Sound waves
- Sound waves are longitudinal. They can travel through solids, liquids and gases, but not through a vacuum.
- Sound travels faster in solids than in liquids, and faster in liquids than in gases.
- (Higher) Sound waves can travel through solids, causing vibrations in the solid. In the ear, sound waves make the eardrum and other parts vibrate, which causes the sensation of sound.
- Humans can hear frequencies from about 20 Hz to 20 kHz. The conversion of sound waves to vibrations of solids only works over a limited range of frequencies.
Ultrasound (Higher)
- Ultrasound waves have a frequency higher than the upper limit of human hearing (above 20 kHz).
- Ultrasound is partially reflected when it meets a boundary between two different media. The time taken for the reflection to reach a detector shows how far away the boundary is.
- This is used in medical imaging (such as scanning a fetus) and industrial imaging (such as finding cracks in metal).
- Distance = speed × time ÷ 2, because the wave travels there and back.
Echo sounding (Higher)
- Ships use echo sounding: high-frequency sound waves are sent down and the echoes detected.
- This finds the depth of water and detects objects such as shoals of fish or wrecks.
Seismic waves (Higher)
- Earthquakes produce seismic waves. P-waves are longitudinal and can travel through solids and liquids. S-waves are transverse and cannot travel through a liquid.
- P- and S-waves travel at different speeds through different materials and are refracted as their speed changes.
- An S-wave shadow zone on the opposite side of the Earth shows that the outer core is liquid. The pattern of P-waves provides evidence for the size of the core.
Key terms
- Sound wave
- A longitudinal wave caused by vibrations, which needs a medium to travel through.
- Ultrasound
- Sound with a frequency above 20 kHz, too high for humans to hear.
- Echo sounding
- Using reflected sound waves to measure water depth or find objects.
- Seismic waves
- Waves produced by earthquakes that travel through the Earth.
- P-waves
- Longitudinal seismic waves that travel through solids and liquids.
- S-waves
- Transverse seismic waves that cannot travel through liquids.
- Shadow zone
- A region where seismic waves are not detected after an earthquake.
- Hearing range
- The frequencies humans can hear: about 20 Hz to 20 kHz.