Echoes

An echo is the sound of your own voice (or any sound) bouncing back to you after hitting a hard surface. Echoes happen when sound waves reflect off solid objects like cliffs, walls or canyon faces, and travel back to your ears a little later. If the reflection takes long enough (at least about a tenth of a second), your brain hears it as a separate repeated sound: an echo. Shorter delays just make a room sound "reverberant" or "live". Echoes are not just a curiosity. They are the basis of sonar, ultrasound scans and bat echolocation.

• What it isSound waves reflecting backOff hard surfaces
• Min distance for echoAbout 17 metresFor a clear separation
• Speed of sound in air343 m/sSets the delay time
• Used inSonar, bats, dolphinsPlus ultrasound
• Hard surfaces echo wellWalls, cliffs, tilesCause sharp returns
• Soft surfaces dampenCarpets, curtains, foamAbsorb sound waves

How echoes form

Sound waves travel through the air in all directions. When they hit a hard, flat surface (like a cliff or a brick wall), most of the energy bounces back instead of being absorbed. The reflected wave travels back the way it came.

If you are close enough to the wall, you will hear the reflection so quickly that it blends with the original sound. Further away, the reflection takes long enough that your brain hears it as a separate, repeated sound: an echo.

How far away is the wall?

Sound travels in air at about 343 metres per second. The time delay for an echo equals twice the distance to the wall divided by the speed of sound (the sound has to go there and back).

• A 50-metre cliff produces an echo about 0.29 seconds after the shout.
• A 170-metre cliff produces an echo about 1 second after the shout.
• A 1-kilometre cliff produces an echo about 5.8 seconds after the shout.

Mountain climbers can roughly estimate the distance to a distant cliff by counting the seconds until they hear their echo: each second of delay represents about 170 metres.

Fact For an echo to be heard as a distinct repetition, the round trip needs to take at least about 1/10th of a second. That means you need to be at least 17 metres away from a wall. Closer than that, your brain merges the original and reflected sounds into one slightly longer noise. Inside a normal room, you do not hear distinct echoes, just a faint "liveness" of sound.

Reverberation

Inside large rooms (churches, cathedrals, concert halls, swimming pools), sound bounces off many surfaces in succession. The result is not a single echo but a smooth, gradually fading tail of sound called reverberation. Reverberation time (how long it takes a sound to fade to inaudibility) is a key measure for buildings.

• Most living rooms: about 0.5 seconds.
• Concert halls: 1.5 to 2.5 seconds (ideal for orchestral music).
• Cathedrals: 4 to 8 seconds (perfect for organ music and choral singing, but bad for speech).

Concert hall architects spend years getting the reverberation right. Too little and the music sounds dry and bare; too much and it sounds muddled.

Sonar: human-made echolocation

Submarines, fishing boats and oceanographers use sonar (SOund Navigation And Ranging) to find things underwater. The basic idea is exactly the same as a shout in a canyon, but underwater.

The ship emits a short loud pulse of sound. The pulse travels through the water in all directions. When it hits something solid (the seabed, a fish school, a sunken wreck, a whale, another submarine), it bounces back. By measuring how long the echo takes to return, the ship can work out how far away the object is. By scanning in different directions, sonar builds up a map of the surroundings.

Ultrasound scans

The same principle helps doctors look inside the human body. Ultrasound scanners emit very high-frequency sound pulses (around 1 to 20 MHz, far above human hearing). The pulses pass through soft tissue but bounce back from any interface between tissues of different density.

By analysing the returning echoes, the scanner can build up real-time images of:

• Unborn babies in the womb.
• The heart and how it is beating.
• Internal organs like the liver, kidneys and bladder.
• Tumours and other unusual masses.
• The thickness of blood vessel walls.

Ultrasound is safe and painless, with no radiation involved. It is one of the most useful and widely used medical imaging techniques.

Did you know? Some buildings are famous for their echoes. St Pauls Cathedral in London has a circular Whispering Gallery where a quiet whisper made on one side of the dome can be heard clearly on the other side, 32 metres away across the open space. The whisper travels around the curved dome wall by repeated bouncing, arriving almost as clearly as if it had crossed a normal room.

Animals using echoes

Many animals use echoes naturally:

• Bats emit ultrasonic clicks and listen for the echoes to find insects in flight, in pitch darkness.
• Dolphins and other toothed whales emit clicks and analyse the returning echoes to find fish, even in murky water.
• Some shrews and tenrecs use simple echolocation in tunnels.
• Cave-dwelling birds like oilbirds use audible clicks to navigate inside dark caves.
• Some blind humans have trained themselves to use mouth clicks and echo perception to navigate, an extraordinary skill called human echolocation.

Try this Stand in front of a large brick wall, at least 20 metres away. Clap your hands once. You should hear a clear echo a moment later. Time the delay with a stopwatch app, then work out the wall distance: distance = (delay in seconds) x 343 / 2 metres. So a 0.2-second delay means a wall 34 metres away. Try different distances and check the results. You can even use this to estimate distances on a hike.

Deeper dive: how the Titanic disaster led to the invention of sonar

The sinking of the RMS Titanic in April 1912, after it struck an iceberg in the North Atlantic, killed over 1,500 people. The tragedy shocked the world and led directly to the invention of one of the most important new technologies of the 20th century: sonar.

The Titanic had no way of detecting the iceberg below the waterline until it was too late. Within weeks of the disaster, English engineer Lewis Fry Richardson and others began proposing ways to detect underwater obstacles using sound. They knew that sound travels well through water, and reasoned that a pulse of sound could be bounced off an iceberg or seabed and timed to reveal the distance.

During the First World War (1914-1918), the technology was rapidly developed. German U-boat submarines were sinking Allied ships, and the Allies needed a way to detect the silent submarines below the water. By 1917 the British Navy had working ASDIC (later renamed sonar), the first practical sound-based submarine detector. Sonar saved countless lives during the Second World War, helping ships locate and attack German U-boats.

After the war, sonar moved into peaceful uses. Fishing boats now find shoals of fish with sonar. Oceanographers map the deep ocean floor with multibeam sonar arrays. Geologists search for oil and gas deposits by analysing returning sound echoes. Modern cars use ultrasonic parking sensors based on the same principle.

Even the most basic sonar works on exactly the same principle as a shout in a canyon. Send out a pulse, time the echo, work out the distance. Echoes have been turning into a powerful tool ever since one terrible night in 1912.

For more, see ultrasound and what is sound.