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Musical Bottles

Using two different media (air and water) in glass bottles show how the speed of vibrations affects pitch. Students will determine which results in a higher pitch: a full bottle or an empty bottle.

Pitch is the way that your ear and brain order sounds based on their frequency (vibrations per second). Rapid vibrations (high frequency) that reach your ear are categorized as a "higher" pitch, while slower vibrations (low frequency) are categorized as a "lower" pitch. Musicians label the pitches with the letters A through G.

When you tap a glass of water with a pencil or spoon the water molecules vibrate and create sound waves through the water. More water means slower vibrations and a deeper tone. Each of the bottles will have a different pitch when hit with the pencil. The bottle with the most water will have the lowest pitch while the bottle with the least water will have the highest.

When you blow across the top of the bottle, you set the air molecules vibrating and produce sound waves. In this case, more water results in a higher pitch, as opposed to tinkling the bottle. When the bottle has a small amount of water in it, the air molecules have lots of room and vibrate slower. Adding more water gives them less space to vibrate in, making them vibrate faster and producing a higher pitch.

Objectives

  • Describe the properties of sound.

  • Describe pitch and how it varies.

Materials

  • Per Class:
    blackboard/whiteboard and chalk/marker

  • Per Student:
    identical glass bottles filled with various amounts of water (glass baby bottles work well)
    spoons or pencils

Key Questions

  • Part 1:
    • Which bottle will produce the highest pitch if we tap the bottle with a spoon? Which bottle will produce the lowest pitch?
    • How do the bottles differ between the highest pitch to the lowest pitch?
  • Part 2:
    • Which bottle will produce the highest pitch if we blow across the top of the bottle?
    • Which bottle will produce the lowest pitch?
    • How do the bottles differ between the highest pitch to the lowest pitch?
    • Why does blowing across the top of the bottle and tapping the bottle produce different sounds? (Hint: Think about which medium is being vibrated in each case.)

What To Do

Preparation

  1. Fill the glass bottles with different amounts of water.
  2. At one end of the blackboard/whiteboard, write “HIGHEST pitch”. At the other end write “LOWEST pitch”.

Part 1: Tapping bottles

  1. Each student taps their bottle and compares their pitch with those of their classmates.
  2. The students put themselves in order along the blackboard from the highest pitch to the lowest pitch.
  3. Once the students are in order, ask each student to tap their bottle to compare the various pitches.

Part 2: Blowing across the bottle

  1. Each student blows across the top of their bottle and compares their pitch with those of their classmates.
  2. The students put themselves in order along the blackboard from the highest pitch to the lowest pitch.
  3. Once the students are in order, ask each student to blow across the top of their bottle to compare the various pitches.

Extensions

  • Play the bottles to interpret a popular children’s song such as “Mary Had a Little Lamb” or “Twinkle Twinkle Little Star”.
  • Separate into teams of 6–7 students, with a "conductor" for each team. Decide as a team whether you will blow across the bottles or tap the bottles to produce your notes. You’ll need some time to fiddle with the amounts of water in the bottles if you want your song to be musical!