Sound Waves
In 1992, Australian researchers measured whale song at 188 dB, louder than a jet engine, travelling 3,000 km through the ocean.
● Know
- Sound is a longitudinal mechanical wave made of compressions and rarefactions
- Pitch depends on frequency; loudness depends on amplitude
- The speed of sound differs in solids, liquids and gases, and it cannot travel through a vacuum
● Understand
- Why sound needs a medium and travels fastest where particles are closest
- How the parts of the ear turn pressure ripples into nerve signals
- How instrument design, including the didgeridoo, controls pitch
● Can do
- Explain sound using the terms compression, rarefaction, frequency and amplitude
- Predict how the speed of sound changes between media
- Trace the path of a sound from the outer ear to the cochlea
Tap a tuning fork and touch it lightly to the surface of a glass of water: the water sprays out. That is your evidence that sound begins with a vibrating object. As the prongs of the fork swing outwards they push the nearby air particles together into a compression, a thin shell of slightly higher pressure. As the prongs swing back they leave a gap, and the air particles spread out into a rarefaction, a shell of slightly lower pressure. The fork repeats this many times each second, so a train of compressions and rarefactions ripples outward in every direction.
Because the air particles oscillate back and forth along the same line the wave travels, sound is a longitudinal wave. This is exactly the pattern you met in Lesson 3, the slinky pushed back and forth, not shaken side to side. Crucially, the particles themselves do not travel from the speaker to your ear; they jiggle about a fixed point and pass the energy on, like a row of dominoes nudging one another.
Sound is also a mechanical wave, meaning it needs a medium of particles to carry it. In the vacuum of space there are no particles to compress and rarefy, so sound cannot travel at all. This is why the famous film tagline "in space no one can hear you scream" is, for once, good physics.
Place a ringing alarm clock inside a sealed glass jar (a "bell jar") and slowly pump the air out. As the air thins, the ringing fades and fades until you can see the hammer striking the bell but hear nothing at all. The clock has not stopped, there is simply less and less medium to carry the compressions and rarefactions. Let the air back in and the sound returns instantly. This classic demonstration is direct evidence that sound needs a material medium and cannot travel through a vacuum.
For tens of thousands of years, Aboriginal peoples across Australia have pressed an ear or a hand to the ground to detect the low-frequency vibrations of distant footsteps, animals or approaching people. The dense, tightly packed particles of solid earth carry these vibrations far better than air, which is exactly why ground-listening works and why sound travels faster in solids. This sophisticated reading of vibration through the land is a longstanding scientific practice, developed and refined long before Western instruments could measure the same effect.
Air particles travel from the speaker all the way to your ear. This is false. The particles only oscillate back and forth about a fixed position; it is the energy, carried as a pattern of compressions and rarefactions, that travels across the room. Think of a Mexican wave in a stadium: the wave sweeps around the ground, but each person stays in their own seat.
Two completely independent properties of a sound wave control what we hear. Keeping them separate is the single most useful idea in this lesson.
Pitch is set by frequency. Frequency is the number of complete vibrations the source makes each second, measured in hertz (Hz). The faster an object vibrates, the more compressions reach your ear each second, and the higher the pitch. A double bass string vibrates slowly, around 40 Hz, and sounds deep; a piccolo vibrates quickly, over 2,000 Hz, and sounds shrill. A healthy young person can hear roughly 20 Hz to 20,000 Hz; sounds above that range are called ultrasound.
Loudness is set by amplitude. Amplitude is the size of each vibration, how far the particles are pushed from their rest position. A bigger amplitude means denser compressions and emptier rarefactions, carrying more energy, so the sound is louder. Loudness is measured on the decibel (dB) scale, which is logarithmic: a whisper is around 30 dB, a normal conversation 60 dB, and a rock concert can exceed 110 dB. Shouting changes the amplitude, not the frequency, so a shout is louder but not higher in pitch.
On a pressure-time graph these two properties are easy to spot: a higher-pitched sound has waves bunched closer together (higher frequency), while a louder sound has taller waves (greater amplitude). Change one and the other can stay exactly the same.
Pluck a guitar string gently and it sounds soft; pluck the same string hard and it sounds loud, but the note is identical. You changed the amplitude (how far the string swings) without changing the frequency (how many times per second it vibrates), so loudness changed while pitch stayed the same. Now press the string against a fret to shorten it: the shorter string vibrates faster, raising the frequency and the pitch, while you can keep the loudness unchanged. This is the cleanest everyday proof that pitch and loudness are independent.
Protecting Australian ears: Safe Work Australia sets a workplace noise limit of 85 dB averaged over an eight-hour day, because prolonged exposure to high-amplitude sound permanently damages the delicate hair cells of the inner ear. Mine sites, music venues and airports across the country monitor decibel levels and provide hearing protection. The fact that loudness is amplitude, and that amplitude carries energy, is exactly why too much of it can injure the ear.
Turning the volume up makes a sound higher. This confuses loudness with pitch. Turning up the volume raises the amplitude, so the sound gets louder, but the frequency, and therefore the pitch, stays exactly the same. To change pitch you must change how fast the source vibrates, for example by shortening a string or tightening a drum skin.
How fast does sound travel? Because sound is passed from particle to particle, its speed depends on how closely packed and tightly bonded those particles are. The closer and more tightly bonded the particles, the faster a compression is handed on. So the speed of sound follows a clear order: solids > liquids > gases. In air sound travels at about 343 m/s, in water about 1,500 m/s, and in steel around 5,000 m/s, nearly 15 times faster than in air. This is why a railway worker can hear a distant train sooner by listening to the steel rail than by listening to the air, and why whale song carries for thousands of kilometres through dense seawater. Temperature also matters: warmer air has faster-moving particles, so sound travels slightly faster on a hot day than a cold one.
