What is sound and how is it classified in Physics?

Sound is a type of mechanical wave that results from the vibration of particles in a medium such as air, water, or solids. In Physics, particularly in the Cambridge IGCSE curriculum, sound is classified as a longitudinal wave because the particles of the medium vibrate parallel to the direction of wave propagation.

How does the frequency of a sound wave affect its pitch?

The frequency of a sound wave determines its pitch. A higher frequency results in a higher pitch, while a lower frequency results in a lower pitch. In the context of the Cambridge IGCSE Physics syllabus, understanding the relationship between frequency and pitch is crucial for explaining how musical notes are produced and perceived.

What factors affect the speed of sound in different media?

The speed of sound is influenced by the medium through which it travels. In general, sound travels fastest in solids, slower in liquids, and slowest in gases. This is because particles in solids are more closely packed, allowing vibrations to transfer more efficiently. Temperature also affects sound speed; as temperature increases, the speed of sound in a gas typically increases due to increased particle energy and movement.

How does the amplitude of a sound wave relate to its loudness?

The amplitude of a sound wave is directly related to its loudness. A larger amplitude means the sound wave carries more energy, resulting in a louder sound. In the Cambridge IGCSE Physics curriculum, students learn that loudness is a subjective perception, but it is quantitatively related to amplitude, which can be measured in decibels (dB).

What is the Doppler Effect and how does it apply to sound waves?

The Doppler Effect refers to the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave. For sound waves, this effect is observed when a sound source moves towards or away from an observer, causing the sound to be perceived at a higher frequency (higher pitch) as it approaches and a lower frequency (lower pitch) as it moves away. This concept is an important part of the Cambridge IGCSE Physics syllabus, illustrating real-world applications such as radar and medical imaging.

Why is "Saying sound travels through a vacuum" a common mistake in Sound?

Why it happens: Confusing with light/EM. How to avoid it: Sound needs a medium. Light does not. Source: 0625/42 — recurring

Why is "Linking pitch to amplitude (or loudness to frequency)" a common mistake in Sound?

Why it happens: Mixing the two properties. How to avoid it: Pitch ↔ frequency. Loudness ↔ amplitude.

Why is "Forgetting to halve the round-trip distance for echoes" a common mistake in Sound?

Why it happens: Computing d = vt once. How to avoid it: An echo is a there-and-back signal — divide the total distance by 2 for the wall distance.

Why is "Saying sound is fastest in air" a common mistake in Sound?

Why it happens: Daily-life experience. How to avoid it: Sound is fastest in SOLIDS, slower in liquids, slowest in gases (closer particles → faster transfer).

WavesCambridge IGCSE Physics (0625)

Sound

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Sound — Cambridge IGCSE 0625 Physics Extended (2026)

Longitudinal waves of compressions and rarefactions. Hearing range, speed comparison across media, echoes, ultrasound, and pitch vs loudness on an oscilloscope.

What you’ll learn

Mapped to the Cambridge IGCSE 0625 syllabus (2026-2028).

3.7 — Describe sound as a longitudinal wave.
3.7 — State the typical speed of sound in air and compare with solid/liquid.
3.7 — State the typical hearing range of humans.
3.7 — Describe an echo and use $v = 2d/t$ .
3.7 — Describe properties and uses of ultrasound.

Sound as a longitudinal wave

Compressions (close particles) and rarefactions (spread out) move along the direction of travel.

Sound is a mechanical, longitudinal wave. A vibrating source (vocal cords, speaker cone) pushes air molecules together, then pulls them apart. The pulse propagates as alternating regions of compression and rarefaction.

No medium = no sound. A bell in a vacuum jar doesn't make audible sound — the air carrying the wave has been removed.

Drawing. Either:

Show particles bunched and spread (compression-rarefaction pattern), OR
Plot pressure vs position — a sine curve, but interpret peaks as compressions.

Speed of sound depends on medium.

Solid (steel): $\sim 5000\,\text{m/s}$ .
Liquid (water): $\sim 1500\,\text{m/s}$ .
Gas (air): $\sim 340\,\text{m/s}$ at $20\degree\text{C}$ .

