What is sound and how is it produced?

Sound is a type of wave that is created by vibrating objects and propagates through a medium such as air, water, or solids. When an object vibrates, it causes the surrounding particles in the medium to vibrate as well, creating a wave of pressure that travels through the medium. This is how sound is produced and transmitted.

How does sound travel through different mediums?

Sound travels through different mediums by causing the particles within those mediums to vibrate. In solids, particles are closely packed, allowing sound waves to travel faster compared to liquids and gases. In liquids, sound travels slower than in solids but faster than in gases, as the particles are less tightly packed. In gases, such as air, sound travels the slowest because the particles are spread out and require more time to transfer the energy of the wave.

What are the key properties of sound waves?

The key properties of sound waves include frequency, amplitude, wavelength, and speed. Frequency refers to the number of vibrations per second and is measured in Hertz (Hz), determining the pitch of the sound. Amplitude is the height of the wave and relates to the loudness of the sound. Wavelength is the distance between two consecutive points in phase on the wave, such as crest to crest. The speed of sound varies depending on the medium through which it travels.

How does the human ear perceive sound?

The human ear perceives sound through a process that involves capturing sound waves and converting them into electrical signals that the brain can interpret. Sound waves enter the ear canal and cause the eardrum to vibrate. These vibrations are transmitted through the ossicles in the middle ear to the cochlea in the inner ear, where they are converted into electrical signals by hair cells. These signals are then sent to the brain via the auditory nerve, allowing us to perceive sound.

What is the difference between pitch and loudness in sound?

Pitch and loudness are two distinct properties of sound. Pitch is determined by the frequency of the sound wave; higher frequencies produce higher pitches, while lower frequencies produce lower pitches. Loudness, on the other hand, is related to the amplitude of the sound wave; larger amplitudes result in louder sounds, while smaller amplitudes result in softer sounds. Both pitch and loudness are important characteristics that help us identify and differentiate sounds.

Why is "Stating that sound can travel through a vacuum" a common mistake in Sound?

Why it happens: Students confuse sound with light or EM waves. How to avoid it: Sound is a mechanical wave requiring a medium (particles) to transmit energy. It cannot travel through a vacuum. This is why space is silent.

Why is "Forgetting to divide the total time by 2 in echo calculations" a common mistake in Sound?

Why it happens: Students use d = vt with the full echo time, giving double the actual distance. How to avoid it: Write d = vt/2 from the start. The sound travels to the reflector AND back — the distance is only half the total path.

Why is "Confusing loudness with pitch" a common mistake in Sound?

Why it happens: Both describe how a sound 'sounds' and students mix the terms. How to avoid it: Pitch = frequency. Loudness = amplitude (or intensity). A high-pitched sound can be quiet; a low-pitched sound can be loud.

Properties of Waves, including Light and SoundCambridge IGCSE Coordinated Sciences 0654

Sound

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Sound — Cambridge IGCSE Coordinated Science 0654

Sound is a longitudinal mechanical wave that requires a medium. Cambridge tests wave properties of sound (frequency = pitch, amplitude = loudness), the speed of sound in different media, echoes, and ultrasound applications.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

P4.4a — Describe sound as a longitudinal wave requiring a medium.
P4.4b — Explain pitch and loudness in terms of frequency and amplitude.
P4.4c — Describe echo and calculate speed of sound using echoes.
P4.4d — Describe uses of ultrasound.

Sound as a Longitudinal Wave

Sound travels as compressions and rarefactions; cannot travel through vacuum.

Nature of sound:

Longitudinal wave: particles of medium oscillate parallel to the direction of wave travel
Consists of compressions (regions of high pressure, particles close together) and rarefactions (low pressure, particles spread out)
Requires a medium (cannot travel through vacuum — no particles to vibrate)

Evidence: Bell jar experiment: bell rings inside evacuated jar → sound fades as air is removed → silence in vacuum.

Speed of sound in different media:

Medium	Speed
Air (20°C)	≈ 330–340 m/s
Water	≈ 1500 m/s
Steel	≈ 5000 m/s

Sound travels faster in denser/more rigid media (particles closer → vibrations transmitted more quickly).

