waves-sound

Sound — IGCSE Physics

1. Production of sound

Sound is produced when a source vibrates.

Examples:

  • Guitar string vibrates → produces sound

  • Speaker cone vibrates → produces sound

  • Vocal cords vibrate → produce sound

The vibrating source causes particles in the surrounding medium to vibrate, transferring the sound energy.



2. Longitudinal nature of sound

Sound is a longitudinal wave.

This means the particles of the medium vibrate parallel to the direction in which the wave travels.

Think:

Wave direction →→→
Particle vibration ↔↔↔



3. Human hearing range

The approximate range of frequencies audible to humans is:

20 Hz to 20 000 Hz\boxed{20\text{ Hz to }20\,000\text{ Hz}}

  • Below 20 Hz → infrasound

  • Above 20 000 Hz → ultrasound



4. Sound needs a medium

Sound cannot travel through a vacuum.

It needs a medium containing particles to transfer the vibrations.

Possible media:

  • Solids

  • Liquids

  • Gases



5. Speed of sound

In air:

330−350 m/s\boxed{330-350\text{ m/s}}

A commonly used approximate value is:

340 m/s\boxed{340\text{ m/s}}


Supplement

10. Compression and rarefaction

Because sound is longitudinal, it consists of alternating regions of:

Compression

A region where particles are close together.

→ High pressure/density

Rarefaction

A region where particles are spread further apart.

→ Low pressure/density

So a sound wave can be represented as:

Compression → Rarefaction → Compression → Rarefaction

The wavelength is the distance from one compression to the next compression (or one rarefaction to the next).



11. Speed in different media

In general:

solid>liquid>gas\boxed{\text{solid}>\text{liquid}>\text{gas}}

So sound travels:

Fastest in solids → slower in liquids → slowest in gases

For example, sound travels much faster through steel than through air.

Memorise these 6 points

Sound = vibration
Longitudinal = particles parallel to wave direction
20 Hz–20 kHz = human hearing
Needs a medium
Air ≈ 340 m/s
Solid > liquid > gas

Sound — IGCSE Exam Notes

7. Amplitude, frequency, loudness and pitch

Property of sound wave

What changes?

Effect

Amplitude

Height of the wave

Greater amplitude → louder sound

Frequency

Number of vibrations per second

Greater frequency → higher pitch

Remember:

  • Amplitude → loudness

  • Frequency → pitch

  • Amplitude is measured in metres (m).

  • Frequency is measured in hertz (Hz).



8. Echoes

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

Example:

You shout towards a cliff → sound travels to the cliff → reflects → returns to you as an echo.

A hard, flat surface produces a clearer echo because it reflects sound well.



9. Ultrasound

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

Humans cannot normally hear ultrasound.



Supplement 12. Uses of Ultrasound

1. Medical scanning

Ultrasound is used to produce images of soft tissue inside the body.

Method:

  1. Ultrasound pulses are sent into the body.

  2. The waves are partially reflected at boundaries between different tissues.

  3. The reflected waves are detected.

  4. The time taken for echoes to return is measured.

  5. A computer uses this information to construct an image.

Why ultrasound?

  • It is non-ionising, unlike X-rays.

  • It can be used to scan soft tissue.

Example: foetal scanning during pregnancy.



2. Non-destructive testing (NDT)

Ultrasound can detect cracks or defects inside materials without damaging them.

Method:

  1. Send ultrasound pulses into the material.

  2. A defect reflects some of the ultrasound.

  3. Detect the reflected pulse.

  4. Measure the time taken for the echo.

  5. Calculate the position/depth of the defect.

This is used to test things such as metal components for internal cracks.



3. SONAR

SONAR uses reflected ultrasound to determine the distance/depth of an object underwater.

genui{"learning_viz":{"type_id":"WAVE_SPEED","initial_values":{"frequency":20000,"wavelength":0.017},"locale_override":"en-US"}}

Method:

  1. An ultrasound pulse is transmitted into the water.

  2. It reflects from the seabed or an underwater object.

  3. The returning echo is detected.

  4. The time for the pulse to travel to the object and back is measured.

  5. Calculate the distance.

Essential equation

distance=wave speed×time2\boxed{\text{distance}=\frac{\text{wave speed}\times\text{time}}{2}}

Why divide by 2?

The measured time is for the complete journey:

transmitter → object → transmitter

So the actual one-way distance is half the total distance travelled.

Example

An ultrasound pulse travels through water at 1500 m/s and returns after 0.40 s.

d=1500×0.402d=\frac{1500\times0.40}{2} d=300 m\boxed{d=300\text{ m}}

Exam trap: Don't forget the ÷2 when the given time is the time for an echo to return