Sound and seismic waves

Sound waves

Sound is a longitudinal mechanical wave: particles vibrate parallel to the direction of travel, creating compressions and rarefactions. Sound needs a medium — it cannot travel through a vacuum.

Typical speeds:

• Air ≈ 340 m/s
• Water ≈ 1500 m/s
• Steel ≈ 5000 m/s

Sound travels fastest in solids because particles are closely packed and tightly bonded, so vibrations transfer quickly. Liquids are intermediate; gases are slowest because particles are far apart.

Sound in solids vs gases (Higher)

When a sound wave travels from one medium to another, its frequency stays the same but its speed and wavelength change (v = f λ). Crossing from air into a solid wall: speed increases → wavelength increases.

The human ear hears 20 Hz to 20 000 Hz. Above 20 kHz is ultrasound — used in medical scans, sonar and cleaning.

Seismic waves

Earthquakes generate two main body waves:

P waves arrive first at a seismometer; S waves arrive later. The time gap is used to locate the epicentre.

What seismic waves reveal about Earth's interior

Detector networks across the planet record where waves arrive and where they don't:

• S-wave shadow zone beyond ~104° from the epicentre → no S waves detected → Earth has a liquid outer core (transverse waves cannot pass through liquid).
• P-wave shadow zone between ~104° and ~140° → P waves bend (refract) as they cross density boundaries, showing layered structure with a denser core.
• Sudden changes in wave speed at boundaries reveal the crust–mantle (Mohorovicic) and mantle–core boundaries.

Edexcel exam tip

For "what do seismic waves tell us about the Earth's structure?" you must (a) name the wave type, (b) describe the observation (shadow zone / refraction), (c) state the conclusion (liquid outer core / layered structure). All three steps score.