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Ultrasound waves have a frequency higher than the upper limit for human hearing – above 20,000 Hertz (Hz). Different species of animal have different hearing ranges. This explains why a dog can hear the ultrasound produced by a dog whistle, but humans cannot.

Uses of ultrasound

Uses of ultrasound include:

• breaking kidney stones
• cleaning jewellery
• foetal scanning
• detecting cracks in machinery

In the first two of these applications, the vibrations caused by the ultrasound shake apart the dirt or kidney stones, breaking them up. The principle is the same as the opera singer's trick, where a glass may shatter if the singer makes a high-pitched sound near to the glass.

Ultrasound imaging creates a picture of something that cannot be seen directly, such as an unborn baby in the womb, or faults and defects inside manufactured parts.

These uses rely on what happens when ultrasound waves meet the boundary between two different materials. When this happens:

1. some of the ultrasound waves are reflected at the boundary
2. the time taken for the waves to leave a source and return to a detector is measured
3. the depth of the boundary can be determined using the speed of sound in the material and the time taken

Example

Ultrasound waves emitted by the source take 0.006 seconds to return to the detector after being reflected by the crack. Calculate the depth of the crack below the surface of the material. The speed of ultrasound in the material under test is 1,200 m/s.

d = v × t

d = 1,200 × 0.006 = 7.2 m

However, since the sound had to travel down to the crack and back again, this distance must be divided by two to find the depth of the crack.

So, depth of crack = 3.6 m.

Echo sounding

High frequency sound waves can be used to detect objects in deep water and to measure water depth. The time between a pulse of sound being transmitted and detected and the speed of sound in water can be used to calculate the distance of the reflecting surface or object. The process is very similar to ultrasound imaging. However, the sound waves used are within normal hearing range, and they are used to identify objects rather than internal structures.

This technique is applied in sonar systems used to find shipwrecks, submarines and shoals of fish. Bats and dolphins use a similar method, called echolocation, to detect their surroundings and to find food.