Now consider the case of an ultrasonic pulse f(t) with a duration of 2T as shown in Figure 3. A second identical pulse is added and that the total delay between the two pulses is 2t0 as shown in Figure 4. They are shown separated in time for clarity. It is worth to note that these two pulses may overlap. In this study, the two pulses represent the reflection waves from the tip and mouth of the surface crack as shown in Figure 2.