Echoes: measuring the speed of sound
Clap in front of a cliff and time the echo. Stand nearer or farther, try a cold and a hot day, and work out how fast sound travels.
Core concept
An echo is sound reflected back; the speed of sound = 2 × distance ÷ echo time, about 340 m/s in air, and a little faster in warmer air
Topic: Sound
Shout towards a hillside or a tall building and you may hear your voice come back a moment later: an echo. Sound travels through the air as a wave, hits the hard surface, reflects and comes back to you. The echo takes time because sound travels at a definite speed: How far something travels in a certain time, like kilometres per hour. All key words. If you know how far away the cliff is and time the echo, you can work out that speed: the sound goes there and back, so speed = 2 × distance ÷ time. Our ears can only hear an echo separately from the original sound if it comes back at least 0.1 s later.
What you will learn
- Explain that an echo is reflected sound.
- Calculate the speed of sound using speed = 2 × distance ÷ echo time.
- Explain why an echo is not heard from a nearby wall.
- Describe how the temperature of the air changes the speed of sound.
Before you start
- Know that speed = distance ÷ time.
Materials
- Two wooden blocks to clap
Equipment
- A stopwatch, a measuring tape, a thermometer
Procedure
- Clap 170 m from a cliff on a 15 °C day and record the echo time.
- Stand only 10 m from the cliff and record.
- Record at four or more different distances.
- At 170 m, record on a 0 °C day and on a 30 °C day.
How this simulation is simplified
- Sound travels at 331 m/s in air at 0 °C, and 0.6 m/s faster for every °C warmer.
- The ear hears an echo separately only if it comes back at least 0.1 s after the clap.
- The cliff reflects the sound straight back, and there is no wind.