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# How does sound travel?

Year 9 Physics · Wave and particle models

<!-- Speaker note: 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). About 25 minutes (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 4 min, Wider about 2 min). -->

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## Today

**We are learning to** sound is a longitudinal wave: a vibrating source makes compressions (particles squashed together) and rarefactions (particles spread out) that travel through a medium, with each particle only vibrating to and fro in place. Frequency sets the pitch and amplitude sets the loudness. Without particles, as in a vacuum, there is nothing to carry the wave.

- 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.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. It follows How does heat get through? and comes before the charge lessons. The hinge: the wave model first appears here. Students who picture particles flowing from speaker to ear cannot go on to light as a wave or to compare the wave and particle models later. The prediction targets it, and the orange particle on the board is the answer. Board notes: the animation is slowed down, the speeds in the three media are not to scale (the numbers shown are the real ones), and the particles are drawn in one row of 7 for clarity. Real sound waves have far more particles per wavelength. Teacher: In your own words: the dot diagram with compression and rarefaction marked. -->

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## Why it matters

> In space, an explosion is completely silent. Yet on Earth you can hear a thunderclap from kilometres away. What is sound actually made of?

<!-- Speaker note: You have heard sounds all your life. This lesson looks at what is happening in the air between a sound and your ear. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**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

<!-- Speaker 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. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. Ask where the 'travels to your ear' group thinks the air at the speaker goes. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Fsound%2Fstep-2&code=widget%3A+wave%0Amode%3A+sound%0Amedium%3A+air%0Afreq%3A+3%0Aamp%3A+3%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-9/sound/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask what moved, the particles or the pattern. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* 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.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: A place where the particles are squashed closer together

<!-- Speaker note: 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. -->

<!-- Speaker note: Teacher: compression, longitudinal and vacuum are the success criteria. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: Longitudinal

<!-- Speaker note: Longitudinal: the vibration is along the direction of travel. That is why sound is squashes and stretches. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: The frequency: more vibrations each second, so shorter waves

<!-- Speaker note: Higher pitch means higher frequency (more compressions pass each second). Loudness depends on amplitude, the size of the vibration. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Matching

**Match the change to the effect on the sound.**

| | |
|---|---|
| `Bigger vibrations (larger amplitude)` | A. Higher pitch |
| `More vibrations each second (higher frequency)` | B. Lower pitch |
| `Fewer vibrations each second` | C. Louder |

* Answer: Bigger vibrations (larger amplitude) → C. Louder; More vibrations each second (higher frequency) → A. Higher pitch; Fewer vibrations each second → B. Lower pitch

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: There are no particles to vibrate and pass the energy on

<!-- Speaker note: Sound is energy passed from particle to particle. With no particles, there is nothing to pass it along. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: different media

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-9%2Fsound%2Fstep-4&code=widget%3A+wave%0Amode%3A+sound%0Amedium%3A+choose%0Afreq%3A+3%0Aamp%3A+3%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-9/sound/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Comparing air, steel and a vacuum links the wave model to the particle model: closer, more tightly bonded particles pass the vibration on faster, and no particles means no sound. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: The particles are closer together and tightly bonded, so each passes the vibration on sooner

<!-- Speaker note: 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. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: an instrument

**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

* Answer: The longer column of air vibrates more slowly, so the frequency is lower and the pitch is lower

<!-- Speaker note: 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. What this step is for: Applying the wave model to a real instrument shows pitch and loudness set by the vibrating air and the size of the instrument. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**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.**

* Large amplitude (loud) and high frequency (high pitch). The particles vibrate to and fro in place while the squashed and spread-out patches travel.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: The charge lessons (the particle model of electricity), then the electromagnetic spectrum, where the wave model has to work without a medium. In class: stretch a slinky across a table and push and pull one end, then shake it sideways; use a tuning fork on water; a bell in a vacuum jar if available. A yidaki or other instrument from a local First Nations community, with the community's guidance, is a strong real example. -->