|MadSci Network: Physics|
You asked a really good question. The reason a ballon or a whoopee cushion make a sound is because when the air rushes out, it makes the lips of the hole vibrate, or move back and forth really fast. In a way, this is like the way clapping your hands makes noise, except it's the rubber in the balloon slapping together that makes the sound.
The sound depends on two things: how hard the air is trying to get out (the pressure inside the balloon) and how tight the hole is. If the hole is big and loose, you get a low flapping sound. If the hole is really tight the sound is much higher, like a bee buzzing. Try this: fill a balloon with air, then hold the nozzle between the index finger and thumbs of both hands, like you are pinching it with both hands. Now pull the nozzle apart so that the hole gets really long, as long as you can. Then relax a bit, letting the air leak out. The sound it makes is a lot higher! Also, it's a lot louder, because the hole is smaller, and the air tries to get out harder. By changing the amount you stretch the hole, you can change the pitch, or note, that the sound makes. Maybe you can even play a tune!
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