This also suggests that the two measurements actually
contain similar information. However, a close examination
of the impact data reveals some interesting points. First, as
the interesting part of the force-time history is much
shorter than the sound duration, there may be some additional
potentially useful information in sound data, and
this additional or redundant information may lead to more
reliable defect detection when properly used. On the other
hand, it is difficult to use the sound signal to deduce the
impact amplitude, so the force-time history profile shape is
still the best indicator both to the under-structure complexity
and to the impact amplitude.