Sound Waves and Hearing

This lesson covers sound waves — their nature, properties, and how humans hear them — as required by AQA GCSE Combined Science Trilogy (8464), Physics Paper 2, section 6.1. You need to know that sound is a longitudinal wave, understand the range of human hearing, and be able to describe echoes and ultrasound.

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Sound as a Longitudinal Wave

Sound is a longitudinal wave. When an object vibrates (e.g. a loudspeaker cone or a guitar string), it pushes and pulls on the surrounding air particles, creating alternating compressions (high-pressure regions) and rarefactions (low-pressure regions) that travel outward from the source.

graph LR
    subgraph "Sound Wave Propagation"
    direction LR
    S["Vibrating source"] --> C1["|||| Compression"]
    C1 --> R1["  |  |  |  Rarefaction"]
    R1 --> C2["|||| Compression"]
    C2 --> R2["  |  |  |  Rarefaction"]
    end

Key Facts About Sound

• Sound is a longitudinal wave — the oscillations are parallel to the direction of energy transfer.
• Sound needs a medium (solid, liquid or gas) to travel through.
• Sound cannot travel through a vacuum because there are no particles to vibrate.
• Sound travels fastest through solids, slower through liquids, and slowest through gases.

Medium	Approximate speed of sound
Air (at 20 °C)	~340 m/s
Water	~1 500 m/s
Steel	~6 000 m/s

Exam Tip: Sound cannot travel through a vacuum. This is because sound needs particles to create compressions and rarefactions, and a vacuum contains no particles. This concept is tested frequently by AQA.

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The Range of Human Hearing

The human ear can detect sounds with frequencies between approximately 20 Hz and 20 000 Hz (20 kHz).

graph LR
    subgraph "Frequency Ranges"
    direction LR
    A["Infrasound < 20 Hz"] --> B["Human hearing 20 Hz – 20 000 Hz"]
    B --> C["Ultrasound > 20 000 Hz"]
    end

    style A fill:#3498db,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#e74c3c,color:#fff

Category	Frequency range	Examples
Infrasound	Below 20 Hz	Earthquakes, elephants, some weather events
Audible sound	20 Hz – 20 000 Hz	Speech, music, everyday sounds
Ultrasound	Above 20 000 Hz	Medical imaging, cleaning, sonar, bat echolocation

Pitch and Loudness

Property	Determined by	Higher value means
Pitch	Frequency	Higher-pitched sound (e.g. a whistle)
Loudness	Amplitude	Louder sound

• A high-frequency sound has a high pitch (e.g. a bird call).
• A low-frequency sound has a low pitch (e.g. a bass drum).
• A large-amplitude sound is loud; a small-amplitude sound is quiet.

Exam Tip: Do not confuse pitch with loudness. Pitch depends on frequency; loudness depends on amplitude. They are independent — a sound can be high-pitched and quiet, or low-pitched and loud.

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Echoes

An echo is a reflection of sound off a hard surface. The sound wave travels to the surface, reflects, and returns to the listener.

Calculating Distance Using Echoes

If you know the speed of sound and the time for the echo to return:

$\text{total distance} = v \times t$total distance=v×t

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

You divide by 2 because the sound travels to the surface and back.

Worked Example

A student claps near a large wall and hears the echo 0.6 s later. The speed of sound is 340 m/s. How far away is the wall?

$\text{total distance} = 340 \times 0.6 = 204 \text{ m}$total distance=340×0.6=204 m

$\text{distance to wall} = \frac{204}{2} = 102 \text{ m}$distance to wall=2204​=102 m

Exam Tip: In all echo / reflection questions, remember to divide by 2. The time measured is for the round trip (there and back).

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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 tissues and return; the time delay is used to build an image
Industrial flaw detection	Pulses detect cracks inside metal castings or welds
Cleaning	High-frequency vibrations dislodge dirt from delicate items (e.g. jewellery, surgical instruments)
Sonar (echo sounding)	Ships send ultrasound pulses to the seabed; the echo time gives the depth

Why Use Ultrasound Instead of X-rays?

• Ultrasound is non-ionising — it does not damage living cells or DNA.
• This makes it safe for imaging unborn babies during pregnancy.
• X-rays are ionising radiation that can damage cells and increase cancer risk.

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Demonstrating That Sound Needs a Medium

The classic demonstration uses a bell jar experiment:

1. Place an electric bell inside a bell jar connected to a vacuum pump.
2. With air inside, you can hear the bell ringing clearly.
3. Gradually pump the air out. The sound becomes quieter and quieter.
4. When most of the air has been removed, you can still see the bell vibrating but can barely hear it.
5. This shows that sound needs air (particles) to travel — without particles, the energy cannot be transferred.

Exam Tip: You will never achieve a perfect vacuum in a school lab, so you may still hear a faint sound. The exam expects you to say the sound gets much quieter as air is removed.

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Common Mistakes

Mistake	Correction
"Sound can travel through a vacuum"	Sound cannot travel through a vacuum — it needs particles
Forgetting to divide by 2 in echo calculations	The sound travels there and back — divide total distance by 2
Confusing pitch and loudness	Pitch = frequency; Loudness = amplitude
"Ultrasound is a type of electromagnetic wave"	Ultrasound is a sound wave (longitudinal, mechanical) — not EM

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Summary