Ultrasonics

Ultrasonic is the word used to describe the range of sound beyond the limit of human hearing. It has a frequency of at least 30 kHz.

Uses and effects of ultrasonics:

1. Pregnancy scan. Ultrasonic scans are used to look at unborn babies because the sound waves are safer than X rays
2. Sonar – depth measurement at sea. The speed of sound in water is greater than that in air because the molecules of water are closer together.
3. Producing fine sprays
4. Physiotherapy – the high frequency sound heats up the damaged tissue and so aids healing
5. Cleaning – the dust is literally shaken off by the vibrations
6. Ultrasonic welding – welding without heat pollution
7. Mixing emulsion paints
8. Burglar alarms – involving a Doppler motion sensor

The Echo Sounder

The echo sounder is used on ships to find the position of the sea bed, to detect sunken wrecks or submarines and to find shoals of fish.

A transmitter at the bottom of the ship sends out a beam of sound waves that is reflected from the sea bed and then picked up by a receiver on the ship. If you know the speed of sound in water and how long it is between transmitting a pulse of sound and receiving back at the ship you can work out how deep the sea is at that point.

Depth of the sea = [Speed of sound in sea water x Time]/2

where the time in the formula is how long it takes the sound to go to the sea bed and back.

There is a distinct difference between a sound pulse reflected from the sea bed and one reflected from a shoal of fish. The one reflected from the sea bed is distinct and not spread out in time while the one from the fish is more diffuse (woolly) and spread out as it reflects from fish at slightly different depths.

schoolphysics echo sounder animation

To see an animation of the use of an echo sounder click on the animation link.

Moving objects like submarines and fish make the pitch of the reflected sound alter - this is called the Doppler Shift

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A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB

© Keith Gibbs 2020