Humming pipes

HOW

1. Place your ear to any pipe.
2. Listen to the humming noise.
3. Listen to the humming noise of the other pipes.

WHY

In each pipe, the air is stimulated to vibrate by sounds coming from the surroundings. Some sounds are amplified in the pipe. Mostly those with a length matching the pipe length. Longer waves are therefore amplified in longer pipes. They are associated with low pitches. Shorter pipes amplify shorter waves and therefore higher pitches.