The Demonstration: The presenter breathes in a gas that makes his/her voice sound different.
Quick Physics: The density of the gas changes the speed of sound, so the voice sounds higher or lower (depending on the gas).
As you know, sound we normally hear travels through air. Sound in air has an important property; it travels with a certain speed, about 750 miles per hour or 1200 kilometers per hour. You can demonstrate this by having a friend shout at you while she is a long way away. You can see her mouth move before you can hear her voice. Or go to a baseball game. You can see the batter hit the ball before you hear the crack of the bat. The speed that sound travels depends on how heavy the gases in the air are. Since the composition of the air is usually a constant mixture of the same gases, you’ve probably never noticed the speed change much. But what if you use a different gas besides air?
When people speak, their vocal chords vibrate, making the air vibrate which makes sound waves. The voice you usually hear sounds that way because people normally breathe air. What if you breathe in a different gas, say helium? The speed of sound in helium is faster than in air because the helium is lighter than air. (This is why helium balloons float.) This makes your voice sound very high, like Donald Duck. On the other hand, suppose you breathe in a gas that is heavier than air? Sulfur hexafluoride makes your voice sound very low because sound travels slower in heavy gases.
DON’T try this yourself. Breathing strange gases in dangerous. People need oxygen to stay alive and some gases are poisonous.