Brainlag PlusUsing echoes of ultrasound to scan bodies and measure depths, and what P-waves and S-waves reveal about the inside of the Earth.
Ultrasound echoes
Ultrasound is sound above 20 kHz, too high for us to hear.
When it reaches a boundary between two different materials, some is reflected and the rest goes on. Timing each echo tells you how far away each boundary is.
- Medical scans: echoes from the front and back of organs build up a picture. Ultrasound is not ionising, so it is used to scan unborn babies.
- Industry: a crack inside a metal part sends back an extra, early echo.
- Echo sounding: ships time pulses to the sea bed to measure depth.
- A pulse returns from the sea bed after 0.6 s; sound travels at 1500 m/s in sea water
- total distance = 1500 × 0.6 = 900 m
- depth = 900 ÷ 2 = 450 m
Watch the prefixes: echo times in scans are in microseconds (1 μs = 10⁻⁶ s), and scanner frequencies are in MHz.
The rest of this lesson and its 9 practice questions are in Plus.
Screen 1 of 3, free to read.
- 9 exam-style questions, marked as you go, each with a worked solution
- Anything you get wrong is saved to your flashcards