Sound and ultrasound
Hearing sound (Higher tier only) (Separate Physics only)
Sound waves are . They can travel through solids, liquids and gases, making the material vibrate, but not through a vacuum.
When sound reaches your ear, it makes your vibrate. This passes the vibrations on to small bones and other parts of the ear, which you sense as sound.
This only works over a limited range of frequencies. Human hearing goes from about 20 Hz to 20 kHz. The top of the range falls as people get older.
Key terms
Ultrasound and echo sounding (Higher tier only) (Separate Physics only)
is sound with a frequency above 20 kHz, too high for humans to hear.
When ultrasound reaches a boundary between two materials, some of it is reflected. The time for the echo to return tells you how far away the boundary is. Uses include:
- medical scans, such as checking an unborn baby
- industrial imaging, such as finding hidden cracks in metal
- , where ships send down high-frequency sound to find the depth of water or detect objects
Example: a ship sends a pulse down. The echo returns 0.4 s later. Sound travels at 1500 m/s in water.
- Total distance = speed × time = 1500 × 0.4 = 600 m
- The sound went down and back, so depth = 600 ÷ 2 = 300 m
Common mistake
Don't forget to halve the distance: the time is for the journey there and back.
Key terms
Seismic waves and the Earth's core (Higher tier only) (Separate Physics only)
Earthquakes produce that travel through the Earth:
- are . They travel through solids and liquids.
- are . They travel through solids only, not liquids.
Seismometers around the world record these waves. On the far side of the Earth from an earthquake, there is a large region where no S-waves arrive. This S-wave shadow zone shows that the S-waves are blocked, so part of the core, the outer core, must be liquid. P-waves also refract at the core's boundaries, which shows its size.
Seismic waves gave scientists evidence about parts of the Earth no one can see directly.