Pitch and loudness:

Pitch: determined by frequency. Higher frequency → higher pitch.
Loudness: determined by amplitude. Larger amplitude → louder sound.
Timbre (quality): determined by waveform shape → how we distinguish instruments

Human hearing range: 20 Hz – 20 000 Hz (20 kHz)

Below 20 Hz: infrasound (earthquakes, whales)
Above 20 kHz: ultrasound (bats, dolphins, medical)

Oscilloscope display:

Shows sound as a transverse wave (vibration up/down represents pressure)
Taller wave → louder (greater amplitude)
More waves on screen → higher frequency → higher pitch

Sound: longitudinal; compressions + rarefactions; needs medium (not vacuum).
Faster in solids > liquids > gases (particles closer → faster transmission).
Pitch ∝ frequency. Loudness ∝ amplitude.

Echoes and Ultrasound

Reflected sound (echo) is used to measure distance; ultrasound has medical and industrial uses.

Echoes:

Reflected sound waves
Used to calculate the distance to a reflecting surface:

distance = (speed × time) / 2 OR: speed = 2 × distance / time

Factor of 2 because sound travels TO the reflector and BACK

Example: A ship sends a sonar pulse that returns after 0.4 s. Speed of sound in water = 1500 m/s. Find depth.

d = (1500 × 0.4) / 2 = 300 m

Ultrasound (f > 20 000 Hz):

Cannot be heard by humans
Used where precision is needed or sound must pass through tissue

Medical uses:

Foetal scanning: safe alternative to X-rays; ultrasound pulses sent into body; reflected at boundaries between different tissues; timing of reflections → image constructed
Kidney stone detection: locate position of stones without surgery
Echocardiography: imaging the heart

Industrial uses:

SONAR (Sound Navigation And Ranging): locating submarines, shoals of fish, mapping ocean floor
Flaw detection: ultrasound reflects from cracks inside metal components (e.g., aircraft wings, pipelines)

Why ultrasound is preferred over X-rays for foetal imaging:

No ionising radiation → no DNA damage risk to developing foetus
Safe to use repeatedly

Echo: reflected sound. distance = speed × time / 2.
Ultrasound (>20 kHz): medical scanning (foetus, kidneys), SONAR, flaw detection.
Ultrasound safer than X-ray: no ionising radiation.

How it’s examined

Paper 4: 'A bat emits an ultrasound pulse that reflects from a wall and returns after 0.02 s. Speed of sound = 340 m/s. Calculate the distance to the wall' (2 marks — d = 340 × 0.02/2 = 3.4 m). 'State TWO uses of ultrasound in medicine' (2 marks — foetal scanning; kidney stone detection). 'Explain why sound cannot travel through a vacuum' (2 marks — sound needs particles to transmit vibrations; vacuum has no particles). Oscilloscope: 'Describe how the trace would change if the note becomes louder but the same pitch' (2 marks — greater amplitude; same frequency/number of waves).

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (P4); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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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.

1Calculating distance using echo
Core• echo, speed of sound, time
Question
A ship sends out a sound pulse and receives the echo from the seabed after $0.40\,\text{s}$ . The speed of sound in sea water is $1500\,\text{m/s}$ . Calculate the depth of the seabed.
Step-by-step solution
1. Step 1
  The pulse travels to the seabed and back, so the distance to the seabed = total distance / 2.
  $\text{Total distance} = v \times t = 1500 \times 0.40 = 600\,\text{m}$
2. Step 2
  Depth = total distance / 2.
  $d = \dfrac{600}{2} = 300\,\text{m}$
Answer
Depth $= 300\,\text{m}$
Examiner tip
The time given is the round-trip time. Always divide by 2 for the one-way distance.
2Frequency, pitch, and amplitude
Extended• pitch, loudness, frequency, amplitude
Question
Two musical notes are played. Note A has a higher frequency than note B. Note B is louder than note A. Sketch the waveforms for both notes on the same time axis and compare their pitch and loudness.
Step-by-step solution
1. Step 1
  Note A: higher frequency → more cycles per second → waves closer together on the time axis (shorter period). Lower amplitude → smaller peak-to-trough distance.
2. Step 2
  Note B: lower frequency → waves further apart on the time axis. Higher amplitude → larger peak-to-trough distance.
3. Step 3
  Pitch: Note A is higher pitched (higher frequency). Loudness: Note B is louder (larger amplitude).
Answer
A: high frequency (closely spaced waves), small amplitude. B: low frequency (widely spaced waves), large amplitude. A is higher pitched; B is louder.
3Ultrasound in medical imaging
Challenge• ultrasound, medical imaging, pulse-echo
Question
Explain how ultrasound is used to produce an image of a foetus. Include the meaning of 'ultrasound' and why it is preferred over X-rays for this purpose.
Step-by-step solution
1. Step 1
  Ultrasound is sound with a frequency above the upper limit of human hearing, $> 20\,000\,\text{Hz}$ .
2. Step 2
  A transducer (probe) emits pulses of ultrasound directed into the body. At each boundary between tissues of different densities (e.g., tissue/bone, tissue/fluid), some of the pulse is reflected as an echo.
3. Step 3
  The time between emission and reception of each echo is measured. Using $d = vt/2$ with the known speed of ultrasound in tissue, the depth of each boundary is calculated. A computer uses these data to construct a 2D image.
4. Step 4
  Advantage over X-rays: ultrasound is non-ionising — it does not damage DNA or increase cancer risk. X-rays are ionising and could harm the developing foetus.
Answer
Ultrasound ( $> 20\,000\,\text{Hz}$ ) pulses reflect at tissue boundaries; echo timing gives depth. Preferred over X-rays because it is non-ionising and safer for the foetus.

