1.18 - Ultrasound

What ultrasound is

• Ultrasound is sound with a frequency higher than 20,000 Hz (20 kHz).
• This is above the upper limit of human hearing, so people can't hear ultrasound.
• Ultrasound can be produced electronically: an electrical oscillator generates oscillations at the required frequency, which are converted into mechanical vibrations to create the sound wave.

Partial reflection and ultrasound echoes

When a wave reaches a boundary between two different media, part of it is transmitted and part of it is reflected back. This is called partial reflection.

Using echoes to find a boundary

1. A short pulse of ultrasound is sent into a material where the wave's speed is known.
2. Measuring the time it takes for the reflection to return allows the distance to the boundary to be calculated using:

Distance = speed × time

Uses of ultrasound

Medical imaging

• Ultrasound passes through the body, but at every boundary between two different tissues (e.g. between fluid in the womb and the skin of a foetus), some of the wave is reflected back and detected.
• A computer processes the timing and distribution of these echoes to produce an image of internal structures or a developing foetus.
• Also used to examine soft tissues and organs (kidneys, liver, bladder).
• Safer than X-rays because it doesn't use ionising radiation.

Industrial flaw detection

• Ultrasound entering a material is usually reflected by the far side. A flaw such as an internal crack reflects the wave sooner.
• Used to detect flaws in pipes, railway tracks, and metal castings without cutting them open.

Echo sounding and sonar

• Used by boats and submarines to find water depth or locate objects in deep water.
• A pulse of ultrasound sent into the water reflects off the seabed or an object, and the time taken for the reflection to return gives the distance.