Ultrasound (Oxford AQA IGCSE Physics)
Revision Note
Ultrasound
Ultrasound is the name given to frequencies of sound waves above the range of human hearing
The range of human hearing is between 20 Hz and 20 000 Hz
Infrasound and ultrasound
Uses of Ultrasound
Electronic systems can produce ultrasound waves
When ultrasound reaches a boundary between two media, some of the waves are partially reflected
The remainder of the waves continue through the material and are transmitted
Ultrasound transducers can:
Emit ultrasound
Receive ultrasound
The time taken for the reflections to reach a detector can be used to determine how far away a boundary is
This is because ultrasound travels at different speeds through different media
This is calculated using the speed, distance, and time equation
Where:
s = distance in metres (m)
v = speed in metres per second (m/s)
t = time in seconds (s)
This allows ultrasound waves to be used for both medical and industrial imaging
Ultrasound in medicine
In medicine, ultrasound can be used:
To construct images of a foetus in the womb
To generate 2D images of organs and other internal structures (as long as they are not surrounded by bone)
As a medical treatment such as removing kidney stones
An ultrasound detector is made up of a transducer that produces and directs a beam of ultrasound waves into the body
The ultrasound waves are reflected back to the transducer by 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
Foetal imaging using ultrasound
Ultrasound in industry
In industry, ultrasound can be used to:
Check for cracks inside metal objects
Generate images beneath surfaces
A crack in a metal block will cause some waves to reflect earlier than the rest, shown as pulses on an oscilloscope trace
Each pulse represents a moment when the wave crosses a boundary
The speed of the waves is constant, so measuring the time between emission and detection can allow the distance from the source to be calculated
Cracked surface detection using ultrasound
Worked Example
In the diagram below, a high-frequency sound wave is used to check for internal cracks in a large steel bolt. The oscilloscope trace shows that the bolt does have an internal crack.
Each division on the oscilloscope represents a time of 0.000002 s. The speed of sound through steel is 6000 m/s.
Calculate the distance, in cm, from the head of the bolt to the internal crack.
Answer:
Step 1: List the known quantities
Speed of ultrasound,
Time taken,
Step 2: Write down the equation relating speed, distance and time
Step 3: Calculate the distance
Step 4: Convert the distance to cm
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