Aquatic Notes

Objective

Students will explore how the mass of water can affect sound vibrations.

Materials

assorted shapes of plastic cups or glass soda bottles (one shape for each student group)
water
pencils
Background

Sound is a form of energy (a vibration) that travels through the air in waves. Sound vibrations have different frequencies. Scientists measure frequency in vibrations per second, or Hertz (Hz). Our sense of hearing interprets the changes in frequencies as a change in pitch. A low-pitched sound has a low frequency A high-pitched sound has a high frequency You can change the frequency of a vibrating object by changing its structure. A more secure object may vibrate faster than a loose object when struck.

An easy way to change the structure of a cup is to add weight - just fill it with water. As the glass is filled, it becomes more stable and vibrates slower when struck. Sound travels 1,450 meters per second in seawater that has a density of 1.025 grams per cm3 and 334 meters per second through air at 20*C that has a density of 0.001293 grams per cm3.

Action

Divide students into groups and assign each group to one shape of cup. Fill cups with water to different levels.
Students tap the rim of each cup with a pencil. They should hear different pitches; that is, the cups should vibrate at different frequencies.
After they've tapped the rims several times, ask students why they think there is a difference. (More water makes the sides of the cups more stable and the vibration is slower, which results in a lower pitch).
Student groups may hear different pitches for different shaped cups filled to the same level of water. Tall glasses may vibrate faster than short, wide glasses.
Deeper Depths

Test the pitch of the cups above using a less dense liquid such as rubbing alcohol (density of 0.791 grams per cm3) and a more dense liquid of glycerin (1.26 grams per cm3). Is there a difference in the pitch?

Students can also determine what notes each glass produces by comparing sounds with a known musical instrument.