What are sound waves and how do they travel?

Sound waves are a type of mechanical wave that travels through a medium, such as air, water, or solids. They are created by vibrating objects, which cause the surrounding particles to vibrate as well. These vibrations transfer energy through the medium in the form of longitudinal waves, where particles move parallel to the direction of the wave propagation. This is a key concept in the IB MYP Science curriculum under the topic of Waves.

How do frequency and amplitude affect sound waves?

Frequency and amplitude are two crucial characteristics of sound waves. Frequency, measured in hertz (Hz), determines the pitch of the sound; higher frequencies produce higher pitches. Amplitude, on the other hand, affects the loudness of the sound; greater amplitudes result in louder sounds. Understanding these properties helps students in the IB MYP program grasp how sound waves are perceived and manipulated in various applications.

What is the difference between longitudinal and transverse waves in the context of sound?

Sound waves are longitudinal waves, meaning the particle displacement is parallel to the direction of wave propagation. In contrast, transverse waves, such as light waves, have particle displacement perpendicular to the direction of wave travel. This distinction is important in the IB MYP Science curriculum as it helps students differentiate between various types of waves and their behaviors.

Why can't sound waves travel through a vacuum?

Sound waves require a medium to travel because they are mechanical waves that rely on particle interactions to propagate. In a vacuum, there are no particles to transmit the vibrations, so sound cannot travel. This concept is fundamental in the study of sound waves within the IB MYP Science curriculum, highlighting the necessity of a medium for sound transmission.

How do humans perceive sound, and what role do sound waves play in this process?

Humans perceive sound through the auditory system, where sound waves enter the ear canal and cause the eardrum to vibrate. These vibrations are transmitted through the ossicles in the middle ear and converted into electrical signals by the cochlea in the inner ear. The brain interprets these signals as sound. Understanding this process is crucial for IB MYP students studying the interaction between sound waves and human perception.

Why is "Saying sound can travel through space." a common mistake in Sound Waves?

Why it happens: Sci-fi movies make space explosions audible. How to avoid it: Sound needs particles to bump into each other. Space is essentially a vacuum — silence. EM waves DO travel through space; sound waves don't.

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

Why it happens: Both describe sound and both can be 'high' (high pitch / high volume). How to avoid it: Pitch = how high/low (frequency). Loudness = how strong/quiet (amplitude). High frequency + small amplitude = quiet high-pitched note.

Why is "Forgetting to divide by 2 in echo distance." a common mistake in Sound Waves?

Why it happens: Students reach for d = vt and stop. How to avoid it: Echo time covers a ROUND TRIP. Distance to reflector = vt / 2.

WavesIB MYP Sciences (Years 1-5)

Sound Waves

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Sound Waves — IB MYP Sciences: longitudinal vibrations, pitch, loudness and the speed of sound

Sound is energy travelling as longitudinal vibrations of particles. This MYP Sciences note covers what causes sound, how pitch and loudness relate to frequency and amplitude, the speed of sound in different media, and the human hearing range.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Describe sound as a longitudinal wave and state the media it can travel through.
MYP Sciences A — Relate frequency to pitch and amplitude to loudness.
MYP Sciences A — Recall the speed of sound in air and explain why it is faster in solids.
MYP Sciences C — Calculate distance from echo timing.
MYP Sciences D — Evaluate uses of ultrasound (medical imaging, cleaning, range-finding).

Sound as a longitudinal wave

Vibrating particles bunch and stretch — that's a sound wave.

When a guitar string is plucked, it vibrates. The vibrating string pushes against neighbouring air molecules, compressing them. They then bump into the next layer of molecules, and so on — passing the disturbance along as a series of compressions (close-packed regions) and rarefactions (spread-out regions).

A loudspeaker pushes air molecules into alternating compressions (C) and rarefactions (R) — that's sound, travelling outwards.

Because sound needs particles to do the bumping, it CANNOT travel through a vacuum. A bell ringing inside a vacuum chamber makes no sound however vigorously it vibrates.

Sound = longitudinal wave of compressions and rarefactions.
Needs a medium (solid/liquid/gas). No sound in vacuum.
Generated by a vibrating object — speaker cone, string, drum, vocal cords.

