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Modelling A Sound Wave

Use a stretched Slinky to model sound waves moving through a material.

When you squeeze the Slinky's coils together at one end (compression), this causes the coils in front of them to spread out (expansion). When the squeezed coils are released they spread out and squeeze the coils in front of them together. The squeezed coils in turn move forward, pushing on the coils in front of them and so on.

Squeezing the end coils gave them energy that was transferred from one end of the slinky to the other. As the energy goes through the Slinky, all the coils do not move at once, some of the coils are crowded together and some are spread apart.

Objectives

  • Describe the properties of sound.

Materials

  • Slinky

Key Questions

  • Does the coil move?
  • Do air molecules bump against each other like the coils on the slinky?

What To Do

  1. Get 2 volunteers to hold each end of a Slinky, stretched out along the floor.
  2. Instruct one of the students (representing the sound source) to push the slinky.
  3. Each coil should compress (or bump) against its neighbour, sending a wave along the spring to the other student (representing the eardrum).
  4. Repeat a few times.
  5. Explain to the class that each coil represents an air molecule, and the pulse of compression represents a sound wave. This is known as a longitudinal wave because the vibration of each coil is parallel to the direction of the sound.
  6. Relate the Slinky model to sound. When a sound is created (because vibrations have been made) the sound travels away from the source like ripples spreading out from a pebble dropped in a pond. The sound travels through the air but not with the air as it moves to your ear. For example, if you listen to someone speaking, the sound travels through the air to your ear but the air from their lungs doesn’t come too. This can be seen with the Slinky — each coil vibrates in place, it does not move along the spring.

Extensions