Can sound travel through space?

No. Sound is a mechanical wave, so it needs particles to pass the vibration along. Space is a near-vacuum, so sound can't cross it. Light and radio can, because they are electromagnetic waves and need no medium.

What is the sign convention for the Doppler effect equation?

For f' = f(vwave + vobserver)/(vwave − vsource), each speed is positive when that object moves towards the other, negative when it moves away, and zero if it is still. Then check: getting closer gives a higher f', moving apart a lower one.

What is the difference between a node and an antinode?

A node is a point on a standing wave that never moves, because the two opposite-travelling waves always cancel there. An antinode is midway between nodes, where they always add, so the amplitude is biggest. Neighbouring nodes are half a wavelength apart.

What speed of sound should I use in Year 11 Physics?

Use 340 m s^−1 for sound in air unless the question gives another value. The real value rises a little as air warms, and sound is much faster in water and solids.

Why does a siren's pitch drop as it passes you?

While the siren approaches, its crests are bunched up, so you hear a higher frequency than it sends out. Once it passes and moves away, the crests are spread out, so you hear a lower frequency. The siren itself never changes.

Astronauts working outside a space station can't hear a tool strike a metal panel nearby. Why?

There is no medium in space to carry the sound to them.

Sound travels too fast in space to be heard.

Space is too cold for sound waves to form.

The tool vibrates at too high a frequency in space.

The absorption lines in a galaxy's spectrum are at slightly longer wavelengths than the same lines measured in a lab. What does this show?

The galaxy is moving away from us.

The galaxy is moving towards us.

The galaxy is hotter than the Sun.

The light slowed down on its way to Earth.

Year 11 Physics · Waves · Light and sound, wave behaviours · 14 min read

Sound, standing waves and the Doppler effect

Sound is a longitudinal wave that needs particles to travel through. Its frequency sets the pitch you hear and its amplitude sets how loud it is. Pluck a string fixed at both ends and you get a standing wave, which is why each string plays its own set of notes.

This guide covers how sound moves through a medium, pitch and loudness, standing waves and harmonics on a string, and the Doppler effect: why a siren sounds higher as it comes towards you and lower as it goes away, and how the same idea gives redshift and blueshift for light. It uses 340 m s⁻¹ for the speed of sound in air unless a question says otherwise.

What you need to know

Identify that mechanical waves cannot travel without a medium.
Explain how a medium carries sound wave energy.
Link frequency with wavelength, pitch with frequency and loudness with amplitude by conducting a practical investigation.
Conduct a practical investigation showing how a sound wave's energy depends on amplitude and frequency.
Show how a standing wave forms.
Explain how a standing wave comes to have nodes and antinodes.
For standing waves on a string fixed at each end, use v = fλ to analyse wavelength and frequency.
Explain the shift in frequency of sound and light waves when a source approaches an observer (blueshift) or recedes from one (redshift).
Use f' = f(v_wave + v_observer)/(v_wave − v_source) for sound frequency-shift problems.

The equations you will use:

v = fλFor sound in air, v = 340 m s⁻¹ unless the question gives another value.

λ_n = 2L/n f_n = nv/2L not on the data sheetA string of length L fixed at both ends; n is the number of loops (n = 1 is the fundamental). Not on the data sheet: it comes from L = nλ/2 and v = fλ.

f' = f(v_wave + v_observer)/(v_wave − v_source)Doppler shift for sound. A speed is positive when that object moves towards the other one, negative when it moves away.

Dot points: PY11-WV-08, PY11-WV-09, PY11-WV-10, PY11-WV-11, PY11-WV-34, PY11-WV-35, PY11-WV-36, PY11-WV-37, PY11-WV-38 (2025 Physics syllabus, Year 11), in our own words.

Mini lesson

Sound needs a medium

A vibrating object (a speaker cone, a guitar string, your vocal cords) pushes on the air next to it. As it moves out it squashes the air into a compression; as it moves back it leaves a rarefaction. The particles in a compression collide with their neighbours and pass the push along, so the energy moves outward while each particle only oscillates back and forth about its rest position.

