*2.2. Action Points and Sensors Distribution*

Four sensors (No. 1–4) were distributed at the interfaces of each layer of combined coal and rock mass, and each sensor was at the midpoint of each interface. The lower surface of the sensor (signal receiving surface) coincided with each interface of coal and rock mass. An action point was set every 100 mm on the upper surface of the fine sandstone for percussion excitation. An iron block of 80 × 80 mm was placed at action points to prevent damage to fine sandstone during excitation; a total of five vertical action points (1–5#) and four inclined action points with an angle of 45◦ to the horizontal direction (6–9#) were arranged, as shown in Figure 2.

**Figure 2.** Action points and sensors distribution in combined coal and rock mass.
