Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
5. Things That Catch Fire or Go Bang
Why are explosives so loud?
We hear sounds when air molecules push on our eardrums. The harder they push, the louder the sound.
To create sounds, air must be pushed. The faster the air is pushed, the more pressure that is created, since the air has less time to get out of the way.
If you push on the air with your hand, you can feel the pressure of the air on your face, but you won’t hear any sound because your hand is moving too slowly. To make a sound, the air must move faster than it can get away. You can do this by clapping your hands together. That traps air between our hands, increasing the pressure. The air molecules can’t get away fast enough, so they all get crowded together.
When an explosive is set off, it produces a lot of molecules of gas in a very short time. These molecules bounce into air molecules and make sound waves. The faster the explosive burns, the louder the sound waves will be.
A firecracker moves air faster than your clapping hands can move air, so it sounds louder than hand clapping. Dynamite explodes faster than gunpowder, so it makes an even louder sound.