Ultrasound
Introduction

Ultrasound is very high-pitched sound. We cannot hear ultrasound (it has a frequency above 20,000 Hz, the upper limit of human hearing). However animals, such as dogs, bats, and dolphins, can hear these very high-pitched sounds. Bats use ultrasound to 'see' trees and insects at night.
Ultrasound
A bat sends out an
ultrasound
Ultrasound is very high-pitched sound. It has a frequency which is above the audible range.
ultrasound
shriek. The shriek travels out in all directions. It bounces off a nearby insect and travels back to the bat. After 0.2 s the bat detects the echo from the insect.

Ultrasound travels at the same
speed
Speed is a measure of how fast something is moving. It is calculated by dividing the distance travelled by the time taken.
speed
as sound, 330 m/s in air. Use this equation to answer the questions below.

How far does the ultrasound travel in 0.2 s?
  •  m
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How far away is the insect? (Hint: Remember that the time given above is the time taken for the bat to detect the echo off the insect.)
  •  m
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Boats and submarines use echo sounding, or sonar. This can tell the sailors how deep the water is or whether there are other objects, such as mines, submarines, or fish beneath them.

A boat sends out a sonar 'ping' straight down into the sea. Four seconds later the echo is detected. The speed of sound in water is 1500 m/s.

How far does the ultrasound wave travel in 4 s?
  •  m
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How deep is the water?
  •  m
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The
frequency
In general, the frequency of an event describes how often it occurs. When talking about waves, the frequency is a measure of how many waves go past a fixed point in a given time.
frequency
of ultrasound is higher than audible sound, but the speeds in different materials are the same.

What can you say about the wavelengths of sound and ultrasound?
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Waves with long wavelengths spread out or diffract around obstacles. They do not travel in straight lines.
Which type of wave would diffract less?
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Uses of ultrasound
Ultrasound can be used to clean objects. The fast, energetic ultrasound vibrations are absorbed by dirt which is then shaken free. This method of cleaning is used in industry for fragile equipment or to clean in corners which are otherwise difficult to get into. It is also used by dentists to clean teeth. Doctors often use X-rays to look inside people.
However X-rays are dangerous to soft tissue and can severely damage cells. For this reason, doctors use ultrasound scans to look inside pregnant women.

Figure 1.   An ultrasound scan of a foetus, and the mother receiving a scan.
Display as fullscreen
Select the phrases in the correct order to describe how an ultrasound scan works.
  • Stage 1:
    Stage 2:
    Stage 3:
    Stage 4:
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Summary


Ultrasound is very high-pitched sound. It has a frequency which is above the audible range.

Ultrasound has many applications, including cleaning delicate objects, measuring distances, and pre-natal scanning.
Exercises
1. Compare the properties below in a sound wave and an ultrasound wave.
  • Wavelength
    Frequency
    Speed
    Amplitude
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Figure 2.  
Display as fullscreen
2. Fig.2 above shows a range of sound frequencies. Select the correct labels for each part of the graph.
  • A:
    B:
    C:
    D:
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3. Bats use ultrasound to look for insects in the dark. Put the phrases below into order to describe this process.
  • Stage 1:
    Stage 2:
    Stage 3:
    Stage 4:
    Stage 5:
    Stage 6:
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4. when doctors are examining a baby in a womb, why do they prefer to use ultrasound rather than other types of radiation?
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