How sound travels
Sound is produced by vibrating objects and travels as a longitudinal wave. The vibrations push particles of the medium together to form compressions (regions of high pressure) and pull them apart to form rarefactions (regions of low pressure). Because sound needs particles to pass on the vibration, it cannot travel through a vacuum.
Key idea
Speed of sound v = distance ÷ time. In air it is about 330–340 m/s. Sound travels faster in liquids and fastest in solids, because the particles are closer together.
Pitch, loudness and hearing
- Pitch depends on the frequency — higher frequency gives a higher pitch.
- Loudness depends on the amplitude — larger amplitude gives a louder sound.
- The human audible range is about 20 Hz to 20 000 Hz.
- Sound above 20 000 Hz is ultrasound, used in cleaning, sonar and medical scanning.
Sound reflects off hard surfaces to produce an echo. Timing an echo lets us measure distance or the speed of sound.
Worked example
A man shouts at a cliff and hears the echo after 2.0 s. Sound travels at 340 m/s. The sound travels to the cliff and back, a total distance = 340 × 2.0 = 680 m, so the cliff is 680 ÷ 2 = 340 m away.
Remember
- Sound is longitudinal; light is transverse.
- No medium, no sound — there is silence in space.
- Pitch ↔ frequency; loudness ↔ amplitude.