4.15P Explain uses of ultrasound and infrasound, including a sonar b…

The idea

Ultrasound is sound with a frequency above the upper limit of human hearing (above about 20 000 Hz), and infrasound is sound below 20 Hz. Both have practical uses that depend on how they travel and reflect. Sonar sends an ultrasound pulse into water and times the echo; because the speed of sound in water is known, the distance follows from distance = speed × time, halved for the return journey. Foetal scanning uses ultrasound reflected from boundaries between tissues of different density to build an image, and it is preferred to X-rays because ultrasound carries no ionising radiation risk to the developing foetus. Infrasound, with its very low frequency and long wavelength, penetrates rock and can be detected after earthquakes and explosions, which helps scientists study the structure and behaviour of the Earth's interior, including the core.

What earns the marks

• Matches sonar to ultrasound and explains that a reflected pulse and its travel time give distance.
• Matches foetal scanning to ultrasound and explains that echoes from tissue boundaries form an image.
• States that ultrasound is used in foetal scanning because it avoids the ionising radiation of X-rays.
• Matches exploration of the Earth's core to infrasound and explains that low-frequency waves penetrate rock and are detected after seismic events.
• Distinguishes ultrasound (above the audible range) from infrasound (below 20 Hz) when selecting an option.

Common mistakes and how to put them right

• Assigns sonar to infrasound.

  Put it right: Link sonar to ultrasound pulses and their echoes.

• Claims foetal scanning uses X-rays.

  Put it right: State that ultrasound is chosen to avoid ionising radiation.

• Treats infrasound as unable to travel through the Earth.

  Put it right: Explain that its low frequency lets it penetrate rock and be detected after seismic events.

Exam tips

• For each option, name the wave band and the physical reason before deciding.
• Use distance = speed × time and remember to halve it for the echo's return journey.
• Reject options that give a use without a correct supporting reason.