1Echoes
An echo is a sound that has been reflected back to you, for example from a cliff or a large, hard wall.
echo
Lesson 9.4 notes
Echoes and ultrasound
Reflection and absorption of sound, using echoes to measure distance, and the uses of ultrasound.
2Reflect or absorb
Hard, smooth surfaces reflect sound well. Soft surfaces, such as curtains, carpets and foam, absorb sound, so studios use them to stop echoes.
reflectabsorb
3Measuring with echoes
The echo travels there and back. If it takes 2 s at 340 m/s, the sound travels 340 × 2 = 680 m in all, so the cliff is 340 m away.
4Sonar
Ships use sonar: they send pulses of sound down and time the echo from the sea bed. Sound travels at about 1500 m/s in water.
sonar
5Ultrasound scans
Ultrasound reflects where one tissue meets another, and a computer turns the echoes into a picture. It is safe for scanning unborn babies.
ultrasound
6Cleaning with ultrasound
Sound waves are pressure waves that transfer energy. In an ultrasonic bath the vibrations shake dirt off jewellery, glasses and dental tools.
pressure waves
7Physiotherapy
Physiotherapists use ultrasound to treat injured muscles: the waves transfer energy deep into the tissue, warming it and helping it heal.
Explore: Echo sounder
Clap at a cliff or send a sonar ping down to the sea bed. Time the echo and work out the distance.
Common mistake: Halve the distance for an echo
An echo travels there and back. Speed × time gives the whole journey, so halve it to find how far away the surface is.
Remember
- An echo is a reflected sound. Hard, smooth surfaces reflect sound; soft surfaces absorb it.
- Distance to a surface = speed × time for the echo ÷ 2.
- Ultrasound (above 20 000 Hz) is used for scanning (its echoes make a picture), and for cleaning and physiotherapy, where pressure waves transfer energy.
Going further at GCSE