Ultrasound and seismic waves

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. It shows soft tissue well, but it does not pass well through bone or air and its pictures are less detailed; X-rays show bones clearly but are ionising.
• 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.

Edexcel only: infrasound is sound below 20 Hz, too low for us to hear. Earthquakes and volcanoes give out very low-frequency waves, and detecting them at stations far away is one way to monitor eruptions and to explore the inside of the Earth, down to its core. Some animals, such as elephants and whales, use infrasound to communicate over long distances.

1. A pulse returns from the sea bed after 0.6 s; sound travels at 1500 m/s in sea water
2. total distance = 1500 × 0.6 = 900 m
3. depth = 900 ÷ 2 = 450 m

The thickness of a layer comes from the gap between two echoes, one from its front boundary and one from its back. The second echo crosses the layer twice (in and back out), so thickness = speed × gap ÷ 2.

1. Echoes from the front and back of an organ arrive 50 μs apart; ultrasound travels at 1540 m/s in tissue
2. 50 μs = 50 × 10⁻⁶ s
3. extra distance = 1540 × 50 × 10⁻⁶ = 0.077 m
4. thickness = 0.077 ÷ 2 = 0.0385 m = 3.85 cm

Watch the prefixes: echo times in scans are in microseconds (1 μs = 10⁻⁶ s), and scanner frequencies are in MHz.