Coin-tap test reliability should be further improved if
we can fuse these two sets of data. For example, a common
case is that there are multiple interactions between
the hammer and the tested surface in a nominally single
tap. The force-time history is very sensitive to this situation,
but then the curve will become so asymmetric that
Cawley’s single-spring model is no longer valid. Thus
sound data would be more useful, although they are less
sensitive. Figure 4 shows how the force-time history and
the sound data behave in such a situation.