Blowing across the open mouth of a bottle causes the air inside the bottle to vibrate. The vibrating air produces sound. The pitch of the sound is determined by how fast the air vibrates. As the air inside the bottle decreases it vibrates faster and produces a higher pitched sound. You could write the relationship between the volume of air and the pitch of the sound like this: > air volume < pitch . This would be read as: An increase in the volume of air causes a decrease (lowering) in the pitch of the sound produced.
Discover For Yourself
Use the same kind of bottles, such as empty glass soda bottles. Fill the bottles with different amounts of water. Blow across the mouth of each bottle and compare the pitch of the sounds produced. You may have to practice a little to produce a sound. I find that puckering my lips helps me to direct the air.
What’s Happening?
The bottle with the most water has the least amount of air, thus it will produce a sound with the highest pitch. The opposite is true for the bottle that is empty or has the least amount of water. This bottle has the most air, thus will produce a sound with the lowest pitch.
Extension:
Use a metal spoon to tap on the bottles. Does the sound of each bottle change? If so, describe the difference in the pitch. What would cause any pitch change? For information about setting up an experiment where bottles are tapped to produce musical sounds, see MUSICAL BOTTLES.
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