Sound is FASTEST in solids — particles are closer, so collisions transfer energy more rapidly.

Longitudinal wave.
Needs a medium.
Speed: solid > liquid > gas.
Speed in air: $\sim 340\,\text{m/s}$ .

Pitch and loudness — the oscilloscope view

Pitch comes from frequency. Loudness comes from amplitude.

Pitch depends on FREQUENCY.

Higher frequency → higher pitch (treble).
Lower frequency → lower pitch (bass).

Loudness depends on AMPLITUDE.

Larger amplitude → louder.
Smaller amplitude → quieter.

Oscilloscope traces. A microphone connected to an oscilloscope shows sound as a wave on screen.

Higher pitch: more peaks per second → more cycles per division.
Louder: peaks taller → larger amplitude.

Worked qualitative. Two oscilloscope traces:

Trace A: low amplitude, short wavelength on screen → quiet AND high pitch.
Trace B: high amplitude, long wavelength → loud AND low pitch.

Cambridge tip. Oscilloscope time-base controls horizontal scale. To compare pitches, the time-base must be the SAME on both traces.

Pitch from frequency.
Loudness from amplitude.
Oscilloscope: visualise both at once.
Same time-base when comparing two sounds.

Echoes

Echo = sound reflected off a hard surface. $v = 2d/t$ — sound travels there AND back.

Echo. When sound hits a hard surface (cliff, wall, sea floor), it reflects. If you're far enough away, you hear two distinct sounds: the original and the reflection.

Distance from echo timing. $v = \dfrac{2d}{t},$ where $d$ is the distance to the reflector, $t$ is the time between making the sound and hearing the echo. The factor of $2$ accounts for the round trip.

Worked. Standing $170\,\text{m}$ from a cliff. Hear echo $1\,\text{s}$ after shouting.

$v = 2 \times 170 / 1 = 340\,\text{m/s}$ . ✓ (matches speed of sound in air).

Sonar. Same idea on the sea: a ship sends a pulse downward, measures the time before the echo returns. Calculates depth.

Worked. Sonar pulse echoes back from sea floor in $0.4\,\text{s}$ . Speed of sound in water $1500\,\text{m/s}$ .

$d = (v \times t)/2 = (1500 \times 0.4)/2 = 300\,\text{m}$ deep.

Echo = reflected sound.
$v = 2d/t$ (round trip).
Sonar uses this for sea floor depth.

Ultrasound

Sound above $20{,}000\,\text{Hz}$ — beyond human hearing. Used for safe imaging.

Ultrasound. Sound waves with frequency $> 20{,}000\,\text{Hz}$ . Can't be heard by humans.

Uses.

Pre-natal scans: ultrasound waves bounce off internal tissues and a foetus; echoes are detected and converted into an image. Safe — no ionising radiation, unlike X-rays.
Industrial cleaning: ultrasound vibrations dislodge dirt from delicate parts (jewellery, electronics).
Echolocation by bats and dolphins: emit ultrasound and listen for echoes to navigate.
Sonar in ships and submarines: as above.
Quality control / flaw detection: ultrasound passes through metal castings; echoes from internal defects show up.

Why ultrasound for medical scans (not X-rays)?

Non-ionising → no risk of cell damage.
Soft-tissue contrast.

Why ultrasound for cleaning (not water alone)?

Vibrations cause cavitation (bubbles forming and collapsing) which physically dislodges contamination.

Ultrasound: $f > 20{,}000\,\text{Hz}$ .
Pre-natal scans: safe alternative to X-rays.
Industrial cleaning, echolocation, sonar.

How it’s examined

Sound appears every Paper 2 (3-4 marks: hearing range, longitudinal/transverse) and many Paper 4s (5-7 marks: echo timing or oscilloscope trace comparisons). Examiner reports flag forgetting the factor of 2 in echo distance, and confusing pitch (frequency) with loudness (amplitude).

Sources: Cambridge IGCSE Physics 0625 syllabus 2026-2028 (3.7); 0625/42 Oct/Nov 2024 — Q15 (echo, ultrasound); 0625 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

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Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Sound, ready to print or save as PDF.

