How can children study sound? They can't see it. They can't touch it. But they do ask questions about it. What causes different sounds? How does a guitar work? Why do some sounds hurt my ears? Sound, an 8-week, 16-lesson unit for third-grade students, provides a series of activities to help children discover answers to these and other questions they may have about sound.

In the first half of the unit, students explore some basic principles about how sound is produced, how sound travels, and how the frequency of vibrations is related to pitch. They begin by producing sounds with different-sized tuning forks in Lesson 1, discussing the similarities and differences in what they hear. In Lesson 2, they explore how the sounds produced by the tuning forks travel through different materials. The concept that sound is produced by vibrations is reinforced in Lesson 3, when students explore the sounds made by vibrating nails of different sizes.

Lessons 4 and 5 focus on the length of a vibrating object as a specific variable that can affect the pitch of the sound produced. As students change the length of the vibrating part of a ruler, they closely watch the frequency of the vibrations and carefully listen to the corresponding changes in pitch. Discussing the relationship between the length of the vibrating part of the ruler, the vibrations observed, and the resulting pitch introduces the concept that higher pitches result from higher frequencies of vibration.

Students' explorations of varying the pitch of a vibrating ruler lead to their investigation of ways to change the pitch of vibrations that cannot be seen. In Lesson 6, students experiment with the sound produced by the vibrating column of air in a slide whistle. In the embedded assessment in Lesson 7, students design and demonstrate a wind instrument that uses a vibrating reed. This gives them the opportunity to reflect on and apply what they have learned about the relationship between the length of a vibrating column of air and the pitch of the sound produced.

As they begin the second half of the unit, students first focus on the eardrum. In Lesson 8, they make a model eardrum and observe how sounds make it vibrate, just as sounds make a real eardrum vibrate. This experience reinforces children's earlier investigations into how sound travels through different substances. A reading selection at the end of Lesson 8 highlights the importance of hearing safety.

Lessons 9 through 12 engage students in an in-depth exploration of the variables that affect the pitch of a sound produced by a vibrating string. Using a harplike instrument, they observe that changing the tension or the length of a string changes its pitch. Students then observe that strings with the same length and tension but different thicknesses produce sounds of different pitches. In Lesson 13, students focus on factors affecting volume, or loudness of the sounds produced, as they add a bridge to their harps.

In Lesson 14, students investigate the sound-producing "instrument" that enables them to speak, sing, or shout--the human vocal cords. They discover how the observations they have made about vibrating strings also apply to the sounds produced by this organ of the human body. The unit concludes in Lessons 15 and 16 with students designing and constructing a musical instrument or other device to demonstrate what they have learned about sound.

In this unit, not only do students find answers to their questions about sound, but they can begin to appreciate the consistency of the principles they have discovered. This reinforcement continues throughout the unit as they experiment with various devices and make connections between what they hear and what they can observe. Recording results with simple charts and diagrams as well as with words and pictures in their science notebooks enables students to review the results of their previous work and to make connections between what they learn in different investigations.