Uses of high and low frequencies
- Ultrasound and infrasound are useful because their wavelengths, propagation and reflection differ from those of audible sound.
- A complete explanation links the wave property to the way information is obtained.
Sonar
Definition
Sonar
A technique that sends ultrasound pulses through water and uses the time for their echoes to return to find the distance to an object or the depth of water.
- A transmitter sends an ultrasound pulse through water.
- The pulse reflects from the seabed, a shoal of fish or another boundary and returns to a detector.
- The measured time is for the outward and return journeys, so the one-way distance is d=vt2d=\dfrac{vt}{2}d=2vt.
- Short pulses and short wavelengths help distinguish nearby objects and provide better detail.
Example
Finding sea depth
- A sonar pulse returns from the seabed after 0.080 s0.080\ \text{s}0.080 s, and sound travels through seawater at 1500 m s−11500\ \text{m s}^{-1}1500 m s−1.
- The total travelled distance is vt=1500×0.080=120 mvt=1500\times0.080=120\ \text{m}vt=1500×0.080=120 m.
- The water depth is d=1202=60 md=\dfrac{120}{2}=60\ \text{m}d=2120=60 m.
Foetal scanning
- A transducer sends pulses of ultrasound into the body through coupling gel, which reduces reflection at the skin.
- At boundaries between tissues, some ultrasound is reflected while the rest continues deeper.
- The time delay gives the depth of a boundary, and the echo strength helps distinguish different tissues.
- A computer combines many echoes to form an image of the foetus.
- Ultrasound is non-ionising, so it does not carry the ionisation risk associated with X-rays, although exposure is still kept as low as needed.
Exploring the Earth
- Earthquakes produce seismic waves with very low frequencies, including infrasound and vibrations that pass through the Earth.
- Seismic waves change speed, refract and reflect at boundaries between layers.
- Detectors around the Earth record arrival times and wave paths.
- Differences in speed and the presence of shadow zones provide evidence for internal layers, including the mantle and core.
- Different seismic wave types behave differently in solids and liquids, which helps identify whether a layer is solid or liquid.
Exam technique
- For an echo calculation, state that the measured time covers two journeys before dividing by 222.
- For a scanning explanation, link each echo's time delay to depth and its amplitude to the nature of the boundary.
- For the Earth's core, refer to changes in speed, refraction, reflection and detected arrival patterns.
Common Mistake
- Do not describe a foetal scan as an X-ray image; it is formed from reflected ultrasound pulses.
- Do not forget the factor of 222 in pulse-echo distance calculations.
Self review
- How does sonar determine the distance to an object?
- Why is coupling gel used in an ultrasound scan?
- What information is obtained from echo time and echo amplitude?
- How do seismic-wave paths provide evidence about the Earth's interior?
- Why must pulse-echo travel time be divided by two?
