Slap Pipes

Principle of Science: Resonance

Slap the open tubes of varying length. Slapping the tube causes the air inside to vibrate at the tube’s natural resonant frequency. Notice that the pitch of each tube depends on its length. Try to play scales or a musical melody.

The frequency or rate of vibration of an air column (its pitch) depends on the length of the column. The smaller the column of air, the faster it vibrates, and the higher the pitch. The human ear can hear frequencies of sound from approximately 20 hertz, or vibrations per second, to 20,000 hertz. A 20 hertz sound is extremely low pitched, while sounds near 20,000 hertz are so high pitched they are uncomfortable to the ear. Dog whistles produce sounds over 20,000 hertz so they are inaudible to humans, but well within a dog’s hearing range. The pitch of the notes in a musical instrument is governed by the same principle. Smaller instruments usually produce higher notes than larger ones. Think of the difference in sound between a flute and a tuba.