Step-by-step worked examples — Sound

Step-by-step solutions to past-paper-style questions on sound, written exactly the way a tutor would explain them at the board.

1Speed of sound in air
Core• speed
Question
A clap is heard $0.4\,\text{s}$ after it is seen, from $136\,\text{m}$ away. Find the speed of sound.
Step-by-step solution
1. Step 1
  $v = s/t$ .
  $v = \dfrac{136}{0.4} = 340\,\text{m/s}$
Answer
$340\,\text{m/s}$
2Echo distance
Extended• Adapted from 0625/42 May/Jun 2024 Q15• echo
Question
An echo of a clap returns after $1.2\,\text{s}$ . Take $v_{\text{sound}} = 340\,\text{m/s}$ . Find the distance to the wall.
Step-by-step solution
1. Step 1
  Sound travels there AND back. Total distance.
  $d = vt = 340 \times 1.2 = 408\,\text{m}$
2. Step 2
  One-way distance.
  $= \dfrac{408}{2} = 204\,\text{m}$
Answer
$204\,\text{m}$
3Pitch vs loudness
Core• pitch, loudness
Question
Two tuning forks. Fork A produces a higher-pitched sound than B; B produces a louder sound than A. Compare their frequency and amplitude.
Step-by-step solution
1. Step 1
  Higher pitch → HIGHER frequency.
  $f_{A} > f_{B}$
2. Step 2
  Louder → larger amplitude.
  $\text{amplitude}_{B} > \text{amplitude}_{A}$
Answer
$f_{A} > f_{B}$ , $\text{amp}_{B} > \text{amp}_{A}$
4Sound cannot travel in a vacuum
Core• vacuum
Question
Explain why a bell ringing inside a vacuum chamber cannot be heard.
Step-by-step solution
1. Step 1
  Sound is a longitudinal wave needing PARTICLES to compress and rarefy.
2. Step 2
  A vacuum has no particles → no medium → no sound transmission.
Answer
Sound needs a material medium to propagate; a vacuum has no particles to oscillate.
5Speed of sound in air, water and steel
Extended• Adapted from 0625/42 May/Jun 2023 Q15• speed, media
Question
Approximate speeds of sound: air $340\,\text{m/s}$ , water $1500\,\text{m/s}$ , steel $5000\,\text{m/s}$ . (a) Place them in increasing order. (b) Explain in particle terms why sound travels faster in solids than in gases.
Step-by-step solution
1. Step 1
  Increasing order.
  $v_{\text{air}} < v_{\text{water}} < v_{\text{steel}}$
2. Step 2
  In a solid, particles are CLOSE together and strongly bonded. A compression at one particle is passed almost instantly to the next.
3. Step 3
  In a gas, particles are far apart and only interact via collisions, so the compression travels much more slowly.
Answer
(a) air < water < steel. (b) Sound is fastest in solids because particles are closer together and more strongly bonded → quicker transfer of the compression.
6Find frequency from an oscilloscope trace
Extended• period, oscilloscope
Question
An oscilloscope is set to a time-base of $2\,\text{ms}$ per division. One complete waveform of a pure tone fits in exactly $4$ divisions. Find the period and the frequency.
Step-by-step solution
1. Step 1
  Period = width of one wave in seconds.
  $T = 4 \times 2\,\text{ms} = 8\,\text{ms} = 8 \times 10^{-3}\,\text{s}$
2. Step 2
  $f = 1/T$ .
  $f = \dfrac{1}{8 \times 10^{-3}} = 125\,\text{Hz}$
Answer
$T = 8\,\text{ms}$ ; $f = 125\,\text{Hz}$
Examiner tip
