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.
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Page Updated 8/29/08