Ultrasound & Infrasound (Edexcel GCSE Physics)

Higher Tier Only

• Ultrasound is defined as:

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

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:

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

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 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

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

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. 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