If you tap on a glass of water, it will make a sound with a specific pitch. But what happens to this pitch if we change the amount of water in the glass? In this experiment, we’ll use physics to study these changes, learning how sound is made and how scientists such as oceanographers use it!

MATERIALS:

4+ identical drinking glasses
Food coloring
Water
Wooden spoon or skewer

TIME ESTIMATED:

10 minutes

DIRECTIONS:

First, fill the glasses so each one has a little more water than the last. The cup with the least amount of water should be around ¼ full.
Next, add food coloring so the water in each glass is a different color.
Arrange the cups in ascending order (smallest amount of water to the largest) so they’re in a straight line.
Now tap the cups with your wooden spoon. Can you hear the differences in sound between each one?

Think LIke a Scientist!

Why does each glass make a different sound?
What do we call these differences?

HOW DOES IT WORK?

This experiment shows how certain instruments create different sounds! When we struck each glass with the wooden spoon, we made both the cup and water vibrate, which made the air inside the cup vibrate at the same rate. Our ears pick up these vibrations as sound, and we label each sound according to its pitch. Pitch really describes the frequency of a sound wave— because all of our sounds had different pitches, they also had different frequencies. The higher the frequency, the higher the pitch. The only thing changing in our experiment is the height of the water, so we see the change must relate to that! As it turns out, when we strike a cup with a greater amount of water, all of the particles vibrate at a slower rate because there is more mass to move. When the air vibrates at a slower rate, the sound has a lower frequency. This is why the cup with the most water had the lowest frequency (or pitch) and the cup with the least water had the highest frequency!

Further exploration:

Oceanographers are scientists who study the ocean. One of their goals is mapping the sea-floor, which is less than 10% done! Of course, beyond a certain depth, it’s too difficult to see or explore the bottom. Instead of relying on sight, scientists developed SONAR (Sound Navigation and Ranging) so they can “see” using sound waves. Because water has a higher density, sound travels farther (and faster) in water than air. A ship using SONAR will produce a sound wave and time how long it takes the echo to reach the ship. Because the speed of sound in water doesn’t change, they can use this time and speed to calculate the distance from the ship to that object!

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