Speed of sound

Sound travels faster in denser, more elastic media — fastest in solids.

Sound speed depends on the medium:

Medium	Speed (m/s)
Air (room temp)	340
Helium	970
Water	1 480
Steel	5 000

In solids particles are tightly bonded and can pass vibrations along quickly. In gases the particles are far apart and the wave takes longer to propagate.

Sound also travels SLIGHTLY faster in warmer air than cooler air, because warmer molecules vibrate faster and pass collisions on quicker.

Compare with light: at 3 × 10⁸ m/s, light is roughly a million times faster than sound. This is why you see a lightning flash before you hear the thunder. If thunder arrives 3 seconds later, the storm is about 3 × 340 ≈ 1 km away.

Sound speed in air ≈ 340 m/s.
Faster in liquids; fastest in solids.
Light is ~10⁶× faster than sound — see lightning before thunder.

Pitch, loudness and the human ear

Frequency → pitch. Amplitude → loudness. Hearing range 20 Hz–20 kHz.

Pitch depends on frequency. A higher-frequency wave makes a higher-pitched note. Middle C on a piano is ~262 Hz; the highest note on a standard piano is ~4 200 Hz.

Loudness depends on amplitude. A bigger amplitude means more energy in the wave and a louder sound.

The human ear can detect sound roughly from 20 Hz to 20 000 Hz (20 kHz). The upper end drops with age — most adults can no longer hear above ~15 kHz. Sounds with frequencies BELOW 20 Hz are called infrasound (used to monitor earthquakes and volcanoes); ABOVE 20 kHz are ultrasound (used in medical scans, cleaning jewellery, distance ranging).

Pitch = frequency.
Loudness = amplitude.
Human hearing: ~20 Hz – 20 kHz.
Below 20 Hz = infrasound. Above 20 kHz = ultrasound.

Echoes and reflection

Sound bounces off hard surfaces, just like light off a mirror.

An echo is a reflected sound wave. Hard, flat surfaces (walls, cliffs) reflect sound well; soft surfaces (carpets, curtains) absorb it. That's why theatre walls are designed with soft padding — they absorb echoes to reduce reverberation.

Echo timing is a useful measurement trick. If you hear an echo after a time t, the sound has travelled to the surface AND back — so the surface is at distance:

$d = \frac{v \, t}{2}$

where v is the speed of sound and the divide-by-2 accounts for the round trip.

Echo = reflected sound.
Hard flat surfaces reflect; soft surfaces absorb.
Distance from echo: d = vt / 2 (round trip).

How it’s examined

Criterion A tests definitions of pitch, loudness and the audible range. Criterion C presents echo timings or wave diagrams and asks for distance, speed or frequency.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

Master this Topic

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Step-by-step worked examples — Sound Waves

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

1Pitch vs loudness
Getting started• pitch, loudness
Question
Two sound waves are displayed on an oscilloscope. Wave A has small amplitude and many cycles per second; wave B has large amplitude and few cycles per second. Compare their pitch and loudness.
Step-by-step solution
1. Step 1
  Wave A: many cycles/sec = high frequency = HIGH pitch. Small amplitude = QUIET.
2. Step 2
  Wave B: few cycles/sec = low frequency = LOW pitch. Large amplitude = LOUD.
Answer
A is high-pitched and quiet. B is low-pitched and loud.
2Distance from an echo
Building confidence• echo
Question
A boy claps once in front of a cliff and hears an echo 2.0 s later. How far away is the cliff? Take v_sound = 340 m/s.
Step-by-step solution
1. Step 1
  The sound travels TO the cliff and BACK in 2.0 s, so it travels for 1.0 s each way.
2. Step 2
  Or use d = vt/2 = 340 × 2.0 / 2 = 340 m.
Answer
340 m.
3Sound in different media
Building confidence• speed of sound
Question
Why does sound travel faster in steel than in air?
Step-by-step solution
1. Step 1
  Sound propagates by particle collisions passing the vibration on.
2. Step 2
  In steel, particles are tightly bonded and packed close together, so vibrations pass on quickly.
3. Step 3
  In air, particles are far apart — the wave takes longer to travel between collisions, so the speed is slower.
Answer
Steel has tightly bonded, closely-packed particles that pass vibrations on much faster than the loosely-spaced particles in air.
4Why use ultrasound for medical imaging?
Stretch• ultrasound, applications
Question
Give TWO reasons doctors prefer ultrasound over X-rays for imaging an unborn baby.
Step-by-step solution
1. Step 1
  Ultrasound is non-ionising — it does NOT damage DNA or cells.
2. Step 2
  Ultrasound shows soft tissue clearly, whereas X-rays mostly show bone and can damage developing tissue.
Answer
(i) Ultrasound is non-ionising so it doesn't damage developing cells. (ii) Ultrasound images soft tissue well, which is what you want when imaging a fetus.
Examiner tip
MYP criterion D often asks for an evaluation — connect physics (ionising vs non-ionising; soft vs hard tissue) to the medical context.