With no particles there is nothing to pass the push along. Sound, like every mechanical wave, cannot travel through a vacuum: a ringing bell in a jar goes quiet as the air is pumped out, and space is silent. Light is different. It is an electromagnetic wave and needs no medium.

Sound travels faster in liquids and solids than in air (about 1500 m s⁻¹ in water) because they are much stiffer: they are far harder to compress.

Pitch, loudness and energy

Pitch is frequency. A higher frequency sounds higher. In air the speed stays about 340 m s⁻¹, so a higher frequency also means a shorter wavelength (λ = v/f).
Loudness is amplitude. A bigger amplitude means bigger pressure changes, so the sound is louder.
Energy depends on both. At the same frequency, a bigger amplitude carries more energy: doubling the amplitude carries four times the energy. At the same amplitude, a higher frequency carries more energy, because the particles move back and forth faster.

As an oscilloscope shows it: louder means a bigger amplitude (same frequency); higher pitch means a higher frequency (more oscillations in the same time).

In the practical, a signal generator drives a speaker and a microphone feeds an oscilloscope or a phone app. Turn up the frequency and the trace squeezes together; turn up the volume and it grows taller. Read the period off the screen to find f, and compare the amplitudes to compare loudness.

How a standing wave forms

Send a wave along a string fixed at both ends and it reflects at each end. The reflected wave travels back through the incoming one. Two identical waves (same frequency, wavelength and amplitude) travelling in opposite directions superpose, and at the right frequencies the result is a standing wave: a pattern that oscillates in place instead of moving along the string.

Nodes: points that never move. There the two waves always arrive exactly out of step (destructive interference), so their displacements always cancel.
Antinodes: points midway between nodes, where the two waves are always in step (constructive interference) and the string oscillates with the biggest amplitude.
Neighbouring nodes are half a wavelength apart, and so are neighbouring antinodes. A node and the next antinode are a quarter of a wavelength apart.
A fixed end can't move, so it is always a node.

Harmonics on a string fixed at both ends

Because both ends are nodes, only whole numbers of half-wavelengths fit on the string: L = nλ/2, where n is the number of loops. So λ_n = 2L/n, and with v = fλ the allowed frequencies are f_n = nv/2L.

n = 1, the fundamental (first harmonic): one loop, λ = 2L, the lowest frequency.
n = 2, 3, …: two, three … loops, at frequencies 2f₁, 3f₁ …
The wave speed on the string is set by its tension and its mass per metre, not by the frequency. Tighten a guitar string and v rises, so every harmonic rises in pitch.
Example: a 0.65 m guitar string with a fundamental of 196 Hz has λ₁ = 2 × 0.65 = 1.30 m, so waves travel along it at v = 196 × 1.30 ≈ 255 m s⁻¹.

Each loop is half a wavelength. The third harmonic has 3 loops, 3 antinodes (A) and 4 nodes (N), counting both fixed ends.

The Doppler effect

When a source moves towards you, each wave crest is sent out from a little closer than the last, so the crests bunch up: the wavelength is shorter and you hear a higher frequency. When it moves away, the crests spread out and you hear a lower frequency. If you move towards the source, you meet the crests more often, which also raises the frequency you hear.

The source's own frequency never changes. As a siren passes at a steady speed, the pitch you hear is steady and high while it approaches, drops quickly as it goes past, then stays steady and low as it moves away.

Each circle was sent out from where the source was at the time. Ahead of the source the crests bunch up (higher f); behind, they spread out (lower f). The source's speed is exaggerated.

Using the Doppler equation: the sign rule

f is the frequency the source sends out; f' is the frequency the observer hears. For sound in air v_wave = 340 m s⁻¹.
Towards = positive. v_source is positive when the source moves towards the observer; v_observer is positive when the observer moves towards the source.
Away = negative. If one is moving away from the other, its speed goes in with a minus sign.
Not moving = zero.
Sense-check every answer: if they are getting closer, f' must be higher than f; if they are moving apart, f' must be lower.

f' = f(v_wave + v_observer)/(v_wave − v_source)

Decide each sign from that object's own motion: towards the other one is +, away from it is −. Brackets around signed values, such as 340 − (−25), keep the double negatives visible.

