Describe compression and rarefaction

What are Compression & Rarefaction?

Sound travels as a longitudinal wave through a medium (air, water, solids). The particles of the medium oscillate back‑and‑forth along the direction of the wave.

• Compression (C) – particles are pushed together → higher pressure & density.
• Rarefaction (R) – particles are pulled apart → lower pressure & density.

Think of a marching band: when the drummer hits the drum, the air in front of him is compressed, then it expands as the wave moves away. 🎵

How Does the Wave Move?

Each particle oscillates around its equilibrium position. The wave propagates because the compression in one region forces the next region to compress, and so on.

Mathematically, the speed of a sound wave is given by:

$$v = \sqrt{\dfrac{B}{\rho}}$$

where $B$ is the bulk modulus (stiffness) of the medium and $\rho$ is its density.

For everyday air at room temperature, $v \approx 340\,\text{m/s}$ (🚗).

Compression & Rarefaction in a Table

Exam Tips

• Always label compression as a region of high pressure and density.
• Use the symbol $C$ for compression and $R$ for rarefaction in sketches.
• When asked to calculate the speed of sound, remember the formula $$v = \sqrt{\dfrac{B}{\rho}}$$ and plug in the given values.
• For wave‑frequency questions, use $$v = f\lambda$$ and note that $\lambda$ is the distance between successive compressions.
• In multiple choice, look for the option that correctly pairs high pressure with compression and low pressure with rarefaction.