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Ultrasound & Infrasound (Edexcel GCSE Physics)
Revision Note
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:
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 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:
- 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
- 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
- On the opposite side of the Earth to an earthquake, only P-waves are detected, not S-waves, this suggests:
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