Sound

Sound is made by vibrations that pass through a material as a longitudinal wave. Pitch depends on frequency and loudness on amplitude. Sound can’t travel through a vacuum.

Key facts

  • Higher frequency = higher pitch; bigger amplitude = louder.
  • Sound travels about 340 m/s in air, faster in liquids and fastest in solids.
  • Humans hear roughly 20 Hz to 20 000 Hz; above that is ultrasound.
  • An echo is a reflected sound; distance = speed × time ÷ 2.

Worked examples

Example 1

A ship’s sonar pulse returns after 0.8 s. Sound travels at 1500 m/s in water. How deep is the sea?

  1. Total distance: 1500 × 0.8 = 1200 m.
  2. Depth: half of that = 600 m.

Answer: 600 m

Example 2

Why can’t astronauts hear each other in space without radios?

  1. Sound is vibrating particles; space is a vacuum with almost no particles.

Answer: No particles to carry sound