Describe compression and rarefaction

Published by Patrick Mutisya · 14 days ago

IGCSE Physics 0625 – Sound: Compression and Rarefaction

3.4 Sound – Compression and Rarefaction

What is a Longitudinal Wave?

Sound travels through gases, liquids and solids as a longitudinal wave. In a longitudinal wave the particles of the medium vibrate parallel to the direction of wave propagation.

Compression

A compression is a region in the medium where the particles are pushed together, producing a local increase in pressure and density.

Rarefaction

A rarefaction is a region where the particles are spread apart, giving a local decrease in pressure and density.

How Compression and Rarefaction Form a Sound Wave

When a source (e.g., a vibrating tuning‑fork) moves forward it pushes the adjacent air molecules together, creating a compression. As the source then moves back, it pulls the molecules apart, creating a rarefaction. This alternating pattern of compressions and rarefactions travels away from the source at the speed of sound.

Key Characteristics

FeatureCompressionRarefaction
Particle motionParticles are pushed togetherParticles are pulled apart
PressureHigher than ambientLower than ambient
DensityIncreasedDecreased
Phase in wave cycleCorresponds to the crest of a longitudinal waveCorresponds to the trough of a longitudinal wave

Relationship to Wave Parameters

The distance between successive compressions (or successive rarefactions) is the wavelength \$\lambda\$. The number of compressions (or rarefactions) that pass a point each second is the frequency \$f\$. The speed of sound \$v\$ is given by

\$v = f \lambda\$

During a compression the pressure \$p\$ is above the equilibrium pressure \$p0\$, and during a rarefaction it is below \$p0\$.

Everyday Examples

  • Speaking – the vocal cords create rapid compressions and rarefactions in the air that travel to the listener’s ear.

  • Musical instruments – plucking a guitar string sends compressions and rarefactions through the surrounding air.

  • Ultrasound imaging – high‑frequency sound waves produce compressions and rarefactions that reflect from body tissues.

Suggested Diagram

Suggested diagram: A side view of a longitudinal sound wave showing alternating compressions (regions of high particle density) and rarefactions (regions of low particle density) along the direction of propagation.

Summary

  1. Sound is a longitudinal wave consisting of alternating compressions and rarefactions.

  2. Compression = high pressure & high density; Rarefaction = low pressure & low density.

  3. The pattern travels through the medium at the speed of sound, described by \$v = f\lambda\$.

  4. Understanding compressions and rarefactions helps explain how sound is produced, transmitted, and received.