The researchers studied how changes in gas pressure affect the speed of sound waves in coal, which can help monitor changes in coal porosity and permeability. They used a special system to measure sound wave speed and swelling in coal samples. They found that both pressure and swelling affect sound wave speed by changing the amount of empty space in the coal. The results matched well with mathematical models of coal permeability. The main factor affecting sound wave speed changes depending on the pressure conditions. Swelling has a bigger impact at high pressure and low gas pressure, causing the sound wave speed to increase. This matches previous models that predict increased permeability at low porosity levels.