Sound Waves
From the roar of a bushfire to the song of a magpie, from the beat of a didgeridoo to the rumble of thunder — sound is everywhere. But what is sound, really? How does it travel? Why does it sound different underwater? And how do our ears turn vibrations into the rich world of sound we experience?
Before You Begin
Stand near a busy road and cover your ears. The sound becomes quieter. Now place your ear against a solid table and gently tap the other end.
Write down your answers before reading on:
- Why does covering your ears make sound quieter?
- Why can you hear the table tapping clearly through the solid wood, even from the other end?
- What do you think would happen if you tried to hear the tapping underwater?
Know
- Sound is a longitudinal mechanical wave
- Pitch is determined by frequency; volume by amplitude
- The speed of sound differs in solids, liquids and gases
Understand
- How compression and rarefaction in air create sound
- How the ear detects and responds to sound waves
- How Aboriginal instruments use sound wave principles
Can Do
- Explain why sound cannot travel through a vacuum
- Predict how pitch and volume change with wave properties
- Compare the speed of sound in different media
Sound as a Longitudinal Mechanical Wave
Compressions and rarefactions in air
Introduction Waves
Sound is not a thing that moves through air — it is a pattern of pressure changes that travels through air.
When an object vibrates — whether it is a vocal cord, a guitar string or a speaker cone — it pushes air particles together and then lets them spring back. This creates alternating regions of compression (high pressure, particles close together) and rarefaction (low pressure, particles spread apart). These regions travel outward from the source as a longitudinal wave.
Because sound is a mechanical wave, it needs a medium. It cannot travel through a vacuum. This is why explosions in space are silent in films — even though filmmakers often add sound effects for drama, in reality there is no air to carry the vibrations.
Pitch and Volume
Frequency and amplitude in sound
Two sounds can have the same source but sound completely different depending on their wave properties:
Pitch is how high or low a sound seems. It is determined by the frequency of the sound wave:
- High frequency = high pitch (whistle, squeak, mosquito buzz)
- Low frequency = low pitch (bass drum, rumble of thunder, didgeridoo drone)
Volume (or loudness) is how loud or soft a sound seems. It is determined by the amplitude of the sound wave:
- High amplitude = loud sound (shout, jet engine, thunderclap)
- Low amplitude = quiet sound (whisper, rustling leaves)
| Property | Wave feature | Effect on sound | Examples |
|---|---|---|---|
| Pitch | Frequency | Higher frequency = higher pitch | Whistle (high), didgeridoo (low) |
| Volume | Amplitude | Higher amplitude = louder sound | Shout (loud), whisper (quiet) |
How the Ear Responds
From vibration to perception
Your ear is a remarkable transducer — it converts mechanical sound waves into electrical signals your brain can interpret. Here is what happens, step by step:
- Outer ear: The pinna (the visible part of your ear) collects sound waves and funnels them down the ear canal to the eardrum.
- Eardrum: Sound waves make the eardrum vibrate. The eardrum is a thin membrane that moves in and out with the compressions and rarefactions of the sound wave.
- Ossicles: Three tiny bones (hammer, anvil, stirrup) amplify the vibrations and transfer them to the cochlea.
- Cochlea: A fluid-filled, spiral-shaped organ. Different frequencies cause different parts of the cochlea to vibrate. High frequencies stimulate the base; low frequencies stimulate the tip.
- Hair cells: Tiny hair-like cells in the cochlea convert the mechanical vibrations into electrical signals.
- Auditory nerve: These electrical signals travel to the brain, which interprets them as sound.
Speed of sound in different media
Sound travels at different speeds depending on the medium. In general:
- Solids: Fastest — particles are close together and tightly bonded, so vibrations transfer quickly. Speed in steel: ~5000 m/s.
- Liquids: Faster than gases but slower than solids. Speed in water: ~1500 m/s.
- Gases: Slowest — particles are far apart, so vibrations take longer to transfer. Speed in air: ~340 m/s.
This is why you can hear a train approaching by placing your ear on the railway track before you hear it through the air — the sound travels much faster through the steel rail.
Common Misconceptions
"Sound travels at the same speed in all materials." No — sound travels fastest in solids, then liquids, then gases. In steel it is about 5000 m/s; in air it is about 340 m/s. The closer the particles, the faster vibrations transfer.
"Loudness is the same as pitch." No — loudness (volume) depends on amplitude; pitch depends on frequency. A sound can be loud and low-pitched (like a bass drum) or quiet and high-pitched (like a distant mosquito).
Aboriginal and Torres Strait Islander Sound Instruments
Aboriginal and Torres Strait Islander Peoples have developed some of the world's most sophisticated acoustic instruments, demonstrating deep understanding of how sound waves work.
The didgeridoo (yidaki): A long wooden tube, traditionally made from termite-hollowed eucalyptus. When a player vibrates their lips at the mouthpiece, they create a standing wave inside the tube. The length of the didgeridoo determines its fundamental pitch — longer didgeridoos produce lower frequencies. Players use circular breathing to maintain a continuous sound, and vocal techniques to add overtones (higher-frequency harmonics) above the fundamental note. This creates the rich, complex sound that makes each didgeridoo unique.
