CIE IGCSE Physics - Sound

3.4.1 Sound Waves

Describing Sound

• Sound waves are created by vibrating sources.
• Vibrations can transfer to solids, e.g., causing a glass to shatter if vibrations are too strong.
• Sound waves are longitudinal, meaning molecules vibrate in the direction of energy transfer.
• A medium is required for sound waves to travel; they cannot travel through a vacuum.

Compression & Rarefaction

• Longitudinal waves have compressions (high-density regions) and rarefactions (low-density regions).
• Sound waves cause pressure variations, making them pressure waves.
• Pressure changes cause solids to vibrate when hit by sound waves.

3.4.2 Measuring the Speed of Sound

Speed of Sound in Air

• Approximate speed: 340 m/s at room temperature.
• Speed varies with air temperature; range: 330 - 350 m/s.

Speed of Sound in Materials

• Sound speed varies in different media: fastest in solids, slowest in gases.

Measuring the Speed of Sound

Method 1: Measuring Sound Between Two Points

• Two people use a trundle wheel to measure 100m distance.
• One bangs wooden blocks to generate sound; the other uses a stopwatch.
• Repeat to calculate average time to determine speed.

Method 2: Using Echoes

• Use echo from a wall ~50m away.
• Measure time between clap and echo to calculate speed.

Method 3: Using an Oscilloscope

• Two microphones and an oscilloscope measure time difference between sound arrival.
• Provides the most accurate results due to automatic timing.

Measuring Wave Speed in Water

• Create ripples on water; measure speed using time and distance of ripples.

Exam Tip

• Be aware of accuracy differences in methods; use larger distances to minimize errors.

3.4.3 Effects of Sound Waves

Pitch & Loudness

• Frequency relates to pitch: high frequency = high pitch, low frequency = low pitch.
• Amplitude relates to volume: large amplitude = high volume, small amplitude = low volume.

Echoes

• Hard surfaces reflect sound waves, creating echoes.
• Echo sounding is used to measure ocean depth by calculating time for wave reflection.

3.4.4 Ultrasound

Ultrasound Basics

• Human audible range: 20 Hz to 20,000 Hz.
• Ultrasound: frequencies > 20,000 Hz.

Uses of Ultrasound

In Medicine

• Construct images of fetuses, organs, and internal structures.
• Non-invasive and believed harmless.

In Industry

• Detect internal cracks in metals and generate images.
• Reflections from cracks appear as pulses on oscilloscope traces.

Worked Example

• Calculate distance to a crack in a steel bolt using sound speed (6000 m/s) and oscilloscope timing.
• Example calculation provided to convert distance to centimeters.