The examiner report flags candidates often forget to convert milliseconds to seconds before computing $f = 1/T$ , ending up with $f = 125\,\text{kHz}$ instead of $125\,\text{Hz}$ .
7Ultrasound application — sonar depth
Extended• ultrasound, sonar
Question
A ship's sonar emits an ultrasound pulse vertically downwards. The echo returns $0.40\,\text{s}$ later. The speed of sound in seawater is $1500\,\text{m/s}$ . Find the depth of the sea below the ship.
Step-by-step solution
1. Step 1
  Sound travels DOWN and back UP.
  $2d = v t = 1500 \times 0.40 = 600\,\text{m}$
2. Step 2
  One-way depth.
  $d = \dfrac{600}{2} = 300\,\text{m}$
Answer
$d = 300\,\text{m}$
8Pitch vs loudness from oscilloscope traces
Extended• oscilloscope, pitch, loudness
Question
Two oscilloscope traces use the SAME time-base and gain. Trace P has 4 cycles in 10 divisions and peaks at $2$ divisions above zero. Trace Q has 2 cycles in 10 divisions and peaks at $3$ divisions above zero. Compare the pitches and loudnesses.
Step-by-step solution
1. Step 1
  More cycles per division → higher frequency → higher pitch. Trace P has TWICE as many cycles as Q, so $f_{P} = 2 f_{Q}$ .
2. Step 2
  Bigger peak height → larger amplitude → louder. Trace Q has 1.5× the amplitude of P, so Q is louder.
Answer
P is higher in pitch (twice the frequency of Q). Q is louder than P (1.5× amplitude).
9Compressions and rarefactions in a sound wave
Extended• longitudinal
Question
A tuning fork vibrates and sends sound through air. (a) Label what is meant by a COMPRESSION and a RAREFACTION. (b) State the relationship between the wavelength of the sound and the spacing of the compressions.
Step-by-step solution
1. Step 1
  (a) Compression: a region where particles are closer together than average (high pressure).
2. Step 2
  (a) Rarefaction: a region where particles are further apart than average (low pressure).
3. Step 3
  (b) The wavelength of a sound wave is the distance from one compression to the next compression (or one rarefaction to the next).
Answer
(a) Compression = high-density region; rarefaction = low-density region. (b) $\lambda$ = distance between adjacent compressions.
10A* — Distance to a thunderstorm
Challenge• Adapted from 0625/42 Oct/Nov 2024 Q15• synoptic
Question
A student sees a flash of lightning, then hears the thunder $4.5\,\text{s}$ later. Take $v_{\text{light}} \approx 3.0 \times 10^{8}\,\text{m/s}$ and $v_{\text{sound}} = 340\,\text{m/s}$ . (a) Justify ignoring the travel time of light. (b) Find the distance to the lightning, in km.
Step-by-step solution
1. Step 1
  (a) For any reasonable storm distance (a few km), light takes microseconds while sound takes seconds. Light's travel time is negligible compared with sound's.
2. Step 2
  (b) All of the $4.5\,\text{s}$ is taken by sound travelling from the lightning to the student.
  $d = v t = 340 \times 4.5$
3. Step 3
  Compute and convert.
  $d = 1530\,\text{m} \approx 1.5\,\text{km}$
Answer
(a) Light is roughly $10^{6}$ times faster than sound → its travel time is negligible. (b) $d \approx 1.5\,\text{km}$ .

