How does the vibration of a drumhead cause a sound wave to form?

As the drum head moves one way, it disturbs the air molecules near it, causing some of them to bunch closer together, increasing the pressure. This higher pressure area starts to affect the air molecules near it, so the high pressure area starts to move away. When the drum head moves in the other way, air molecules will move toward it to fill the space created. This makes a low pressure region. The alternating high and low pressure regions is the sound wave, traveling through the air.