Key Formulae — Sound

The formulae you need to memorise for sound on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.

Echo distance
$d = \dfrac{v \times t}{2}$
$d$
distance to reflecting surface (m)
$v$
speed of sound in medium (m/s)
$t$
time for round trip (s)
When to use
Finding the depth or distance of a reflecting surface from an echo time.
Wave equation (sound)
$v = f\lambda$
$v$
speed of sound (m/s)
$f$
frequency (Hz)
$\lambda$
wavelength (m)
When to use
Relating the speed, frequency, and wavelength of a sound wave.

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 0654 sitting.

Longitudinal wave
Examiner keyword
A wave in which the oscillations of particles are parallel to the direction of energy transfer. Sound is a longitudinal wave consisting of alternating compressions and rarefactions.
Compression
Examiner keyword
A region in a longitudinal wave where particles are closer together than normal, producing a region of higher pressure.
Related: rarefaction
Rarefaction
Examiner keyword
A region in a longitudinal wave where particles are further apart than normal, producing a region of lower pressure.
Related: compression
Ultrasound
Examiner keyword
Sound with a frequency above the upper limit of human hearing ( $> 20\,000\,\text{Hz}$ or $20\,\text{kHz}$ ). Used in medical imaging, sonar, and cleaning.
Pitch
The perceived highness or lowness of a sound, determined by its frequency. Higher frequency → higher pitch.
Related: frequency, amplitude

Common Mistakes and Misconceptions — Sound

The traps other students keep falling into on sound questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Stating that sound can travel through a vacuum
Why it happens
Students confuse sound with light or EM waves.
How to avoid it
Sound is a mechanical wave requiring a medium (particles) to transmit energy. It cannot travel through a vacuum. This is why space is silent.
✕Forgetting to divide the total time by 2 in echo calculations
Why it happens
Students use $d = vt$ with the full echo time, giving double the actual distance.
How to avoid it
Write $d = vt/2$ from the start. The sound travels to the reflector AND back — the distance is only half the total path.
✕Confusing loudness with pitch
Why it happens
Both describe how a sound 'sounds' and students mix the terms.
How to avoid it
Pitch = frequency. Loudness = amplitude (or intensity). A high-pitched sound can be quiet; a low-pitched sound can be loud.

Practice questions

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

Past paper style quiz

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Sound — frequently asked questions

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

Sound

Short Study Notes in the form of Slides

Short Study Notes — Sound

Detailed Notes

What you’ll learn

How it’s examined

1Calculating distance using echo

2Frequency, pitch, and amplitude

3Ultrasound in medical imaging

Echo distance

Wave equation (sound)

Longitudinal wave

Compression

Rarefaction

Ultrasound

Pitch

✕Stating that sound can travel through a vacuum

✕Forgetting to divide the total time by 2 in echo calculations

✕Confusing loudness with pitch

Practice questions

Past paper style quiz

Assess your understanding

Sound — frequently asked questions