What is ultrasound and how can it be used?

Ultrasound is high frequency sound that cannot be heard by humans. You should have seen a demo where the frequency of a note produced by a signal generator and loud speaker is increased until it is so high that you cannot hear it. For most people this happens at about 20 kHz.

You may be asked to compare the frequencies and amplitudes of oscilloscope traces produced by ultrasound just as with audible notes.

Pre-Natal Scanning

How can we check a baby is OK before its born. X rays only show bones well and, because they are ionising radiation, they may harm the foetus. Ultrasound can be used instead. The ultrasound reflects off the boundaries between different types of tissue back to a detector. A computer then uses these reflections to produce an image we can see.

Other uses

These include cleaning and quality control. The waves can shake off tiny dirt particles off delicate instruments such as the inside of watches. Ultrasound can also be used to clean teeth.

We can use ultrasound to look for cracks in objects, e.g. in concrete pillars. The sound reflects off a crack inside the object and from the time to reflect we can calculate the position of the crack.

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Revision Questions

1. A pulse of ultrasound is sent into the wall of a nuclear reactor and any reflections are detected. A large pulse is detected after 8 ms (0.008s)

a) Show that the speed of the ultrasound in the concrete is 3750 m/s

b) Why is there a smaller pulse?

2. What exactly do ultrasound waves reflect off inside a pregnant woman?