Key Formulae — Sound

The formulae you need to memorise for sound on the Cambridge IGCSE 0625 paper, with every variable defined in plain English and a note on when to use it.

Speed of sound
$v_{\text{air}} \approx 340\,\text{m/s} \text{ (varies with temperature, humidity)}$
When to use
Air at room temperature.
Wave equation (sound)
$v = f\lambda$
When to use
Connect frequency, wavelength and speed of sound.

Key Definitions and Keywords — Sound

Definitions to memorise and the exact keywords mark schemes credit for sound answers — sharpened from recent examiner reports for the 2026 0625 sitting.

Sound wave
Examiner keyword
A longitudinal mechanical wave caused by the vibration of particles in a medium.
Pitch
Examiner keyword
Determined by frequency. Higher frequency → higher pitch.
Loudness
Examiner keyword
Determined by amplitude. Larger amplitude → louder sound.
Audible range
Examiner keyword
Approximately $20\,\text{Hz}$ to $20{,}000\,\text{Hz}$ for a healthy young human.
Ultrasound
Examiner keyword
Sound with frequency above $20{,}000\,\text{Hz}$ . Used in medical imaging and sonar.

Common Mistakes and Misconceptions — Sound

The traps other students keep falling into on sound questions — taken from recent Cambridge IGCSE 0625 examiner reports and mark schemes — and how to avoid them.

✕Saying sound travels through a vacuum
0625/42 — recurring
Why it happens
Confusing with light/EM.
How to avoid it
Sound needs a medium. Light does not.
✕Linking pitch to amplitude (or loudness to frequency)
Why it happens
Mixing the two properties.
How to avoid it
Pitch ↔ frequency. Loudness ↔ amplitude.
✕Forgetting to halve the round-trip distance for echoes
Why it happens
Computing $d = vt$ once.
How to avoid it
An echo is a there-and-back signal — divide the total distance by 2 for the wall distance.
✕Saying sound is fastest in air
Why it happens
Daily-life experience.
How to avoid it
Sound is fastest in SOLIDS, slower in liquids, slowest in gases (closer particles → faster transfer).

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Sound — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

WavesCambridge IGCSE Physics (0625)

Sound

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

Previous Next

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Page 1 / 0

Detailed Study Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Sound, ready to print or save offline.

Sound — Cambridge IGCSE 0625 Physics Extended (2026)

Longitudinal waves of compressions and rarefactions. Hearing range, speed comparison across media, echoes, ultrasound, and pitch vs loudness on an oscilloscope.

What you’ll learn

Mapped to the Cambridge IGCSE 0625 syllabus (2026-2028).

3.7 — Describe sound as a longitudinal wave.
3.7 — State the typical speed of sound in air and compare with solid/liquid.
3.7 — State the typical hearing range of humans.
3.7 — Describe an echo and use $v = 2d/t$ .
3.7 — Describe properties and uses of ultrasound.

Sound as a longitudinal wave

Compressions (close particles) and rarefactions (spread out) move along the direction of travel.

No medium = no sound. A bell in a vacuum jar doesn't make audible sound — the air carrying the wave has been removed.

Drawing. Either:

Show particles bunched and spread (compression-rarefaction pattern), OR
Plot pressure vs position — a sine curve, but interpret peaks as compressions.

Speed of sound depends on medium.

Solid (steel): $\sim 5000\,\text{m/s}$ .
Liquid (water): $\sim 1500\,\text{m/s}$ .
Gas (air): $\sim 340\,\text{m/s}$ at $20\degree\text{C}$ .

Sound is FASTEST in solids — particles are closer, so collisions transfer energy more rapidly.

Longitudinal wave.
Needs a medium.
Speed: solid > liquid > gas.
Speed in air: $\sim 340\,\text{m/s}$ .

Pitch and loudness — the oscilloscope view

Pitch comes from frequency. Loudness comes from amplitude.

Pitch depends on FREQUENCY.

Higher frequency → higher pitch (treble).
Lower frequency → lower pitch (bass).

Loudness depends on AMPLITUDE.

Larger amplitude → louder.
Smaller amplitude → quieter.

Oscilloscope traces. A microphone connected to an oscilloscope shows sound as a wave on screen.

Higher pitch: more peaks per second → more cycles per division.
Louder: peaks taller → larger amplitude.

Worked qualitative. Two oscilloscope traces:

Trace A: low amplitude, short wavelength on screen → quiet AND high pitch.
Trace B: high amplitude, long wavelength → loud AND low pitch.

Cambridge tip. Oscilloscope time-base controls horizontal scale. To compare pitches, the time-base must be the SAME on both traces.

Pitch from frequency.
Loudness from amplitude.
Oscilloscope: visualise both at once.
Same time-base when comparing two sounds.

Echoes

Echo = sound reflected off a hard surface. $v = 2d/t$ — sound travels there AND back.

Echo. When sound hits a hard surface (cliff, wall, sea floor), it reflects. If you're far enough away, you hear two distinct sounds: the original and the reflection.

Worked. Standing $170\,\text{m}$ from a cliff. Hear echo $1\,\text{s}$ after shouting.

$v = 2 \times 170 / 1 = 340\,\text{m/s}$ . ✓ (matches speed of sound in air).

Sonar. Same idea on the sea: a ship sends a pulse downward, measures the time before the echo returns. Calculates depth.

