--- title: "How does sound travel?" subtitle: "Year 9 Physics · Wave and particle models · Lesson 2" --- # How does sound travel? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** In space, an explosion is completely silent. Yet on Earth you can hear a thunderclap from kilometres away. What is sound actually made of? :::teacher-note **In one sentence:** Students describe sound as a longitudinal wave that needs a medium, use compressions and rarefactions to explain how energy is transferred, and link frequency to pitch and amplitude to loudness (AC9S9U04, node 2, a hinge node). **Students should be able to say:** I can describe a sound wave with compressions and rarefactions. I can say what changes when a sound gets higher or louder. I can explain why sound cannot travel through a vacuum. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **A speaker plays a note. As the sound passes through the air, what does one particle of air do?** ☐ It travels from the speaker all the way to your ear ☐ It vibrates to and fro in about the same place ☐ It speeds up, then stops ☐ It doesn't move: only the sound does My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It vibrates to and fro in about the same place. Each particle bumps its neighbour and returns. What travels is the pattern of squashing, not the particles. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: the wave model of sound :::keep-together ### Compression **What is a compression in a sound wave?** ☐ The sound getting quieter ☐ A place where the particles are spread further apart ☐ A place where the particles are squashed closer together ::: :::teacher-note **Answer:** A place where the particles are squashed closer together A compression is a squashed patch; a rarefaction is a stretched, spread-out patch. A sound wave is a line of them, one after the other. ::: :::keep-together ### Longitudinal **In sound, the particles vibrate backwards and forwards along the direction the wave travels. What do we call this kind of wave?** ☐ Transverse ☐ Stationary ☐ Longitudinal ::: :::teacher-note **Answer:** Longitudinal Longitudinal: the vibration is along the direction of travel. That is why sound is squashes and stretches. ::: :::keep-together ### Pitch **You raise the pitch. What has increased?** ☐ The frequency: more vibrations each second, so shorter waves ☐ The speed of every particle ☐ The amplitude: bigger vibrations ::: :::teacher-note **Answer:** The frequency: more vibrations each second, so shorter waves Higher pitch means higher frequency (more compressions pass each second). Loudness depends on amplitude, the size of the vibration. ::: :::keep-together ### Matching **Match the change to the effect on the sound.** A. Higher pitch B. Lower pitch C. Louder {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Bigger vibrations (larger amplitude)` | | | `More vibrations each second (higher frequency)` | | | `Fewer vibrations each second` | | ::: :::teacher-note **Answer:** Bigger vibrations (larger amplitude) → C. Louder; More vibrations each second (higher frequency) → A. Higher pitch; Fewer vibrations each second → B. Lower pitch ::: :::keep-together ### Vacuum **Why can't sound travel through a vacuum?** ☐ Sound is too slow in a vacuum ☐ A vacuum absorbs the sound ☐ There are no particles to vibrate and pass the energy on ::: :::teacher-note **Answer:** There are no particles to vibrate and pass the energy on Sound is energy passed from particle to particle. With no particles, there is nothing to pass it along. ::: :::typical ## Elaborate: different media :::keep-together ### Why faster **Sound travels at about 340 m/s in air, 1500 m/s in water and 5000 m/s in steel. Why is it fastest in steel?** ☐ Steel is warmer ☐ The particles are closer together and tightly bonded, so each passes the vibration on sooner ☐ Steel particles are heavier ::: :::teacher-note **Answer:** The particles are closer together and tightly bonded, so each passes the vibration on sooner In a solid the particles are packed close together and joined to their neighbours, so a vibration is passed on very quickly. In a gas they are far apart, so it takes longer. ::: ::: :::wider ## Elaborate: an instrument :::keep-together **A yidaki (didjeridu) is a hollow wooden tube. The player's lips vibrate the air inside it. A longer tube makes a lower note. Using the wave model, which is the best explanation?** ☐ A longer tube makes the sound louder, so it sounds lower ☐ Sound travels slower in longer tubes ☐ The longer column of air vibrates more slowly, so the frequency is lower and the pitch is lower ::: :::teacher-note **Answer:** The longer column of air vibrates more slowly, so the frequency is lower and the pitch is lower A longer air column vibrates at a lower frequency, so the pitch is lower. The material and shape of the tube also affect how the sound carries. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Sound is a **longitudinal wave**: particles vibrate to and fro along the direction the wave travels. - A wave is a line of **compressions** (squashed) and **rarefactions** (spread out). The pattern travels; the particles stay in place. - **Frequency** (vibrations per second) sets the **pitch**; **amplitude** (size of the vibration) sets the **loudness**. - Sound needs a **medium**: it cannot travel through a vacuum. It is faster in solids than in gases because the particles are closer together and more tightly bonded. ## Exit ticket :::keep-together **A loud, high note is played. Say what is large or fast about the wave, and what the particles of air in between are doing.** {{lines-3}} ::: :::teacher-note **Answer:** Large amplitude (loud) and high frequency (high pitch). The particles vibrate to and fro in place while the squashed and spread-out patches travel. :::