PHYSICS · Stage 7 · Chapter 3: Sound
Physics 3.3 Echoes — Sound Reflection, Sonar and Echolocation
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Lesson summary
3.3 Echoes — Sound Reflection, Sonar and Echolocation — Alhussan International Academy — Physics (Stages 7–9) notes: Describe how echoes are formed and how humans and animals use echoes, and calculate distance from the time for an echo to return.Echo = a reflection of sound, hear…
- Time delay
- Not a new sound
- Large rooms
- Reverberation
- Reducing echoes
- Ultrasound
Section 1 of 7What an echo is
- Echo = a reflection of sound, heard a short time after the original sound.
- The sound wave spreads out and reflects off surfaces such as the walls of a cave.
- Time delay
- sound travels relatively slowly, so there is a time delay between a shout and the echo.
- Not a new sound
- an echo is the original sound reflected, not a new sound made by the wall.
Section 2 of 7Problems with echoes
- Large rooms
- echoes can be a nuisance in cinemas and theatres, where sound reflects off all surfaces and echoes can last for several seconds, muddying the sound.
- Reverberation = the muddying of sound caused by many echoes lasting several seconds.
- Reducing echoes
- cinema walls and ceilings are covered with soft, sound-absorbing materials.
Section 3 of 7Ultrasound
- Ultrasound = sound with a frequency higher than 20 000 Hz, above the range of human hearing.
- Use
- humans and animals use sound that we cannot hear.
- Why ultrasound
- beams of ultrasound are more focused than beams of audible sound, and they will not be confused with sounds made by people, animals or other boats.
Section 4 of 7Sonar
- Sonar
- the word comes from sound navigation and ranging.
- Submarines use it to navigate underwater and ships use it to measure the depth of water.
- Fishermen use it to find fish.
- A transmitter (an underwater loudspeaker) on the ship sends out pulses of ultrasound.
- The waves travel through the water and reflect off objects such as rocks, fish or the sea bed.
- The echoes return to the ship and are detected by underwater microphones (the receiver).
- The device uses the time between sending and receiving to work out the depth of the water.
Section 5 of 7Calculating distance from an echo
- Formula
- distance = speed × time.
- Halve the time
- the measured time is for the journey there and back, so the time to the object is half of it.
- distance = speed × (time ÷ 2).
- Speeds of sound
- about 330 m/s in air and 1500 m/s in water.
- Reason for halving
- if the echo takes 1 second, the wave takes 0.5 s to reach the object and 0.5 s to come back.
Section 6 of 7Echolocation in animals
- Echolocation = finding the distance to an object by using echoes.
- Dolphins and whales
- they use echolocation more than sight, to find food or recognise family members in the dark or when the water is not clear.
- The dolphin makes a series of clicking sounds.
- The sound reflects off a fish.
- The dolphin detects the echoes and uses the time to work out the location of the fish.
- Bats
- they find the distance to objects using echolocation.
- Distance
- sound travels much faster in water than in air, so dolphins and whales can also communicate over very long distances.
- Dolphins can find food at greater distances than bats because sound travels faster and further in water.
Section 7 of 7Ultrasound scanning
- Safety
- pregnant women are not given X-rays because they could harm both the fetus and the mother,
- so doctors use ultrasound to check the development of the fetus.
- Method
- a transducer transmits ultrasound waves into the woman's body.
- The waves are reflected by boundaries between soft tissue and by hard surfaces such as bone.
- The transducer detects the echo and uses the time delay to calculate where the boundary is.
- Image
- by taking lots of pictures it is possible to build up a three-dimensional picture of the baby.
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