Worked. Sonar pulse echoes back from sea floor in $0.4\,\text{s}$ . Speed of sound in water $1500\,\text{m/s}$ .

$d = (v \times t)/2 = (1500 \times 0.4)/2 = 300\,\text{m}$ deep.

Echo = reflected sound.
$v = 2d/t$ (round trip).
Sonar uses this for sea floor depth.

Ultrasound

Sound above $20{,}000\,\text{Hz}$ — beyond human hearing. Used for safe imaging.

Ultrasound. Sound waves with frequency $> 20{,}000\,\text{Hz}$ . Can't be heard by humans.

Uses.

Pre-natal scans: ultrasound waves bounce off internal tissues and a foetus; echoes are detected and converted into an image. Safe — no ionising radiation, unlike X-rays.
Industrial cleaning: ultrasound vibrations dislodge dirt from delicate parts (jewellery, electronics).
Echolocation by bats and dolphins: emit ultrasound and listen for echoes to navigate.
Sonar in ships and submarines: as above.
Quality control / flaw detection: ultrasound passes through metal castings; echoes from internal defects show up.

Why ultrasound for medical scans (not X-rays)?

Non-ionising → no risk of cell damage.
Soft-tissue contrast.

Why ultrasound for cleaning (not water alone)?

Vibrations cause cavitation (bubbles forming and collapsing) which physically dislodges contamination.

Ultrasound: $f > 20{,}000\,\text{Hz}$ .
Pre-natal scans: safe alternative to X-rays.
Industrial cleaning, echolocation, sonar.

How it’s examined

Sources: Cambridge IGCSE Physics 0625 syllabus 2026-2028 (3.7); 0625/42 Oct/Nov 2024 — Q15 (echo, ultrasound); 0625 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Sound, ready to print or save as PDF.

Step-by-step worked examples — Sound

Step-by-step solutions to past-paper-style questions on sound, written exactly the way a tutor would explain them at the board.

