Ultrasound | GCSE Physics | Doodle Science
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Doodle Science teaches you high school physics in a less boring way in almost no time!
GCSE Science
Script:
The range of human hearing is 20Hz to 20,000Hz. Ultrasound waves are just sound waves with a frequency above 20,000Hz.
When ultrasound reaches a boundary between two media with different densities, some are partially reflected back and the rest pass through.
This fact is used in industry to detect cracks or flaws in materials such as metal. They calculate the distance travelled by the ultrasound wave using the simple equation you probably all know, s=vt, where s is the distance in m, v is the speed in m/s and t is the time in seconds.
We can work this out by using an oscilloscope trace, take this one for example showing the waves being rebounded from a crack in a piece of aluminium.
There are 120 microseconds between the generated pulse and the reflected pulse. 120 microseconds is 120x10^-6 seconds. Now all we need to do is sub this value along with the speed of sound in aluminium into the equation to find the distance of 0.758m. But remember this is double the distance because the time we measured was the time it took the waves to travel to the crack and back. So the actual distance is half this.
Because ultrasound partially reflects at the boundary of different media, it is extremely useful for use in pre-natal scanning. It partially reflects at all the different types of tissue, which can be interpreted by a computer to put together an image.
Very high frequency ultrasound waves have other medical uses too like breaking up kidney big stones into smaller pieces so that they can be passed out with the urine with minimal pain.