Redshift and blueshift for light

Light shows the same effect. Light from a source moving towards us arrives with a slightly higher frequency and shorter wavelength, shifted towards the blue end of the spectrum: blueshift. Light from a source moving away arrives with a lower frequency and longer wavelength: redshift.

Astronomers see this in the dark absorption lines of a star's or galaxy's spectrum. Each line has a known wavelength measured in the lab, so if the whole pattern is shifted to longer wavelengths, the object is moving away. In Year 11 this is qualitative only: the sound equation doesn't apply to light, which needs no medium.

Common mistakes

Making v_source positive whenever the source is moving, whichever way it goes.The sign depends on direction: towards the observer is +, away is −. A source moving away at 20 m s⁻¹ goes in as −20, so the bottom line becomes 340 + 20.
Saying the pitch of an approaching siren rises steadily as it gets closer.At a steady speed the pitch heard is constant while it approaches and drops as the siren passes. Getting closer makes it louder, not higher.
Taking the distance between neighbouring nodes as one wavelength.Node to node is half a wavelength. The fundamental on a string of length L has λ = 2L.
Counting nodes to find the harmonic number.Count the loops: n loops is the nth harmonic. The third harmonic has 3 loops but 4 nodes, counting both ends.
Saying sound can travel through space, or that light needs a medium.Sound is a mechanical wave and needs particles. Light is an electromagnetic wave and travels through a vacuum.
Saying a redshifted galaxy has turned redder in colour or cooler.Redshift means its spectral lines are shifted to longer wavelengths because it is moving away from us. It is about motion, not temperature.

Worked example

An ambulance siren sends out sound at 960 Hz. The ambulance travels at 25 m s⁻¹ along a straight road, behind a cyclist riding in the same direction at 6.0 m s⁻¹. Take the speed of sound as 340 m s⁻¹.

(a) Calculate the frequency the cyclist hears while the ambulance is catching up. (3 marks)

(b) Calculate the frequency the cyclist hears after the ambulance has passed and is moving ahead. (2 marks)

(5 marks)

I'll think out loud, the way you should in the exam, using KNOW-UD: Knowns, Not known, Organise, Work it out, Units, Direction.

Prediction first. The ambulance gains on the cyclist at 25 − 6 = 19 m s⁻¹, about 6% of 340. So before it passes I expect about 6% above 960 Hz, roughly 1015 Hz. After it passes they separate at the same 19 m s⁻¹, so I expect roughly 5% below, around 910 Hz.

K · Knowns

I sketch both moments with the speeds on them. In the Doppler equation each sign depends on whether that object is moving towards or away from the other, so I read each sign off the sketch rather than using 'right is positive'.

f = 960 Hz, v_wave = 340 m s⁻¹
(a) ambulance (source) behind, moving towards the cyclist: v_source = +25 m s⁻¹
(a) cyclist (observer) moving away from the ambulance: v_observer = −6.0 m s⁻¹
(b) ambulance ahead, moving away: v_source = −25 m s⁻¹; cyclist moving towards it: v_observer = +6.0 m s⁻¹

Knowns on the sketch: f = 960 Hz, ambulance 25 m s⁻¹, cyclist 6.0 m s⁻¹, both moving the same way, sound at 340 m s⁻¹. In (a) the ambulance closes in; in (b) it pulls away.

N · Not known

Two frequencies in hertz: one while the ambulance is catching up, one after it has passed.

(a) f' = ? Hz, (b) f' = ? Hz

O · Organise

Only the signs change between (a) and (b), so I write the equation once in symbols.

f' = f(v_wave + v_observer)/(v_wave − v_source)

W · Work it out

I put each signed speed in brackets so the double negatives are visible, and keep the full calculator values.

