Echoes

1. Echoes D. Crowley, 2008

2. Echoes To know what causes echoes and how these are used in sonar

3. Solid Speed • What is the speed of sound? • Sound travels at 330m/s through air • Sound travels faster / slower depending on what medium it goes through • Generally, the denser the medium, the faster it travels as the particles are closer together, so vibrations are more easily passed on from particle to particle: - • 330m/s through air • 1480m/s through water • 5100m/s through steel

4. Echoes • What are echoes? • Sound can reflect from the surface of an object - this is called an echo • Which surfaces reflect sound better? • Hard surfaces reflect sound better than soft surfaces – which is one reason why classrooms without carpets or curtains can be noisy places

5. Bathroom Singers • Why are some rooms / natural environments better at producing echoes than others? • Remember, sound reflects off a surface – sound reflects best of shiny hard surfaces • Bathrooms are good rooms to sing in as the sound bounces well off tiled walls • Singing in the living room results in most of the sound energy being lost, as the sound energy is absorbed by the carpet, furniture, curtains etc…

6. Echoes In Caves • Why do you hear echoes in caves? • Hard surfaces reflect sound better than soft surfaces • Sound is reflected well by large solid objects, so in caves the sound is reflected well from the solid cave wall • Why do echoes usually sound quieter? • As the sound wave travels some energy is lost, so you usually hear your echo with less amplitude (volume)

7. Distance • You can work out how far away something is using the reflection of waves • If you stood at the entrance of a cave and shouted, how could you work out how far away the back of the cave was? Distance = Speed x Time • If the echo took 20 seconds to be heard after you shouted, how far back is the cave? Distance = 330m/s x 20 seconds Distance = 6600m • However, the distance is from your mouth, to the back of the cave, and back again! So we need to halve this result – distance to back of cave is therefore 3300m

8. There & Back Again Distance Speed Time 1. Sound echo took 10 seconds to bounce back Distance = 330m/s x 10 seconds Distance = 3300m 2. Sound echo took 30 seconds to bounce back Distance = 330m/s x 30 seconds Distance = 4950m 3. Sound echo took 1 minute to bounce back Distance = 330m/s x 60 seconds Distance = 9900m For echo ÷ by 2

9. Sonar • Sonar stands for sound navigation and ranging – a technique used to measure how far away something is • The boat sends out a sound wave which hits the sea bed • The sound wave reflects off the surface, back to the receiver – the time it takes can be used to calculate the distance • Sound travels at 1480m/s through water - if the sound took 0.3 seconds to get back to the boat, what is the distance?

10. Sonar • Sound travels at 1480m/s through water • If the sound took 0.3 seconds to get back to the boat, what is the distance? • Distance = Speed x Time • Distance = 1480m/s x 0.3 seconds • Distance = 444m • However, this is the sound travelling there and back, so we need to ÷ 2 • Distance = 222m

11. Sonar In Nature • Sonar is used by bats to help them navigate • They send out very high pitched sounds, and detect the echoes – the quicker the echo comes back, the nearer the object is • Bats are so good at this, they are able to fly in total darkness (useful when living in a cave)!

12. Temperature • How do you think water temperature changes affect how quickly the sound travels • As water gets colder, the particles get closer together (density increases) so the speed of sound increases • Density is at its greatest when water is 4o (so at this temperature, sound travels the quickest)

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