2.5 Echoes and ultrasound
Summary: Where is the quietest place in the world? Scientists have designed a room so quiet that you can hear your own heartbeat. The surfaces are designed to absorb take in sounds. There are no echoes. Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- explain how echoes are used to find distance
- describe uses of ultrasound
- compare Sound and ultrasound.
Think back
- What happens to waves when they hit a barrier?
- What is sound with a frequency higher than 20 000 Hz called?
- How does the Speed of sound in water compare with the speed of sound in air?
Key idea We hear echoes when sound is reflected off a surface. Ultrasound technology uses echoes to gather information.
Key words echo, transmitter, receiver, sonar
Where is the quietest place in the world? Scientists have designed a room so quiet that you can hear your own heartbeat. The surfaces are designed to absorb (take in) sounds. There are no echoes.
What is an echo?
When sound reflects off a surface it produces an echo. Sound takes time to travel. There is a time delay (short wait) between making a sound and hearing an echo. Soft surfaces like curtains, or specially designed surfaces like those in Figure 1, can reduce echoes.
Figure 1These walls are designed to produce no echoes.
A Suggest one place where echoes might be annoying.
Measuring distances
Imagine someone is standing a long distance from a building, as in Figure 2. They clap their hands and hear an echo two seconds later. The time delay depends on the distance.
Figure 2A sound will bounce off objects nearby to make an echo. This can be used to measure the distance to objects. Full text alternative to the content
You can use the time it takes to hear an echo to work out how far away objects are from the source of the sound.
B Describe what happens to the time it takes to hear an echo if the person moves closer to the building.
What are the uses of ultrasound?
Ultrasound is sound with a frequency above 20 000 Hz. An ultrasonic transmitter produces a beam, or line, of ultrasound. You can produce a narrow beam of ultrasound more easily than a narrow beam of sound waves.
When radiographers make images of unborn babies (see Figure 3), a beam of ultrasound travels through the body and reflects off the fetus. The ultrasonic receiver detects the echo. It uses the time it takes for the echo to return to build up an image of the fetus.
Figure 3An ultrasound image (left) of a baby (right).
C Suggest whether ultrasound travels faster in bone or in tissue.
Physiotherapists can use ultrasound to reduce the pain and swelling of a damaged tendon. Doctors can use ultrasound to identify cancer. You can also use ultrasound to clean dirty objects.
Where is ultrasound used to measure distance?
Figure 4 shows ultrasound being used on ships at sea. This is called sonar. A transmitter under the ship sends out a beam of ultrasound. This travels through the water and reflects off the seabed. A receiver detects the reflection. It uses the time taken to work out the depth of the water.
Figure 4Ships use ultrasound to work out the depth of the ocean below them.
Maths skills A ship’s sonar detects an echo 1.6 s after it sends the pulse. The speed of sound in water is 1500 m/s. Work out how deep the water is.
Animals also use ultrasound. Bats and dolphins use ultrasound to find their food. They use sound and ultrasound to communicate with each other.
Stretch zone
- Copy and complete the sentences. An echo is a rarefaction/reflection of sound. You can use the distance/time between making a sound and hearing its echo to calculate the distance/time to an object. Soft materials absorb/reflect sound and so reduce echoes. Sailors use ultrasound to find the depth/temperature of the ocean.
- Describe how to use sound to calculate how far away a building is.
Stretch zone
- A fishing boat owner uses an ultrasonic transmitter and detector to find fish. The transmitter sends out beams. Suggest why the captain might think the fish were below where they actually are. Explain your answer.