(a) f' = 960 × (340 + (−6.0)) ÷ (340 − (+25)) = 960 × 334 ÷ 315 = 1017.9 Hz
(b) f' = 960 × (340 + (+6.0)) ÷ (340 − (−25)) = 960 × 346 ÷ 365 = 910.0 Hz

U · Units

Hertz throughout. The data have two or three significant figures, so I give three.

(a) f' ≈ 1.02 × 10³ Hz (1020 Hz)
(b) f' ≈ 910 Hz

D · Direction

Frequency has no direction, so instead I sense-check against the motion. In (a) they are getting closer, so f' should be above 960 Hz. In (b) they are moving apart, so f' should be below 960 Hz.

(a) 1020 Hz > 960 Hz: closing in, higher
(b) 910 Hz < 960 Hz: moving apart, lower

(a) About 1020 Hz (1.02 × 10³ Hz) while the ambulance catches up. (b) About 910 Hz after it has passed.

Check against the prediction. I predicted roughly 1015 Hz and 910 Hz from the closing speed of 19 m s⁻¹; I got 1018 Hz and 910 Hz. The drop the cyclist hears (about 108 Hz) is smaller than a person standing still would hear (about 142 Hz), because the cyclist is moving the same way as the ambulance.

Exam advice

What markers look for

Write the sign of each speed beside the Doppler equation before you substitute ('source towards: +25'), then check whether f' should come out higher or lower than f.CortexHSC teaching notes
For 'explain' questions (how a medium carries sound, how nodes form), link every step with cause and effect: the cone pushes the air, which makes a compression, which pushes the next layer. HSC marking feedback asks for clear cause-and-effect links in explanations, and better answers show them.NESA marking feedback, 2023, summarised
Add a labelled diagram wherever it helps: N and A marked on a standing wave with the loops counted, or wavefronts bunched ahead of a moving source. HSC feedback asks for annotated diagrams where they help explain a set-up or what happens.NESA marking feedback, 2025, summarised
Keep the full ratio in your calculator and round only the final frequency. Rounding 334 ÷ 315 to 1.1 turns a 1018 Hz answer into 1056 Hz. Early rounding is a mark-loser HSC markers report regularly.NESA marking feedback, 2022, summarised
For redshift, say what has shifted and which way: 'the spectral lines are shifted to longer wavelengths, so the galaxy is moving away'. 'It is redshifted' on its own explains nothing.CortexHSC teaching notes

Summaries in our own words. Read NESA's own feedback with the exam packs: 2022 · 2023 · 2025.

Where this leads in Year 12

In Year 12 the Doppler shift becomes a tool in astrophysics: the shift of a star's spectral lines reveals how fast it is moving towards or away from us, and the redshift of galaxy spectra is evidence that the universe is expanding. Standing waves come back in the atom, where electrons in stationary energy levels can be pictured as standing waves, and superposition explains the interference patterns of light.

FAQ

Can sound travel through space?: No. Sound is a mechanical wave, so it needs particles to pass the vibration along. Space is a near-vacuum, so sound can't cross it. Light and radio can, because they are electromagnetic waves and need no medium.
What is the sign convention for the Doppler effect equation?: For f' = f(v_wave + v_observer)/(v_wave − v_source), each speed is positive when that object moves towards the other, negative when it moves away, and zero if it is still. Then check: getting closer gives a higher f', moving apart a lower one.
What is the difference between a node and an antinode?: A node is a point on a standing wave that never moves, because the two opposite-travelling waves always cancel there. An antinode is midway between nodes, where they always add, so the amplitude is biggest. Neighbouring nodes are half a wavelength apart.
What speed of sound should I use in Year 11 Physics?: Use 340 m s⁻¹ for sound in air unless the question gives another value. The real value rises a little as air warms, and sound is much faster in water and solids.
Why does a siren's pitch drop as it passes you?: While the siren approaches, its crests are bunched up, so you hear a higher frequency than it sends out. Once it passes and moves away, the crests are spread out, so you hear a lower frequency. The siren itself never changes.

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