Today’s “Moment of Science” is not typical of our usual programing. Since we have the time, we’re combining the content of two programs. You also have the chance to apply what you learn right here on the web site.
If you blow across the top of an empty soda pop bottle it makes a fairly low-pitched tone. If you add a little liquid and then blow, the pitch is higher; the more liquid, the higher the pitch. But if you tap the sides of the same bottles you get the opposite effect: the empty bottle has the highest pitch, the fuller bottle a lower pitch. It seems odd, doesn’t it? This Moment of Science will explain why this happens.
When you blow across the top of an open bottle the air inside it vibrates, producing sound waves we can hear. When we blow across an empty bottle there is more space for the air to vibrate in, so the air vibrates more slowly. We hear a lower pitch. As we add water there is less space in the bottle for the air to vibrate, so the air vibrates faster. Now we hear a higher pitch.
Try it yourself and listen to these three identical bottles.
First with no water [BLOW #1].
Now one-third full [BLOW #2].
And finally two-thirds full [BLOW #3].
After adding water, the pitch goes up. If we tap the bottles, we get a different set of notes. This time, the one with the most water has the lowest pitch. The reason adding water raises the pitch when you blow across the top but lowers it when you tap the side is that different parts of the bottle are producing the sound. When you blow, sound waves come from vibrating air. When you tap on it, the bottle itself vibrates.
When the bottle vibrates, the water inside has to vibrate with it. Having to move all that water slows down the vibrations of the glass and that in turn slows down the frequency of the sound waves, producing a lower pitch. Adding more water slows down the bottle’s vibrations even more, creating an even lower pitch. On the other hand, when the bottle is empty there’s no water to move so the vibrations are faster and the pitch is higher.
You can now try this for yourself: click on the butterfly in the top left corner of this page to return to the home page, then click on Laboratory, and select “Jugband Physics.”