Bottle music: high and low notes
Blow across the top of a bottle and tap it with a spoon, with more and more water inside. Find what vibrates each time, and how the water changes the note.
Core concept
In short: Tap bottles with different amounts of water and hear the note change.
Sound is made when something vibrates; the faster it vibrates, the higher the note (pitch)
Topic: Sound
Every sound is made by something vibrating, moving quickly to and fro. A madal skin, a guitar string and the air in a bansuri all vibrate when they play. How fast something vibrates sets the pitch of the note: fast vibrations give a high note, slow ones a low note. We count vibrations in hertz (Hz): one vibration a second is 1 Hz. In this lab you play a glass bottle two ways. Blow across its top and the air inside the bottle vibrates. Tap it with a spoon and the glass and the water in it vibrate. Adding water changes the note, but not in the same way each time!
What you will learn
- Explain that sound is made by vibration.
- Find what vibrates when a bottle is blown and when it is tapped.
- Explain how the amount of water changes the pitch in each case.
- Link the length of a vibrating air column to its pitch.
Before you start
- Know that we hear sounds with our ears.
Materials
- A glass bottle (30 cm tall), water
Equipment
- A metal spoon, a ruler, a sound-meter app that shows the pitch in Hz (if you have one)
Procedure
- Blow across the top of the empty bottle and record the note.
- Fill the bottle almost to the top, blow again and record.
- Blow across the bottle at four or more different water levels and record each.
- Tap the empty bottle and the full bottle with a spoon, and record both.
How this simulation is simplified
- The bottle is 30 cm tall from the bottom to the mouth.
- Blown, the air above the water vibrates with a quarter of a wave in it, and sound travels at 340 m/s.
- Tapped, the pitch falls by 24 Hz for every centimetre of water (a simple rule for this bottle).