Show the audience what sound vibrations look like and prove that sound is moving air by making a "sound visualizer" out of a tin can and a balloon. Remove both ends of the tin can, making sure to cut off all jagged edges. Cut off the balloon's opening and fit it snugly across one of the openings in the tin can. Glue a mirror onto the center of the taut balloon using super glue. During the science fair shine a light into the mirror so that it reflects onto the wall. As people visit the experiment they talk into the can. The sound vibrations make the surface of the balloon vibrate, which in turn vibrates the mirror causing the reflection to move. The moving reflection proves that sound is movement.
Demonstrate the effects of helium on sound waves using a balloon and a Slinky. Helium allows sound waves move more than twice as fast as air does, which is why people's voices sound high and squeaky after they inhale helium. To demonstrate this concept, provide guests at the science fair with balloons and ask them to try inhaling helium before speaking. After experimenting with the balloons explain the concept using a Slinky. With one volunteer holding one end of the Slinky, and your child holding the other, the slinky is waved up and down slowly at first to show how sound vibrates in air, then quickly to demonstrate how it moves through helium. This visual aid helps people understand the effects of helium on sound vibration.
Allow science fair guests to feel sound vibration by making a foil vibration helmet. Make the helmet by shaping two large sheets of foil around the top of someone's head. The helmet should cover the entire face, but not so close that it is difficult to breathe. It is best if the foil ends just past the chin. The presenter invites guests to try on the helmet and make a "hoo" sound. When the volunteer makes noise, they will feel the helmet vibrate. The presenter then explains that the vibrations come from the moving air, which is also how the sound is created.