Topic 6

 

 What is ultrasound and how can it be used?

 

In the exam you need to know how:

  • to compare the amplitudes and frequencies of ultrasounds from diagrams of oscilloscope traces

    •

  • to determine the distance between interfaces in various media from diagrams of oscilloscope traces.

In the exam you need to know that:

  • Electronic systems can be used to produce ultrasound waves which have a frequency higher than the upper limit of hearing for humans.

  • Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector is a measure of how far away such a boundary is.

  • Ultrasound waves can be used in industry for cleaning and quality control.

  • Ultrasound waves can be used in medicine for pre-natal scanning.

 

Key Words

Ultrasound

Media

Boundary

 

 

Ultrasound

Ultrasound is sound of any frequency above the range of human hearing, about 20 000 Hz.  Many animals can hear ultrasound.  Early remote controllers for TV sets used ultrasound, which meant that each time you changed channel, the cat would leap up the curtains.  Modern remote controllers use infra red.

 

Bats and dolphins are animals that use ultrasound as SONAR for both hunting and navigation.  It is believed that they can make a picture in their brains from the ultrasound echoes they pick up.

 

 

Note the large ears that funnel the sound down to the animal.  The shape of the mouth and nose focus the sound waves.

 

 

 Bats and dolphins use SONAR (Sound navigation and ranging), not RADAR (radio detection and ranging).  This is a common bear trap in exams.

 

 

Ultrasound in Medicine

Ultrasonic devices are used primarily for non-destructive investigation:

The most obvious use in human and veterinary medicine is the use of the ultrasound test in pregnancy.  The equipment can nowadays reveal very detailed pictures:

 

 

The ultrasound equipment consists of:

The transducer is coupled to the skin using a jelly.  This is because ultrasound is reflected as soon as it hits a material-air boundary.  Therefore without a jelly the waves would reflect as soon as they were transmitted, and not show anything.  The sonographer (operator) moves the transducer about to get patterns of signals from a wide area.

 

 

The beam of ultrasound can be focused to improve the picture.  The higher the frequency, the better the resolution (how much detail can be picked up).

 

Question 1  The speed of sound through the body is about 1400 m/s.  What is the wavelength of an ultrasound signal of frequency 2 000 000 Hz?  ANSWER

 

If you want to go into ultrasound in medicine in more detail, go to:

 

http://electronics.howstuffworks.com/ultrasound.htm

 

Ultrasound can also be used for treatment.  Ultrasound probes can be used to break up gallstones.  In dentistry, ultrasonic probes can clean plaque off your teeth.

 

 

Ultrasound in Industry

A common use for ultrasound is to check for flaws in castings.  Flaws are bubbles of gas that can occur while metal is cast.  As the metal solidifies, the bubbles get trapped.  They can seriously weaken the casting.  If they are bad enough, the casting may have to be scrapped.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The ultrasound probe send pulses through the casting.  Where the pulses meet a boundary, for example, if there are two layers of metal, they are partially reflected.  If there is a flaw, most of the waves get reflected.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Question 2  What else happens to the sound waves as they pass the boundary?  Why?  ANSWER

 

Now look at what the CRO shows:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

You can see that there is a transmitted pulse, which is quite big, and a smaller received echo, which is the reflected pulse.

 

Question 3.  Why is the reflected pulse smaller than the transmitted pulse?  ANSWER.

 

Question 4.  Each square on the horizontal axis represents 1 microsecond (1 × 10-6 s).  How long does it take for the echo to arrive back at the probe after the pulse has left?  ANSWER

 

Question 5.  The speed of sound in the casting is 6000 m/s.  What is the depth of the flaw?  ANSWER

 

 

Ultrasound is often used for cleaning.  In water and other solvents small bubbles are formed by cavitation.  These are very effective at removing dirt from components.  Ultrasonically cleaned components are very clean indeed.  Here is a picture of an ultrasonic component cleaner used to clean instruments:

 

 

Here is another ultrasound device used to clean wires.

 

 

Now answer Question 6.

 

Now try the Crossword that gets you to think about sound and ultrasound.

 

Summary

  • Ultrasound is any sound of frequency greater than 20 000 Hz

  • Ultrasound is used in medical imaging

  • Ultrasound can be used in industry for checking the quality of components.

  • The ultrasound probe transmits pulses and receives the echoes.

  • The distance to a boundary is half the distance worked out from the time between pulse and echo.

  • Ultrasound can be used to clean components.

 

Now try the Topic Quiz

 

 

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