CIE IGCSE Physics Notes: Sound

Sound is a form of energy that travels as a wave. It is produced by vibrating objects and travels through a medium such as air, water, or solids.

1. Production of Sound by Vibrating Sources

Definition: Sound

Sound is a wave produced by a vibrating source and transmitted through a medium.

When an object vibrates, it moves back and forth rapidly. This movement causes the particles in the surrounding medium to vibrate, producing sound waves.

Examples of vibrating sources

Source	What vibrates
Guitar	Strings
Drum	Drum skin
Speaker	Speaker cone
Human voice	Vocal cords
Tuning fork	Metal prongs

Example: Tuning fork

When struck:

• The prongs vibrate.
• Air particles around it vibrate.
• Sound waves travel to your ear.

2. Longitudinal Nature of Sound Waves

Definition: Longitudinal wave

A longitudinal wave is a wave in which particles vibrate parallel to the direction of wave travel.

Sound waves are longitudinal waves.

This means:

• Particles move back and forth.
• Energy moves forward.

Key features of longitudinal waves

Compression

Region where particles are close together.

High pressure region.

Rarefaction

Region where particles are spread apart.

Low pressure region.

Diagram representation

Compression → Rarefaction → Compression → Rarefaction

3. Audible Frequency Range of Humans

Definition: Frequency (f)

Frequency is the number of vibrations per second.

Unit: Hertz (Hz)

Human hearing range

Humans can hear frequencies between:

20 Hz and 20 000 Hz (20 kHz)

Frequency	Sound type
Below 20 Hz	Infrasound (cannot hear)
20 Hz – 20 kHz	Audible sound
Above 20 kHz	Ultrasound

4. Sound Requires a Medium

Definition: Medium

A medium is a substance through which sound travels.

Examples:

• Air
• Water
• Metal
• Glass

Sound cannot travel through a vacuum because there are no particles to vibrate.

Example:

• In space, no sound can be heard.

Exam tip:
Sound is a mechanical wave and requires particles.

5. Speed of Sound in Air

The speed of sound in air is approximately:

330–350 m/s

Exam value usually used:
340 m/s

Comparison in different materials

Medium	Speed
Air	340 m/s
Water	1500 m/s
Steel	5000 m/s

Conclusion:

Sound travels fastest in solids
Sound travels slower in liquids
Sound travels slowest in gases

6. Measuring Speed of Sound Using Distance and Time

Formula:

Speed = Distance ÷ Time

Where:

• v = speed (m/s)
• d = distance (m)
• t = time (s)

Method 1: Echo method

Step-by-step:

1. Stand a known distance from a wall.
2. Clap hands.
3. Measure time for echo to return.
4. Sound travels to wall and back.

Distance travelled = 2 × distance to wall

Example:

Distance to wall = 170 m
Total distance = 340 m
Time = 1 s

Speed = 340 ÷ 1
Speed = 340 m/s

Method 2: Direct method

The speed of sound in air can be found directly by measuring the time t taken for a sound to travel past two microphones separated by a distance d:

speed of sound in air(v) = distance travelled by the sound(d) ÷ time taken(t)

7. Amplitude and Frequency Effects

Amplitude affects loudness

Definition: Amplitude is the maximum displacement of particles.

Large amplitude → loud sound
Small amplitude → quiet sound

Example:

• Shouting → large amplitude
• Whispering → small amplitude

Frequency affects pitch

Definition: Pitch is how high or low a sound is.

High frequency → high pitch
Low frequency → low pitch

Examples:

Sound	Frequency	Pitch
Whistle	High	High pitch
Drum	Low	Low pitch

8. Echo (Reflection of Sound)

Definition: Echo

An echo is the reflection of sound waves from a surface.

Examples:

• Shouting near a mountain
• Sound reflecting from a wall

Echo occurs when reflected sound returns after a delay.

Condition to hear an echo

Minimum time gap needed = 0.1 s

Minimum distance from wall ≈ 17 m

9. Ultrasound

Definition: Ultrasound

Ultrasound is sound with frequency higher than 20 kHz (20 000 Hz).

Humans cannot hear ultrasound.

Animals that use ultrasound:

• Bats
• Dolphins

10. Compression and Rarefaction

These are parts of longitudinal waves.

Compression

• Particles close together
• High pressure

Rarefaction

• Particles far apart
• Low pressure

Exam tip:
Sound waves consist of alternating compressions and rarefactions.

11. Speed of Sound in Different States of Matter

Speed depends on how closely packed particles are.

State	Particle spacing	Speed
Solid	Very close	Fastest
Liquid	Close	Medium
Gas	Far apart	Slowest

Reason:
Particles transfer vibrations faster when closer.

12. Uses of Ultrasound

Ultrasound has many important uses.

(A) Medical scanning

Used to scan soft tissues.

Examples:

• Pregnancy scans
• Heart scans

Advantages:

• Safe
• No harmful radiation

(B) Non-destructive testing

Used to detect cracks in metals.

Example:

• Aircraft wings
• Bridges

Ultrasound reflects from cracks.

(C) SONAR (Sound Navigation and Ranging)

Used to measure distance underwater.

Used in:

• Ships
• Submarines

SONAR calculation

Formula:

Distance = Speed × Time ÷ 2

Divide by 2 because sound travels to object and back.

Example:

Speed = 1500 m/s
Time = 4 s

Distance = (1500 × 4) ÷ 2
Distance = 3000 m

Important Definitions Summary

Sound
A wave produced by vibrating objects and transmitted through a medium.

Longitudinal wave
Wave where particle vibration is parallel to wave direction.

Frequency
Number of vibrations per second.

Amplitude
Maximum displacement of particles.

Echo
Reflection of sound waves.

Ultrasound
Sound with frequency above 20 kHz.

Compression
Region where particles are close together.

Rarefaction
Region where particles are far apart.

Exam Tips (Very Important)

Students often lose marks here.

Remember:

• Sound is longitudinal
• Sound needs a medium
• Sound cannot travel in vacuum
• Speed of sound in air = 340 m/s
• Humans hear 20 Hz – 20 kHz
• Ultrasound > 20 kHz
• Loudness depends on amplitude
• Pitch depends on frequency
• Sound travels fastest in solids

Example Exam Questions

Question 1

What produces sound?

Answer:
Vibrating objects produce sound.

Question 2

What type of wave is sound?

Answer:
Longitudinal wave.

Question 3

State speed of sound in air.

Answer:
340 m/s

Question 4

What determines pitch?

Answer:
Frequency.

Question 5

What determines loudness?

Answer:
Amplitude.

Common Mistakes

Students confuse:

Pitch and loudness

Correct:

Pitch → frequency
Loudness → amplitude

Real Life Applications Summary

Application	Description
Medical scans	Scan babies
SONAR	Measure sea depth
Crack detection	Detect faults
Communication	Speaking