Unit 9, lesson 43. About 12 minutes.

Echoes, bats and ultrasound

How can a bat hunt tiny moths in total darkness?

Play

Getting the experiment ready…

Explain

Sounds can bounce back.

Bats use echoes to find their way.

An echo is a sound bouncing off something and coming back to you.

Bats squeak and listen for echoes. The longer the echo takes, the further away a moth is.

Doctors use very high sounds, called ultrasound, to see a baby before it is born.

Echo distance: sound travels there and back, so d = v × t / 2.

Ultrasound means frequencies above 20 000 Hz. High frequencies have short wavelengths, so they show small details.

Worked example: an echo returns after 2 s. So d = 343 × 2 / 2 = 343 m.

Send out a sound
It bounces back
Time tells the distance

Want the full story?

Bats use a very wide range of calls, and can even detect how fast a moth is moving. Ships use the same idea, called sonar, but sound in water travels at about 1500 m/s. The sim runs at a quarter of real speed.

The Formula

distance = speed of sound × time ÷ 2

d = \frac{v \times t}{2}

$d$: the distance to the wall, measured in metres (m)
$v$: the speed of sound in air, about 343, measured in metres per second (m/s)
$t$: the time until the echo comes back, measured in seconds (s)

Try the numbers

Echo time: 1 s

Distance to the wall: 171.5 m

Try it at home

Ask a grown-up to help

You need a big wall outdoors, a stopwatch, a tape measure.

You need: a big wall outdoors, a stopwatch, a tape measure.

Clap near a big wall. Listen!

Stand far from a large wall outdoors, in a safe place.
Clap steadily, so that each clap lands on the echo of the last.
Time 10 claps, then work out the speed of sound.

What's happening: each gap between claps is the time for sound to go there and back.

Physics is life

Your body

Doctors use ultrasound to see inside.

Ultrasound scans show babies, hearts and other organs safely.

Daily life

Ships use echoes to see the sea bed.

Ships use sonar to map the sea bed and find fish.

Nature

Bats and dolphins use echoes.

Dolphins use echoes to find food in dark water.

Space

Radar is an echo using radio waves.

Astronomers bounced radio waves off the Moon and Venus to measure their distance.

Quick check

Question 1 of 3

What is an echo?

Words to know

Echo: A sound that bounces back to you.

Echo: A sound that bounces back to you.
Ultrasound: Sound too high for people to hear.
Sound: Vibrations that travel through air or other materials.

Next up

Light travels in straight lines: shadows