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# Pop Bottle Sounds

## Making sounds by blowing air and tapping the glass

Submitted by Mark Spangler

When you blow on a soda bottle or clink it with a spoon, the bottle makes a noise. Sometimes the sound is high and sometimes the sound is low. I want to find out how the liquid in the bottle makes the sound change. Here's my hypothesis... When I clink the bottle with a spoon, I think the bottle will make a low sound when it is full and a higher sound when it is empty.

### Materials

• 8 glass Orange Crush bottles (all of the bottles must be the same!)
• Water
• Spoon
• Good ears

Test #1 Clink Two Bottles
You need two bottles for this experiment. Fill one bottle full with water and leave the other bottle empty. Clink both bottles. Are the sounds different?

Test #2 Clink Three Bottles
You need three bottles for this experiment. Fill the first bottle full of water. Fill the second bottle half full. Don’t put water in the third bottle. Will the sound of bottle #2 (half full) be in the middle of the other two sounds?

Test #3 Blowing Air
This test used the three bottles from test #2. Instead of “clinking” the bottle, I want to blow air across them. My hypothesis is that they will make the same sound as clinking the bottles.

### How does it work?

What I Learned About Sound
Sound comes from vibrations. When you hit the bottle with the spoon, it makes the glass vibrate. When you fill the bottle with water, the glass cannot vibrate as much. Fast vibrations make a high sound and slow vibrations make a low sound. A full bottle will produce a slow vibration and a low sound. An empty bottle will have a faster vibration and a higher sound.

My Big Discovery

I thought that blowing into the bottle would be the same as hitting it with a spoon, but I was wrong. Blowing into the empty bottle made a low sound. I learned that when you blow into the bottle, you are making the air vibrate - not the glass! When you put more water into the bottle, there is less air to vibrate. This means the air will vibrate faster and the sound is higher.

How it Works
As you blow air across the lip of the bottle, the air inside the bottle flows out as new air flows back into the bottle. This is actually an application of Bernoulli's Principle - fast-moving air creates an area of low pressure. As the air leaves the bottle, the molecules vibrate and that vibration creates a sound.

Pitch is a measure of the speed of the vibration. Rapid vibrations create a high pitch while slower vibrations result in a lower tone. You probably noticed that the pitch of the sound from the bottle changes as you add or subtract water. An empty bottle produces a lower pitch because there's lots of air in the bottle to vibrate. Adding water to the bottle decreases the amount of air space which means there is less air to vibrate. These vibrations happen more quickly and produce a higher pitch.

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