P3aL10  Did you hear that?

Key Words

Amplitude - height of wave from the crest to the rest position

Echo - a reflected sound wave

Frequency - number of waves per second.

Media - materials that carry waves.

Reflection - a wave bouncing off a boundary.

Refraction - the bending of waves as they pass over a boundary at an angle.

Ultrasound - sound that is above the range of human hearing

Vibrating - moving forward and backward at a high frequency.

Grade E

Sound is caused by vibrating objects.   Like all waves, sound waves can be reflected and refracted.  Sound wave reflections, echoes, are heard best in empty rooms, where they bounce off solid surfaces.  Reflection happens at the boundary of two different media.

Geophysicists use explosions to send sound waves through rocks.  By looking at the reflection and refraction patterns, they can tell the structure of the rocks beneath.

Sound travels at 340 m/s in air, at 1500 m/s through water, but 6000 m/s through rocks.  The precise speed depends on the rocks.  Just as light refracts as it passes from air to glass, sound refracts as it passes rock boundaries.  The interpretation of seismograms is not easy, but it's done by computer.

Grade C

Ultrasound is any sound with a frequency above the range of human hearing, which is from 20 Hz to 20 000 Hz.  Sounds below the range of human hearing are called infrasound.  Many big animals like elephants use infrasound to communicate over long distances.

Many animals can hear well into the ultrasound range.  Very old TV remote controllers used ultrasound.   These sent the cat up the curtains and the dog howling from the room.  Nowadays they use infra red.  Bats and dolphins use ultrasound for navigation and locating prey.

  Bats and dolphins use SONAR (Sound Navigation and Ranging), not RADAR (Radio navigation and ranging).

Ultrasound in Medicine

Industrial and medical devices use ultrasound  with a frequency of millions of Hertz, way above the range of any animal.

Ultrasonic devices are used primarily for non-destructive investigation:

• In engineering to test that a structure doesn't have flaws in it.

• In medicine to diagnose a range of conditions.

The most obvious use in human and veterinary medicine is the use of the ultrasound test in pregnancy.  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.

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.

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.

SONAR

Echoes are examples of reflected sound.  A ship using SONAR (sound navigation and ranging) sends pulses (pings) of high frequency sound to the bottom of the sea.  It has microphones that pick up the echoes, and the depth of the water can be worked out from the time taken between sending the pulse and receiving the echo.

Grade A

There is research going on that involves applying an electric field to tumours.  This makes the cancerous cells more prone to absorbing ultrasound.  When ultrasound is applied, the cells vibrate and burst, killing the cancer.