Ultrasound (Oxford AQA IGCSE Physics)

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