Figure 8.1.1: The recording of a near field (N wave form) sonic boom at DIA. Coherency as a function of time and frequency is plotted in the lower frame. The best beam for the resolved source characteristics is shown in the top frame.

Based on its N wave form, the energy is identified to result from a plane flying through the sound barrier. The N wave is formed at a certain distance from the source, generating the characteristic double bang. The actual wave shape follows from compression at the front of the plane and expansion of the air at the back. The amplitude of the N wave (i.e. 5 Pa) is large, indicating a direct or carpet zone arrival.
The source is located through cross bearing. The bearing of DIA is combined with the bearing obtained in De Bilt (DBN), see figure 8.1.2.

Figure 8.1.2: Cross bearing for source localization

The distance from the source to DIA is 76 km as derived from the cross bearing analysis shown in figure 8.1.2.
ECMWF atmospheric data is used to build a velocity model of the atmosphere. The data from ECMWF is validated by balloon measurements at the KNMI in De Bilt. The correlation between ECMWF and KNMI data gives confidence in the model up to 25 km height. Balloon measurements are not available for heights larger than 25 km.

Figure 8.1.3: Results of modeling through raytracing. The velocity model is shown in the lower frame. The effective sound speed (wind and temperature effects) in brown, and the temperature depend sound speed, in green, are shown to the right of the lower frame. In white are the various rays plotted. The rays depart from an estimated source height of 10 km, at intervals of 5 degrees from the vertical.
The top frame shows the travel times for rays reaching the surface, in red, also the best beam for DIA is plotted in this frame.

The results from raytracing through the atmospheric model are shown in figure 8.1.3. The lower frame shows the effective sound speed indicated by various colors. To the right of the lower frame, the effective sound speed (in brown) and temperature depend sound speed (in green) are plotted. It follows from the velocity model that at a height between 25 and 35 km, a velocity gradient is present. Therefore, upward traveling rays will refract from these height, bending towards the surface. The rays are also plotted in lower frame. The source is locate at 10 km height and emits infrasound through rays at a 5 degree interval with respect to the vertical.
The travel times for rays reaching the surface are shown in the top frame, in red. The carpet zone exists up to a distance of 50 km. The zone where the first refracted waves arrive, starts at 95 km. DIA and its best beam are plotted in the top frame, at a distance of 76 km as follows from the cross bearing analysis. Although, DIA recorded a clear and strong N wave arrival, it appears to be located within in the shadow zone. Either the velocity model or applied raytrace theory fails to explain the clear observation. Accounting for turbulence and/or inhomogeneities might solve this ambiguity.