Clapsticks (bilma): Two wooden sticks struck together produce a sharp, percussive sound. The hardness and density of the wood affect the frequency and amplitude of the sound produced — harder woods create brighter, louder sounds. Clapsticks are used to keep rhythm in ceremonies and songs, with different patterns carrying different meanings.
Message sticks and sound signals: Some Aboriginal groups used sound-based communication systems, including knocking rhythms on hollow logs or using bullroarers (aerophones that produce a whirring sound when spun). These systems demonstrate understanding that sound can carry information across distances — a principle that underlies all modern communication technology.
✍ Copy Into Your Books
▾Sound Wave Basics
- Sound is a longitudinal mechanical wave
- Travels as compressions and rarefactions
- Needs a medium — cannot travel in vacuum
Pitch and Volume
- Pitch = frequency (high f = high pitch)
- Volume = amplitude (high A = loud)
- They are independent properties
Speed of Sound
- Solids fastest (~5000 m/s in steel)
- Liquids faster than gases (~1500 m/s in water)
- Gases slowest (~340 m/s in air)
Pitch and Volume Predictor
Speed of Sound Calculator
Test Your Understanding
1. Which wave feature determines the pitch of a sound?
2. Why does sound travel faster through steel than through air?
3. A musician plays a note on a didgeridoo and then on a clapstick. The didgeridoo produces a sustained low note; the clapstick produces a brief, sharp sound. Which statement is correct?
4. An astronaut on the Moon fires a gun. Another astronaut 500 m away sees the flash but hears nothing. Which explanation is correct?
5. A student claims: "If I shout louder at a wall, the echo will come back faster." Is the student correct?
Short Answer Questions
1. Explain why sound is described as a longitudinal mechanical wave. In your answer, describe how sound travels through air and why it cannot travel through a vacuum. 4 MARKS
2. A singer sings a high note and then a low note at the same volume. A second singer sings the same two notes but much more quietly. For each singer, describe which wave property changes and which stays the same. Use the terms frequency, amplitude, pitch and volume. 4 MARKS
3. Describe how the didgeridoo produces sound, and explain how its length affects the pitch of the note produced. How does this demonstrate an understanding of sound wave properties? 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain why sound travels faster through a table than through air?
- Would sound travel faster or slower underwater compared to air? Why?
Answers
▾MCQ 1
B — Pitch is determined by frequency. Higher frequency means higher pitch; lower frequency means lower pitch.
MCQ 2
A — In solids like steel, particles are close together and tightly bonded, so vibrations transfer from particle to particle very quickly. In gases like air, particles are far apart, so vibrations take longer to transfer.
MCQ 3
C — The didgeridoo produces a sustained low note because its long tube creates a low-frequency standing wave. A clapstick produces a brief, sharp percussive sound with a broad range of frequencies, including higher-frequency components.
MCQ 4
D — The Moon has virtually no atmosphere, so there is no medium for sound to travel through. The astronaut sees the flash because light is an electromagnetic wave that does not need a medium.
MCQ 5
B — The speed of sound in air depends on the temperature and properties of the air, not on the amplitude of the sound. Shouting louder increases the amplitude (making the echo louder) but does not change the speed or the time it takes to return.
Short Answer 1
Model answer: Sound is a longitudinal mechanical wave because the particles of the medium oscillate parallel to the direction the wave travels. As sound travels through air, vibrating objects push air particles together to form compressions (high pressure) and then let them spring back to form rarefactions (low pressure). These compressions and rarefactions travel outward from the source. Sound cannot travel through a vacuum because it is a mechanical wave that requires a medium. In a vacuum there are no particles to compress and rarefy, so no sound wave can form or propagate.
Short Answer 2
Model answer: For the first singer, singing high then low notes at the same volume: the frequency changes (high note = high frequency, low note = low frequency), which means the pitch changes. The amplitude stays the same, so the volume stays the same. For the second singer singing the same notes much more quietly: the frequency still changes between high and low notes (so pitch still changes), but the amplitude is lower for both notes, meaning the volume is quieter for both. In summary, the first singer changes pitch only; the second singer changes both pitch and volume.
Short Answer 3
Model answer: The didgeridoo produces sound when the player vibrates their lips at the mouthpiece, setting up a standing wave inside the hollow tube. The air column inside vibrates with compressions and rarefactions travelling along its length. The length of the didgeridoo affects its pitch because a longer tube supports a longer wavelength standing wave, which corresponds to a lower frequency and therefore a lower pitch. This demonstrates understanding that sound is a longitudinal wave whose properties (frequency, wavelength, pitch) depend on the dimensions of the resonating chamber — the same principles that govern all wind instruments in Western and non-Western music traditions.
Wave Jumper
Jump through the sound wave platforms while testing your knowledge of pitch, volume and the speed of sound. Can you hear your way to the top?
Mark lesson as complete
Tick when you have finished all activities and checked your answers.