How do we detect sound? The human ear is a beautiful machine for turning pressure ripples into nerve signals, in four stages:
- Outer ear: the curved flap called the pinna funnels sound waves down the ear canal.
- Eardrum: the compressions and rarefactions push and pull on this thin membrane, making it vibrate at the same frequency as the sound.
- Middle ear: three tiny bones (the hammer, anvil and stirrup, the smallest bones in the body) act as levers that amplify the vibration and pass it on.
- Inner ear: the vibration enters a fluid-filled, snail-shaped tube called the cochlea, where thousands of tiny hair cells convert the movement into electrical nerve signals that travel along the auditory nerve to the brain.
A glimpse ahead, the Doppler effect. When a siren races past you, its pitch drops noticeably as it goes by, even though the siren itself never changes note. As the source approaches, each compression is released a little closer than the last, so they bunch up and the frequency you hear rises; as it recedes they stretch out and the frequency falls. This change in pitch caused by relative motion is the Doppler effect, and you will study it in depth in Lesson 9.
You see a distant lightning flash and then, several seconds later, hear the thunder. Light reaches you almost instantly, but sound crawls across the air at only about 343 m/s. Counting the seconds between flash and crash and dividing by 3 gives a rough distance to the storm in kilometres, because sound covers very nearly 1 km every 3 seconds in air. The delay is direct evidence of how slowly sound moves compared with light, and of how its speed in a gas is far lower than in a solid or liquid.
Listening to the Southern Ocean: Australian marine scientists at CSIRO and the Integrated Marine Observing System deploy underwater microphones (hydrophones) that catch whale calls travelling hundreds of kilometres through seawater. Because sound moves at about 1,500 m/s in water, far faster and further than in air, a single listening station can monitor migrating humpback and blue whales across vast stretches of ocean, helping protect them from ship strikes.
Shouting louder makes an echo come back sooner. False. The speed of sound depends on the medium and its temperature, not on the amplitude of the sound. A louder shout has greater amplitude, so the returning echo is louder, but it travels at the same speed and so takes exactly the same time to return.
Wrong: "Sound is a transverse wave with crests and troughs like a water wave." No. Sound is longitudinal: the air moves back and forth along the direction of travel, forming compressions and rarefactions, not crests and troughs.
Right: Sound is a longitudinal mechanical wave. Air particles vibrate parallel to the direction the wave travels, creating regions of high pressure (compressions) and low pressure (rarefactions). The wavy line we draw is just a graph of pressure over time, not a picture of the particles moving up and down.
Wrong: "A higher-pitched sound is just a louder sound." No. Pitch and loudness are separate. Pitch comes from frequency; loudness comes from amplitude. You can raise one and leave the other unchanged.
Right: Pitch is controlled by frequency (vibrations per second, in Hz), while loudness is controlled by amplitude (the size of the vibration, in dB). A piccolo and a tuba can play at the same loudness yet sound very different in pitch, and the same note can be played soft or loud.
Wrong: "Sound travels fastest through air because gases are light and easy to move." No. Sound is slowest in gases. It travels fastest in solids, where particles are close together and tightly bonded.
Right: The speed of sound follows solids > liquids > gases, because closely packed, tightly bonded particles pass a compression on more quickly. Sound moves at about 343 m/s in air, 1,500 m/s in water and 5,000 m/s in steel.
The Didgeridoo and the Physics of Wind Instruments
The didgeridoo, or yidaki, is one of the world's oldest wind instruments, traditionally made by Aboriginal peoples of northern Australia from a length of eucalyptus naturally hollowed out by termites. When the player buzzes their lips at one end, they set up a standing sound wave in the air column inside the tube.
The length of that air column sets the pitch. A longer didgeridoo supports a longer wavelength standing wave, which means a lower frequency and a deeper drone; a shorter tube gives a higher note. This is the very same physics that decides why a tuba sounds lower than a trumpet, or why the long pipes of a church organ produce the deepest notes. Skilled players also use circular breathing to sustain the drone for minutes at a time, while shaping higher-frequency overtones with their tongue and cheeks.
✍ Copy Into Your Books
▾What Sound Is
- A longitudinal mechanical wave
- Made of compressions (high pressure) and rarefactions (low pressure)
- Needs a medium; cannot travel through a vacuum
Pitch and Loudness
- Pitch = frequency (Hz); faster vibration = higher pitch
- Loudness = amplitude (dB); bigger vibration = louder
- The two are independent of each other
Speed and the Ear
- Speed: solids > liquids > gases (air ~343, water ~1500, steel ~5000 m/s)
- Ear path: pinna → eardrum → 3 bones → cochlea → nerve
- Doppler effect changes pitch of a moving source (Lesson 9)
Pitch or Loudness?
Trace the Sound
At the start of this lesson you were shown sound travelling nearly 17 times faster through steel than air, and how Indigenous Australians pressed their ears to the ground to detect distant vibrations long before European contact.
Now that you've worked through the lesson, how has your thinking shifted? Can you explain that hook idea more precisely using what you've learned today?
Q1. Explain why sound is described as a longitudinal mechanical wave, using the terms compression and rarefaction. Then explain why sound cannot travel through a vacuum. (4 marks)
Q2. Two singers each sing a high note then a low note. The first sings both notes at the same volume; the second sings both notes much more quietly than the first. Explain what changes for each singer in terms of frequency and amplitude, and therefore pitch and volume. (4 marks)
Q3. Explain how a didgeridoo produces its sound as a standing wave, and how the length of the instrument affects its pitch. Link your answer to wind instruments in other cultures. (4 marks)
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?
Model answers (click to reveal)
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.