1Speed of sound in air
Core• speed
Question
A clap is heard $0.4\,\text{s}$ after it is seen, from $136\,\text{m}$ away. Find the speed of sound.
Step-by-step solution
1. Step 1
  $v = s/t$ .
  $v = \dfrac{136}{0.4} = 340\,\text{m/s}$
Answer
$340\,\text{m/s}$
2Echo distance
Extended• Adapted from 0625/42 May/Jun 2024 Q15• echo
Question
An echo of a clap returns after $1.2\,\text{s}$ . Take $v_{\text{sound}} = 340\,\text{m/s}$ . Find the distance to the wall.
Step-by-step solution
1. Step 1
  Sound travels there AND back. Total distance.
  $d = vt = 340 \times 1.2 = 408\,\text{m}$
2. Step 2
  One-way distance.
  $= \dfrac{408}{2} = 204\,\text{m}$
Answer
$204\,\text{m}$
3Pitch vs loudness
Core• pitch, loudness
Question
Two tuning forks. Fork A produces a higher-pitched sound than B; B produces a louder sound than A. Compare their frequency and amplitude.
Step-by-step solution
1. Step 1
  Higher pitch → HIGHER frequency.
  $f_{A} > f_{B}$
2. Step 2
  Louder → larger amplitude.
  $\text{amplitude}_{B} > \text{amplitude}_{A}$
Answer
$f_{A} > f_{B}$ , $\text{amp}_{B} > \text{amp}_{A}$
4Sound cannot travel in a vacuum
Core• vacuum
Question
Explain why a bell ringing inside a vacuum chamber cannot be heard.
Step-by-step solution
1. Step 1
  Sound is a longitudinal wave needing PARTICLES to compress and rarefy.
2. Step 2
  A vacuum has no particles → no medium → no sound transmission.
Answer
Sound needs a material medium to propagate; a vacuum has no particles to oscillate.
5Speed of sound in air, water and steel
Extended• Adapted from 0625/42 May/Jun 2023 Q15• speed, media
Question
Approximate speeds of sound: air $340\,\text{m/s}$ , water $1500\,\text{m/s}$ , steel $5000\,\text{m/s}$ . (a) Place them in increasing order. (b) Explain in particle terms why sound travels faster in solids than in gases.
Step-by-step solution
1. Step 1
  Increasing order.
  $v_{\text{air}} < v_{\text{water}} < v_{\text{steel}}$
2. Step 2
  In a solid, particles are CLOSE together and strongly bonded. A compression at one particle is passed almost instantly to the next.
3. Step 3
  In a gas, particles are far apart and only interact via collisions, so the compression travels much more slowly.
Answer
(a) air < water < steel. (b) Sound is fastest in solids because particles are closer together and more strongly bonded → quicker transfer of the compression.
6Find frequency from an oscilloscope trace
Extended• period, oscilloscope
Question
An oscilloscope is set to a time-base of $2\,\text{ms}$ per division. One complete waveform of a pure tone fits in exactly $4$ divisions. Find the period and the frequency.
Step-by-step solution
1. Step 1
  Period = width of one wave in seconds.
  $T = 4 \times 2\,\text{ms} = 8\,\text{ms} = 8 \times 10^{-3}\,\text{s}$
2. Step 2
  $f = 1/T$ .
  $f = \dfrac{1}{8 \times 10^{-3}} = 125\,\text{Hz}$
Answer
$T = 8\,\text{ms}$ ; $f = 125\,\text{Hz}$
Examiner tip
The examiner report flags candidates often forget to convert milliseconds to seconds before computing $f = 1/T$ , ending up with $f = 125\,\text{kHz}$ instead of $125\,\text{Hz}$ .
7Ultrasound application — sonar depth
Extended• ultrasound, sonar
Question
A ship's sonar emits an ultrasound pulse vertically downwards. The echo returns $0.40\,\text{s}$ later. The speed of sound in seawater is $1500\,\text{m/s}$ . Find the depth of the sea below the ship.
Step-by-step solution
1. Step 1
  Sound travels DOWN and back UP.
  $2d = v t = 1500 \times 0.40 = 600\,\text{m}$
2. Step 2
  One-way depth.
  $d = \dfrac{600}{2} = 300\,\text{m}$
Answer
$d = 300\,\text{m}$
8Pitch vs loudness from oscilloscope traces
Extended• oscilloscope, pitch, loudness
Question
Two oscilloscope traces use the SAME time-base and gain. Trace P has 4 cycles in 10 divisions and peaks at $2$ divisions above zero. Trace Q has 2 cycles in 10 divisions and peaks at $3$ divisions above zero. Compare the pitches and loudnesses.
Step-by-step solution
1. Step 1
  More cycles per division → higher frequency → higher pitch. Trace P has TWICE as many cycles as Q, so $f_{P} = 2 f_{Q}$ .
2. Step 2
  Bigger peak height → larger amplitude → louder. Trace Q has 1.5× the amplitude of P, so Q is louder.
Answer
P is higher in pitch (twice the frequency of Q). Q is louder than P (1.5× amplitude).
9Compressions and rarefactions in a sound wave
Extended• longitudinal
Question
A tuning fork vibrates and sends sound through air. (a) Label what is meant by a COMPRESSION and a RAREFACTION. (b) State the relationship between the wavelength of the sound and the spacing of the compressions.
Step-by-step solution
1. Step 1
  (a) Compression: a region where particles are closer together than average (high pressure).
2. Step 2
  (a) Rarefaction: a region where particles are further apart than average (low pressure).
3. Step 3
  (b) The wavelength of a sound wave is the distance from one compression to the next compression (or one rarefaction to the next).
Answer
(a) Compression = high-density region; rarefaction = low-density region. (b) $\lambda$ = distance between adjacent compressions.
10A* — Distance to a thunderstorm
Challenge• Adapted from 0625/42 Oct/Nov 2024 Q15• synoptic
Question
A student sees a flash of lightning, then hears the thunder $4.5\,\text{s}$ later. Take $v_{\text{light}} \approx 3.0 \times 10^{8}\,\text{m/s}$ and $v_{\text{sound}} = 340\,\text{m/s}$ . (a) Justify ignoring the travel time of light. (b) Find the distance to the lightning, in km.
Step-by-step solution
1. Step 1
  (a) For any reasonable storm distance (a few km), light takes microseconds while sound takes seconds. Light's travel time is negligible compared with sound's.
2. Step 2
  (b) All of the $4.5\,\text{s}$ is taken by sound travelling from the lightning to the student.
  $d = v t = 340 \times 4.5$
3. Step 3
  Compute and convert.
  $d = 1530\,\text{m} \approx 1.5\,\text{km}$
Answer
(a) Light is roughly $10^{6}$ times faster than sound → its travel time is negligible. (b) $d \approx 1.5\,\text{km}$ .

