Building directly on Lesson 16, learners investigate what actually happens when sound leaves its source and travels to a listener. Students discover that sound moves as a longitudinal wave (a traveling pattern of compression and rarefaction) and that it requires a medium of particles to travel through. Through demonstrations and discussion, the lesson establishes that sound moves fastest through solids, slower through liquids, and slowest through gases, and cannot travel through a vacuum at all. The lesson concludes by turning to light: students compare sound waves to light waves and discover a key difference, light does not need a medium and travels incomparably faster, which is why we see lightning before we hear thunder.

Key Ideas

• Sound travels as a longitudinal wave. The particles of the medium push back and forth in the same direction the wave travels.
• Sound requires a medium (a solid, liquid, or gas) to travel through. Without particles there is nothing to carry the wave.
• Sound travels fastest through solids, slower through liquids, and slowest through gases because the closer the particles, the faster energy passes between them.
• In a vacuum, like outer space, there are no particles, so sound cannot travel at all. Explosions in space are completely silent.
• Light also travels in waves, but it is a transverse wave and does not need a medium. Light can travel through empty space.
• Light travels incomparably faster than sound, about 186,000 miles per second versus about 1,100 feet per second, which is why we see lightning before we hear thunder.

Vocabulary

• Medium: The material (solid, liquid, or gas) that sound travels through.
• Longitudinal wave: A wave in which particles move back and forth in the same direction as the wave travels.
• Compression: A zone in a sound wave where particles are pushed close together.
• Rarefaction: A zone in a sound wave where particles are spread apart.
• Vacuum: Space that contains no particles. Sound cannot travel through it; light can.

Hands-On Activity: Sound Through the Table

Supply List

• A sturdy wooden table
• A hand for knocking

Instructions

• Sit at a wooden table. Knock gently on the underside and listen to how it sounds through the air. Notice the volume and clarity.
• Press one ear flat against the tabletop and close your other ear with your finger. Knock again with the same force.
• Compare the two experiences. The knock heard through the solid wood should be noticeably louder and clearer than through the air.
• Discuss why: molecules in a solid are packed closely together and pass vibrations from one to the next very efficiently, while air molecules are spread far apart and lose energy quickly over distance.
• Extension: try the same experiment on different surfaces — a metal desk, a plastic table, a carpeted floor. Does the material matter?

Supplies for Live Class

• Soundwaves Printable or paper
• Homemade Harmonica from previous class
• Slinky (optional)

Sources

• Nebel, Bernard J. Building Foundations of Scientific Understanding. C-2 Sound, Vibrations, and Energy
• Sound Facts for kids. (n.d.). https://kids.kiddle.co/Sound
• Waves: Light and Sound | Next Generation Science Standards. (n.d.). https://www.nextgenscience.org/topic-arrangement/1waves-light-and-sound
• Traveling sound. (2023, March 1). [Video]. TeachEngineering.org. https://www.teachengineering.org/activities/view/cub_energy2_lesson05_activity2

Sources

• Nebel, Bernard J. Building Foundations of Scientific Understanding. C-2 Sound, Vibrations, and Energy
• Sound Facts for kids. (n.d.). https://kids.kiddle.co/Sound
• Waves: Light and Sound | Next Generation Science Standards. (n.d.). https://www.nextgenscience.org/topic-arrangement/1waves-light-and-sound
• Traveling sound. (2023, March 1). [Video]. TeachEngineering.org. https://www.teachengineering.org/activities/view/cub_energy2_lesson05_activity2
• Britannica Editors. (2026, April 9). How does sound travel? | Waves, speed, frequency, & amplitude. Encyclopedia Britannica. https://www.britannica.com/science/How-Does-Sound-Travel
• Light facts for kids. (n.d.). https://kids.kiddle.co/Light

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