Figure 2 shows signals simulated for a configuration similar to the one of Figure 1 with the probe 10mm away for
the specimen, with two probe positions: above the side-drilled hole and shifted 25mm sideways. All models predict
similar shapes for the signals at the first position, but at the second one the effects of probe diffraction that spread the
signals are only predicted by the finite elements and ray-based models. The plane-wave approximation is unable to
reproduce it because a single plane wave implies a single time-of-flight, whereas probe diffraction is related to the
width of the probe and the range of times-of-flight it implies.