Key Formulae — Sound

The formulae you need to memorise for sound on the Cambridge IGCSE 0625 paper, with every variable defined in plain English and a note on when to use it.

Speed of sound
$v_{\text{air}} \approx 340\,\text{m/s} \text{ (varies with temperature, humidity)}$
When to use
Air at room temperature.
Wave equation (sound)
$v = f\lambda$
When to use
Connect frequency, wavelength and speed of sound.

Key Definitions and Keywords — Sound

Definitions to memorise and the exact keywords mark schemes credit for sound answers — sharpened from recent examiner reports for the 2026 0625 sitting.

Sound wave
Examiner keyword
A longitudinal mechanical wave caused by the vibration of particles in a medium.
Pitch
Examiner keyword
Determined by frequency. Higher frequency → higher pitch.
Loudness
Examiner keyword
Determined by amplitude. Larger amplitude → louder sound.
Audible range
Examiner keyword
Approximately $20\,\text{Hz}$ to $20{,}000\,\text{Hz}$ for a healthy young human.
Ultrasound
Examiner keyword
Sound with frequency above $20{,}000\,\text{Hz}$ . Used in medical imaging and sonar.

Common Mistakes and Misconceptions — Sound

The traps other students keep falling into on sound questions — taken from recent Cambridge IGCSE 0625 examiner reports and mark schemes — and how to avoid them.

✕Saying sound travels through a vacuum
0625/42 — recurring
Why it happens
Confusing with light/EM.
How to avoid it
Sound needs a medium. Light does not.
✕Linking pitch to amplitude (or loudness to frequency)
Why it happens
Mixing the two properties.
How to avoid it
Pitch ↔ frequency. Loudness ↔ amplitude.
✕Forgetting to halve the round-trip distance for echoes
Why it happens
Computing $d = vt$ once.
How to avoid it
An echo is a there-and-back signal — divide the total distance by 2 for the wall distance.
✕Saying sound is fastest in air
Why it happens
Daily-life experience.
How to avoid it
Sound is fastest in SOLIDS, slower in liquids, slowest in gases (closer particles → faster transfer).

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Sound — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Sound

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Speed of sound in air

2Echo distance

3Pitch vs loudness

4Sound cannot travel in a vacuum

5Speed of sound in air, water and steel

6Find frequency from an oscilloscope trace

7Ultrasound application — sonar depth

8Pitch vs loudness from oscilloscope traces

9Compressions and rarefactions in a sound wave

10A* — Distance to a thunderstorm

Speed of sound

Wave equation (sound)

Sound wave

Pitch

Loudness

Audible range

Ultrasound

✕Saying sound travels through a vacuum

✕Linking pitch to amplitude (or loudness to frequency)

✕Forgetting to halve the round-trip distance for echoes

✕Saying sound is fastest in air

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Sound — frequently asked questions

Sound

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Speed of sound in air

2Echo distance

3Pitch vs loudness

4Sound cannot travel in a vacuum

5Speed of sound in air, water and steel

6Find frequency from an oscilloscope trace

7Ultrasound application — sonar depth

8Pitch vs loudness from oscilloscope traces

9Compressions and rarefactions in a sound wave

10A* — Distance to a thunderstorm

Speed of sound

Wave equation (sound)

Sound wave

Pitch

Loudness

Audible range

Ultrasound

✕Saying sound travels through a vacuum

✕Linking pitch to amplitude (or loudness to frequency)

✕Forgetting to halve the round-trip distance for echoes

✕Saying sound is fastest in air

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Sound — frequently asked questions