The “Helmholtz resonator” concept explains the frequencies of sound produced by clapping the hands together in different configurations.

We wanted to explore how we generate the sound depending on how we clap our hands." The researchers used high-speed cameras to track the hand motion, air flow and sound of 10 volunteers clapping ...

You stand a long distance from a wall, clap, and listen for the echo. The distance travelled is twice the distance from you to the wall (because the sound has to travel to the wall and back).