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

Range of human hearing, infrasound and ultrasound for IGCSE & GCSE Physics revision notes
Frequencies below 20 Hz are called infrasound and frequencies above 20 000 Hz are called 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

s space equals space v space cross times space t

  • 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 imaging, for IGCSE & GCSE Physics revision notes
Ultrasound can be used to construct an image of a foetus in the womb

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

Ultrasound  trace of a cracked bolt, for IGCSE & GCSE Physics revision notes
Ultrasound is partially reflected at boundaries, so in a bolt with no internal cracks, there should only be two pulses (at the start and end of the bolt)

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.

An oscilloscope trace showing that the time interval between two pulses is 5 squares where each square represents 0.00002 s

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, v space equals space 6000 space straight m divided by straight s

  • Time taken, t space equals space 5 space cross times space 0.000002 space equals space 0.00001 space straight s

Step 2: Write down the equation relating speed, distance and time

s space equals space v space cross times space t

Step 3: Calculate the distance

s space equals space 6000 space cross times space 0.00001

s space equals space 0.06 space straight m

Step 4: Convert the distance to cm

s space equals space 0.06 space cross times space 100

s space equals space 6 space cm

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