency spectrum obtained for the brass specimen.
