Ultrasound (Cambridge International IGCSE Physics)

Spec point — 3.4.4 · State the approximate range of frequencies audible to humans as 20 Hz to 20 000 Hz Define ultrasound as sound with a frequency higher than 20 kHz

Ultrasound

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

  • The human hearing range decreases with age

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

Spec point — 3.4.4 · Describe the uses of ultrasound in:

• non-destructive testing of materials,
• medical scanning of soft tissue and
• sonar including calculation of depth or distance from time and wave speed

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 depth of a boundary

• 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

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:

$\mathrm{depth}=\frac{1}{2}\times\mathrm{speed}\times\mathrm{time}$

• 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 foetus 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

Worked Example

In the diagram above, a very 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.

[4]

Answer:

Step 1: List the known quantities

• Speed of ultrasound, $v=6000m/s$
• Time taken between emission and detection, $t=5\times0.000002=0.00001s$ [1 mark]

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

$\mathrm{distance}=\mathrm{speed}\times\mathrm{time}$

Step 3: Calculate the total distance travelled by the sound wave

$\mathrm{total} \mathrm{distance}=6000\times0.00001=0.06m$ [1 mark]

Step 4: Calculate the distance travelled by the sound wave to the crack

$\mathrm{distance} \mathrm{to} \mathrm{the} \mathrm{crack}=\frac{0.06}{2}=0.03m$ [1 mark]

Step 5: Convert the distance to cm

$\mathrm{distance} \mathrm{to} \mathrm{the} \mathrm{crack}=3\mathrm{cm}$ [1 mark]