2.2.2. Top Floor Level

The wireless AE system was installed at the top floor level (also known as +48 level) to monitor a vertical crack that may penetrate an exterior wall, which is shown in Figure 4, using four resonant sensors (type PK6I). Sensor layout and spacing is also shown in the figure.

**Figure 4.** Photographs at top floor level (+48 level): (**a**) Sensor grid from interior and (**b**) vertical crack from exterior.
