Section 1 of 6Beyond the Hearing Range
Audible sound
- Frequencies from about 20 Hz to 20 000 Hz can be detected by the human ear.
Ultrasound
- Sound with a frequency above 20 000 Hz, too high for people to hear but audible to some animals such as bats.
Infrasound
- Sound with a frequency below 20 Hz, too low for people to hear, produced by earthquakes and some large animals.
Same nature
- Both are longitudinal pressure waves and obey the same wave equation as audible sound.
| Type | Frequency range |
|---|---|
| Infrasound | below 20 Hz |
| Audible sound | 20 Hz to 20 000 Hz |
| Ultrasound | above 20 000 Hz |
Section 2 of 6How Echo Ranging Works
1
A pulse of ultrasound is sent from a transmitter into the medium, such as sea water or body tissue.
2
When the pulse reaches a boundary between two different materials, part of it is reflected back towards the source.
3
A detector next to the transmitter records the returning echo, and a timer measures the time between sending and receiving.
4
The pulse travelled to the boundary and back, so the one-way distance equals speed × time ÷ 2.
5
Repeating the pulses and combining the echoes from many boundaries builds up a picture of the structure.
Section 3 of 6Uses of Ultrasound
Uses of Ultrasound
Sonar
- Ships send ultrasound downwards to measure sea depth and to locate shoals of fish or wrecks.
Medical imaging
- Echoes from boundaries between tissues produce scans of an unborn baby without the harm caused by ionising radiation.
Industrial testing
- Echoes from cracks inside metal parts show up flaws that cannot be seen from the outside.
Cleaning
- Strong ultrasound vibrations shake dirt off delicate items such as jewellery and surgical instruments.
Section 4 of 6Uses of Infrasound
Earthquakes
- Detecting infrasound waves from the ground helps scientists to locate and study earthquakes.
Volcanoes
- Monitoring infrasound from a volcano can give early signs that an eruption is becoming more likely.
Animals
- Elephants and whales communicate over long distances using infrasound, because low frequencies travel far.
Weather and explosions
- Large explosions and storms produce infrasound that sensitive microphones can detect from far away.
Section 5 of 6Worked Example
Question
- A ship sends an ultrasound pulse to the sea bed and detects the echo 0.40 s later.
- The speed of ultrasound in sea water is 1500 m/s.
- How deep is the water?
- Total distance travelled = speed × time = 1500 × 0.40 = 600 m.
- This is the journey down and back, so depth = 600 ÷ 2.
- Depth = 300 m.
Answer The sea is 300 m deep.
Section 6 of 6Exam Checklist & Tips
4.2.2 – Key Points for the Exam
- Ultrasound Sound waves with frequencies higher than 20 000 Hz.
- Infrasound Sound waves with frequencies lower than 20 Hz.
- Echo A reflected sound wave returning to its source.
- Sonar A method of finding distances underwater by timing reflected ultrasound pulses.
- Boundary The surface where two different materials meet and where partial reflection happens.
Common Mistakes
- Using the total time as the time to reach the boundary; the pulse travels there and back, so divide by two.
- Believing that ultrasound is a different kind of wave to sound; it is simply sound at a higher frequency.
- Using the speed of sound in air for an underwater calculation instead of the speed in water.
- Swapping the definitions, with infrasound above 20 000 Hz and ultrasound below 20 Hz.