Key Formulae — Sound Waves

The formulae you need to memorise for sound waves on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Distance from echo
$d = v × t / 2$
$d$
distance to reflector (m)
$v$
speed of sound (m/s)
$t$
total time for sound to go and return (s)
When to use
When measuring distance using an echo — divide by 2 because the sound makes a round trip.

Key Definitions and Keywords — Sound Waves

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

Sound wave
Examiner keyword
A longitudinal wave consisting of compressions and rarefactions of the particles in a medium.
Pitch
Examiner keyword
How high or low a sound is. Determined by frequency: higher frequency = higher pitch.
Loudness
Examiner keyword
How loud a sound is. Determined by amplitude: bigger amplitude = louder.
Audible range
The range of frequencies the human ear can hear: roughly 20 Hz to 20 kHz.
Ultrasound
Examiner keyword
Sound above 20 kHz. Used for medical imaging, cleaning, and distance measurement.
Infrasound
Sound below 20 Hz. Used for monitoring earthquakes, volcanoes and weather systems.
Echo
A reflected sound wave that arrives back at the listener after bouncing off a surface.

Common Mistakes and Misconceptions — Sound Waves

The traps other students keep falling into on sound waves questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying sound can travel through space.
Why it happens
Sci-fi movies make space explosions audible.
How to avoid it
Sound needs particles to bump into each other. Space is essentially a vacuum — silence. EM waves DO travel through space; sound waves don't.
✕Confusing pitch with loudness.
Why it happens
Both describe sound and both can be 'high' (high pitch / high volume).
How to avoid it
Pitch = how high/low (frequency). Loudness = how strong/quiet (amplitude). High frequency + small amplitude = quiet high-pitched note.
✕Forgetting to divide by 2 in echo distance.
Why it happens
Students reach for d = vt and stop.
How to avoid it
Echo time covers a ROUND TRIP. Distance to reflector = vt / 2.

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.

Sound Waves — frequently asked questions

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

Medium

Speed (m/s)

Air (room temp)

340

Helium

970

Water

1 480

Steel

5 000

Sound Waves

Short Study Notes in the form of Slides

Short Study Notes — Sound Waves

Detailed Notes

What you’ll learn

How it’s examined

1Pitch vs loudness

2Distance from an echo

3Sound in different media

4Why use ultrasound for medical imaging?

Distance from echo

Sound wave

Pitch

Loudness

Audible range

Ultrasound

Infrasound

Echo

✕Saying sound can travel through space.

✕Confusing pitch with loudness.

✕Forgetting to divide by 2 in echo distance.

Practice questions

Past paper style quiz

Assess your understanding

Sound Waves — frequently asked questions

Sound Waves

Short Study Notes in the form of Slides

Short Study Notes — Sound Waves

Detailed Notes

What you’ll learn

How it’s examined

1Pitch vs loudness

2Distance from an echo

3Sound in different media

4Why use ultrasound for medical imaging?

Distance from echo

Sound wave

Pitch

Loudness

Audible range

Ultrasound

Infrasound

Echo

✕Saying sound can travel through space.

✕Confusing pitch with loudness.

✕Forgetting to divide by 2 in echo distance.

Practice questions

Past paper style quiz

Assess your understanding

Sound Waves — frequently asked questions