Sound Waves

This lesson covers sound waves — how they are produced, how they travel, their properties, and their uses including echoes and ultrasound, as required by the Edexcel GCSE Combined Science specification (1SC0).

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How Sound Is Produced

Sound is produced when an object vibrates. The vibrations cause the particles of the surrounding medium to oscillate, creating a longitudinal wave that transfers energy from the source to the listener.

• A drum vibrates when struck → the air particles next to the drum surface vibrate → the vibration passes through the air as a longitudinal wave.
• A guitar string vibrates when plucked → sound wave is produced.
• A loudspeaker cone vibrates back and forth → sound wave is produced.

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Sound as a Longitudinal Wave

Sound is a longitudinal wave:

• The particles oscillate parallel to the direction of energy transfer.
• The wave consists of alternating compressions (high-pressure regions) and rarefactions (low-pressure regions).

graph LR
    subgraph "Sound wave in air"
    direction LR
    A["|||| Compression (high P)"] --- B["  |  |  |  Rarefaction (low P)"]
    B --- C["|||| Compression (high P)"]
    C --- D["  |  |  |  Rarefaction (low P)"]
    end

Feature	Description
Compression	Region where particles are pushed close together; higher pressure
Rarefaction	Region where particles are spread apart; lower pressure
Wavelength	Distance from one compression to the next (or one rarefaction to the next)

Exam Tip: Sound cannot travel through a vacuum because there are no particles to vibrate. This is a classic exam question — the bell-in-a-jar experiment demonstrates this.

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The Bell Jar Experiment

This classic experiment demonstrates that sound needs a medium to travel:

1. Place an electric bell inside a glass bell jar connected to a vacuum pump.
2. Switch on the bell — you can hear it.
3. Gradually pump out the air.
4. The sound gets quieter and quieter.
5. When most of the air is removed, you can see the bell vibrating but can barely hear any sound.

Conclusion: Sound requires a medium (solid, liquid or gas) to travel. It cannot travel through a vacuum.

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Speed of Sound in Different Media

The speed of sound depends on the medium and the temperature.

Medium	Approximate Speed of Sound
Air (20 °C)	340 m/s
Water	1500 m/s
Steel	5000 m/s
Concrete	3400 m/s
Vacuum	Sound cannot travel

Why Does Sound Travel at Different Speeds?

• Sound travels fastest in solids because the particles are closest together and vibrations are passed on most quickly.
• Sound travels slowest in gases because the particles are furthest apart.
• In general: solids > liquids > gases.

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Pitch and Frequency

The pitch of a sound is related to its frequency.

Frequency	Pitch
High frequency	High pitch
Low frequency	Low pitch

• The human ear can typically detect sounds in the range 20 Hz to 20 000 Hz (20 kHz).
• Sounds below 20 Hz are called infrasound.
• Sounds above 20 000 Hz are called ultrasound.

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Loudness and Amplitude

The loudness of a sound is related to its amplitude.

Amplitude	Loudness
Large amplitude	Loud sound
Small amplitude	Quiet sound

• A larger amplitude means more energy is carried by the wave.

Exam Tip: Don't confuse pitch and loudness. Pitch depends on frequency, loudness depends on amplitude. They are independent of each other.

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Echoes

An echo is a reflected sound wave. When sound hits a hard surface, it bounces back.

Calculating Distance Using Echoes

$\text{distance to surface} = \frac{v \times t}{2}$distance to surface=2v×t​

The factor of 2 accounts for the sound travelling there and back.

Worked Example

A ship sends a pulse of sound towards the seabed. The echo returns 0.4 s later. The speed of sound in water is 1500 m/s. Calculate the depth of the seabed.

$d = \frac{v \times t}{2} = \frac{1500 \times 0.4}{2} = \frac{600}{2} = 300 \text{ m}$d=2v×t​=21500×0.4​=2600​=300 m

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Ultrasound

Ultrasound is sound with a frequency above 20 000 Hz (above the range of human hearing).

Uses of Ultrasound

Use	How It Works
Pre-natal scanning	Ultrasound pulses are sent into the body; they reflect off boundaries between different tissues; the reflections are used to build an image of the foetus
Sonar	Ships send ultrasound pulses to the seabed; the time for the echo to return is used to calculate depth
Industrial flaw detection	Ultrasound is sent through materials; unexpected reflections reveal cracks or defects
Cleaning	High-frequency vibrations dislodge dirt and grease from delicate objects (e.g. jewellery, surgical instruments)

Why Use Ultrasound Instead of X-rays?

• Ultrasound is non-ionising — it does not damage DNA or living cells.
• It is safe for use during pregnancy.
• X-rays are ionising and should be used as little as possible.

Exam Tip: In questions comparing ultrasound and X-rays for medical imaging, always mention that ultrasound is non-ionising and therefore safer for the patient, especially for pre-natal scanning.

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Measuring the Speed of Sound in Air

Method 1: Two-Microphone Method

1. Set up two microphones a known distance apart, connected to a data logger or computer.
2. Make a sharp sound (e.g. clap or bang) near the first microphone.
3. The data logger records the time difference between the sound arriving at the first and second microphone.
4. Calculate speed: $v = \frac{d}{t}$v=td​

Method 2: Echo Method

1. Stand a known distance from a large flat wall.
2. Clap in time with the echoes (so each clap coincides with the return of the previous echo).
3. Use a stopwatch to time a large number of claps (e.g. 20).
4. Calculate the time for one round trip, then use: $v = \frac{2d}{t}$v=t2d​

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The Wave Equation Applied to Sound

$v = f \lambda$v=fλ

Worked Example 2

A sound wave has a frequency of 440 Hz and travels at 340 m/s in air. Calculate its wavelength.

$\lambda = \frac{v}{f} = \frac{340}{440} = 0.77 \text{ m (2 s.f.)}$λ=fv​=440340​=0.77 m (2 s.f.)

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Summary