Ultrasound (Cambridge (CIE) IGCSE Physics)

Ultrasound

• The approximate range of frequencies audible to humans is 20 Hz to 20 000 Hz in frequency

  • The human hearing range decreases with age

 Frequencies of ultrasound

Humans can hear sounds between 20 and 20 000 Hz

• Ultrasound is defined as

sound with a frequency higher than 20 kHz

Uses of ultrasound

Extended tier only

• Ultrasound is used in:

  • non-destructive testing of materials

  • medical scanning of soft tissue

  • sonar to calculate the depth or distance from time and wave speed

Measuring the speed of ultrasound

• When ultrasound signals reach a boundary between two media, some of the waves are partially reflected

  • The remainder of the waves are transmitted through the material

• Ultrasound transducers:

  • emit and receive ultrasound

  • display the received signal on an oscilloscope screen

• The depth of the boundary can be determined using:

  • Time taken between the ultrasound being emitted and received

  • Knowing the speed of the ultrasound through the material

Measuring the depth of a boundary

The depth of a boundary is measured using the difference between emitted and received ultrasound pulses

• The depth of the boundary is calculated using the equation:

• Where:

  • depth = half the total distance travelled by the ultrasound, measured in metres (m)

  • speed = speed of the ultrasound in the medium, measured in metres per second (m/s)

  • time = total time from ultrasound emission to receipt, measured in seconds (s)

Ultrasound in material testing

• In industry, ultrasound can be used to:

  • Check for cracks inside metal objects

  • Generate images from beneath surfaces

• Ultrasound waves that reach a crack in an object, such as a metal bar, are reflected back from the crack earlier than the waves that continue to be absorbed through the object
  

  • The reflected waves are displayed as pulses on an oscilloscope trace

• This allows engineers to assess the structure of an object

Oscilloscope display for material imperfection testing

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)

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

• The ultrasound waves are reflected back to the transducer by boundaries between tissues in the path of the beam

  • Examples of boundaries are between fluid and soft tissue or tissue and bone

• 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 image of a baby in the womb

Ultrasound can be used to construct an image of a foetus in the womb

Sonar

• Echo sounding can be used to measure depth or to detect objects underwater

  • A sound wave can be transmitted from the surface of the water

  • The sound wave is reflected off the bottom of the ocean

• 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

Echo sounding

Echo sounding is used to determine water depth