Boomwhackers

When the tube is open, the air vibrates like so:

There is a point in the middle where the air doesn’t move. The air effectively vibrates only for half the length of the tube.

Now when the tube is closed, the air vibrates like this:

The air now vibrates at effectively double the length of the tube. A wavelength of double the size means the frequency is halved, and with that the tone is twice as deep.