Ultrasound & Infrasound (Edexcel GCSE Physics)

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Ultrasound

Higher Tier Only

  • Ultrasound is defined as:

Sound waves with a frequency above the human hearing range of 20 000 Hz

Infrasound

Higher Tier Only

  • Infrasound is defined as:

Sound waves with a frequency below the human hearing range of 20 Hz

  • The spectrum of sound waves, including infrasound and ultrasound, is shown in the image below:

Range of human hearing, IGCSE & GCSE Physics revision notes

The human ear can detect sounds between around 20 and 20 000 Hz in frequency with a peak sensitivity at around 4000 Hz

Uses of Ultrasound & Infrasound

Higher Tier Only

  • Ultrasound and infrasound have multiple applications, including:
    • Sonar
    • Foetal scanning
    • Exploration of the Earth's core

Sonar

  • Sonar uses ultrasound to detect objects underwater
  • The sound wave is reflected off the ocean bottom
  • The time it takes for the sound wave to return is used to calculate the depth of the water
  • The distance the wave travels is twice the depth of the ocean
    • This is the distance to the ocean floor plus the distance for the wave to return

Foetal Scanning

  • In medicine, ultrasound can be used to construct images of a foetus in the womb
    • An ultrasound detector is made up of a transducer that produces and detects a beam of ultrasound waves into the body
    • The ultrasound waves are reflected back to the transducer by different boundaries between tissues in the path of the beam
    • For example, the boundary between fluid and soft tissue or tissue and bone

  • When these echoes hit the transducer, they generate electrical signals that are sent to the ultrasound scanner
  • Using the speed of sound and the time of each echo’s return, the detector calculates the distance from the transducer to the tissue boundary
  • By taking a series of ultrasound measurements, sweeping across an area, the time measurements may be used to build up an image
  • Unlike many other medical imaging techniques, ultrasound is non-invasive and is believed to be harmless

Ultrasound Medical Imaging, downloadable IGCSE & GCSE Physics revision notes

Ultrasound can be used to construct an image of a foetus in the womb

Exploration of the Earth's Core

  • Earthquakes produce two types of waves:
    • P-waves (primary waves, named so because they travel faster and so these waves are felt first in an earthquake)
    • S-waves (secondary waves, named so because these travel slower and so these waves are felt second in an earthquake)

  • These waves pass through the Earth’s centre and can be detected at various points around the Earth using seismometers
  • By carefully timing the arrival of the waves at each point, the location of the earthquake, along with its magnitude, can be pinpointed

P-Waves

  • P-waves are longitudinal waves, the direction of the oscillation is parallel to the direction of energy transfer
  • P-waves are faster than S-waves
    • Therefore, P-waves are felt first during an earthquake
    • P-waves produce a forward and backward motion
  • P-waves can pass through solids and liquids
    • Longitudinal waves can travel through gases, but P-waves do not
  • P-waves are very low frequency sound waves known as infrasound
    • Infrasound is any sound below the frequency of human hearing (<20 Hz)

  • P-waves refract as they pass through the different layers of the Earth
  • This refraction affects the regions in which waves can be detected, yielding important information about the nature and size of the Earth’s various layers

P-waves, IGCSE & GCSE Physics revision notes

Low frequency sound waves (P-waves) produced by earthquakes, pass through the centre of the Earth, revealing useful information about its structure

S-Waves

  • S-waves are transverse waves, the direction of the oscillation is perpendicular to the direction of energy transfer
  • S-waves are slower than P-waves
    • Therefore, S-waves are felt after P-waves during an earthquake
    • S-waves produce a side-to-side motion 
  • Unlike P-waves, S-waves are unable to travel through liquids
    • Longitudinal waves can travel through solids, liquids and gases, but S-waves only travel through solids
  • This means that they are unable to travel through the Earth’s molten (liquid) outer core – providing important evidence about its state and size

S-Wave, downloadable IGCSE & GCSE Physics revision notes

Transverse S-Waves are unable to pass through the Earth’s liquid outer core

Discoveries from Seismic Waves

  • The interior of the Earth is not directly observable as it is not physically possible to drill that far
    • The furthest humans have managed to drill down is 12.2 km - whereas the radius of the Earth is over 6000 km!

  • Seismic waves provide vital evidence that has led to a greater understanding of the structure of the Earth
  • The two main discoveries are:

    1. On the opposite side of the Earth to an earthquake, only P-waves are detected, not S-waves, this suggests:
      • The mantle is solid – this is because both types of wave can pass through it
      • The outer core of the Earth is liquid – hence no S-waves can penetrate it

    2. Refractions between layers cause two shadow zones, where no P-waves are detected, this suggests:
      • The inner core is solid – this is due to the size and positions of these shadow zones which indicate large refraction taking place

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.