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Sound = Vibration, Vibration, Vibration

In this demonstration, students use their bodies to model vibrations that lead to sound waves.

Three things vibrate when sound is created:

  1. the source object
  2. the molecules in the air (or another medium e.g. water)
  3. the eardrum

When a sound is produced, it causes the air molecules to bump into their neighbouring molecules, who then bump into their neighbours, and so on. There is a progression of collisions that pass through the air as a sound wave.

Air itself does not travel with the wave (there is no gush or puff of air that accompanies each sound); each air molecule moves away from a rest point and then, eventually, returns to it.

When we hear something, we are sensing the vibrations in the air. The number of vibrations per second is known as the frequency, measured in Hertz (1 Hz = 1 vibration per second).

These vibrations enter the outer ear and cause the eardrum to vibrate too. We cannot hear the vibrations that are made by waving our hands in the air because they are too slow. The slowest vibration our human ears can hear is 20 times a second. That would be a very low sound.

The fastest vibration we can hear is 20,000 times per second, which would be a very high sound. Animals can hear different frequencies from humans. Cats can hear even higher frequencies than dogs, and porpoises can hear the fastest vibrations of all (up to 150,000 times per second).

It takes 3 different vibrations to hear a sound, since sound is made when things vibrate (or wiggle) :

  1. The object that makes the noise vibrates (our bell).
  2. The air molecules vibrate as the sound moves through the air.
  3. The eardrum vibrates when the sound wave reaches it.

When sound waves move through the air, each air molecule vibrates back and forth, hitting the air molecule next to it, which then also vibrates back and forth. The individual air molecules do not "travel" with the wave. They just vibrate back and forth.

When the vibrations are fast (high frequency), you hear a high note. When vibrations are slower, you hear a lower note.

Objectives

  • Describe how sound is produced.

Materials

  • anything that makes a sound e.g. bell, whistle, hand clap
    drum

Key Questions

  • How many different vibrations are needed to hear a sound?
  • All objects have the potential to vibrate.
  • Can we hear all of them?
  • If a tree falls in a forest and there is nobody around to hear it, does it still make a sound?

What To Do

Part 1: Introducing the concept of vibration

  1. Ask the students to wave their hands in front of their faces.
  2. What is this motion called (waving, shaking, jiggling, wiggling, etc.)? The scientific name for this motion is vibration. All sounds begin with vibration.
  3. Can you hear the vibrations of your hands?

Part 2: Illustrating Sound = Vibration, Vibration, Vibration

  1. Invite 5 or 6 students to the front of the class to be your volunteers.
  • 1 student is the sound source (holding something that makes a sound, such as a bell).
  • 1 student is the eardrum (holding the drum).
  • 4 students are air molecules between the source and the eardrum.
  1. Place all of the volunteers in a line, facing the class, with 3–4 feet of space separating them.
  2. Ask the students to act out the following sequence of events:
  • The first student rings the bell (or other sound-making device).
  • The second student (air molecule) begins to vibrate (wiggle), moves toward the third student (air molecule) and touches them.
  • The third student, feeling the touch of the second student, starts to vibrate and move towards the fourth student. In the meantime, the second student returns to their original place.
  • The vibration continues down the line until it reaches the last student, who bangs the drum (eardrum) when the last air molecule reaches them.
  1. Run through it a second time with more speed.

Teacher Tip: This demonstration is a good way to introduce the topic of sound. It’s a simple visual demo and allows for volunteer participation right away.

Extensions

  • Have the students use the same kind of “human model” to act out high and low frequency sounds (more or fewer vibrations per second).
  • Can sound exist in space outside of the space shuttle? Hint: No. Sound needs a medium (e.g. air, water), but space is a near vacuum without enough molecules for sound waves to move through.