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HOW ECHOES WORK

Shout, wait,
listen.

An echo is a measuring tape made of sound.

A small survey boat on a calm grey-blue sea at dawn, with a wide stretch of still water and soft sky beside it — AI-generated image, not a photograph of a real person or place.

Ships and bats use the same trick. Send out a pulse of sound, time how long the echo takes to come back, and you know how far away something is. Ocean scientists map the seabed this way, and bats hunt insects in the dark.

THEREand back again

Time an echo

Keep in mind

Send a pulse
Time the echo
Halve the trip
Distance from sound

THE SHORT ANSWER

Time the echo,
find the distance.

The method is the same in water and in air.
Only the speed of sound changes.

SONAR

Seeing with sound

Active sonar sends a pulse into the water. The time between the pulse and its echo gives the range of whatever it bounced off.

Time an echo

BATS

Calls we cannot hear

Bats send out short, loud, high-frequency pulses from their mouths or noses and use the returning echoes to map their surroundings.

Bat calls

LISTENING

Bringing calls into range

A bat detector converts ultrasonic calls into sounds people can hear, so walkers can tell some species apart by their calls.

How to listen

A QUICK ECHO CHECK

How far away
is it?

Pick an echo time to see how far away the object is. The sound has to go there and back, so the distance is half the total trip. The bar shows how far.

Echo timer

Choose an echo0.1 second, in air1 second, in air0.1 second, in seawater1 second, in seawater4 seconds, in seawater

0.1 second, in airAbout 17 m

In a tenth of a second sound travels about 34 metres in air: 17 metres out to the wall and 17 back.

1 second, in airAbout 170 m

A one-second echo in air means a cliff or building around 170 metres away.

0.1 second, in seawaterAbout 75 m

Sound is much faster in seawater, so the same tenth of a second covers about 150 metres there and back.

1 second, in seawaterAbout 750 m

A one-second echo from the seabed means water about 750 metres deep.

4 seconds, in seawaterAbout 3,000 m

A four-second wait for the echo means the seabed is roughly three kilometres down.

Speeds from NOAA National Ocean Service, Basics of underwater sound, and NOAA Ocean Exploration, Ocean acoustics. Distance = speed × time ÷ 2.

Worked out from NOAA's round figures for the speed of sound: about 340 metres a second in air and 1,500 in seawater. Real speeds vary with temperature and pressure.

TWO NUMBERS WORTH KNOWING

Fast in water,
high in the air.

One figure for the sea,
one for the bats in a British garden.

4× faster

How much faster sound travels in seawater than air

NOAA gives about 1,500 metres a second in seawater against about 340 in air: generally over four times faster. Low sounds can travel hundreds of kilometres underwater.

SPEED · NOAA

≈ 110 kHz

Peak frequency of the lesser horseshoe bat

The greater horseshoe bat calls at just above 80 kHz. Common, soprano and Nathusius' pipistrelles are best heard at about 45, 55 and 35 kHz.

BAT CALLS · BCN WILDLIFE TRUST

Most British bats make very brief calls, usually under 10 milliseconds, in rapid series of up to 25 a second. Horseshoe bats instead give long whistles of up to 50 milliseconds or more at a near constant frequency.

LISTENING FOR BATS

Tune in
at dusk.

UK bat groups describe the basics of using a bat detector. Some ways to get started:

Hands holding a small handheld bat detector in a garden at dusk, with trees and a pale evening sky behind — AI-generated image, not a photograph of a real person or place.

Step 1

Start with a heterodyne detector

London Bat Group calls these affordable detectors perfect for beginners. You tune them to one frequency at a time.

Step 2

Set it to 50 kHz

Pipistrelles are usually the first bats you meet, and are usually listened for with the detector set to 50 kHz.

Step 3

Listen for the sound

Pipistrelles sound like irregular smacks. A feeding buzz, like someone blowing a raspberry, means a bat is closing on prey.

Step 4

Join a bat walk first

The group suggests going on a few bat walks with experienced people and trying their detectors before you buy one.

ACTIVE AND PASSIVE

Speak, or
only listen.

A small bat in flight at dusk against a deep blue sky, silhouetted beside the dark outline of trees — AI-generated image, not a photograph of a real person or place.

Active sonar sends out its own pulse and listens for the echo, which tells it the range and direction of an object. NOAA uses it to make nautical charts, find hazards, search for shipwrecks and map the seafloor.

Passive sonar sends nothing. It only listens for sounds such as ships or whales. One passive listener cannot measure range, but several together can locate a sound source from the time delays between them.

Questions

Questions on the way

What does sonar stand for?

Sound Navigation and Ranging. NOAA explains that sonar is useful at sea because sound waves travel farther in water than radar and light waves do.

How do you work out distance from an echo?

Multiply the speed of sound by the time the echo took, then halve it, because the sound travelled out and back. In seawater that is about 750 metres for a one-second echo.

Are bats blind?

No. London Bat Group notes that bats can see very well, probably better than we do at dusk, but they also use echolocation to move and hunt in the dark.

Can a bat detector tell me which bat I heard?

Sometimes. The frequency and type of call help, but the BCN Wildlife Trust key warns that accurate identification from calls alone is not always possible.

Why does sound go so far underwater?

NOAA says temperature and pressure bend sound into a sound channel in the deep ocean, letting it travel thousands of miles without losing much energy.

Destination

Where the facts come from

Updated 16 October 2026

Shout, wait,listen.

Time the echo,find the distance.