Echoes and Ultrasound

When sound bounces back

Shout "HELLO!" across a valley or inside a large empty hall, and a moment later the word comes back to you. That returning sound is an echo — and it is one of the most useful tricks in all of physics. The very same idea lets ships measure the ocean floor, lets bats hunt in pitch darkness, and lets doctors see a baby before it is born. To understand all of that, we just need to understand what an echo really is.

What an echo is

Sound travels as a wave through the air. When that wave hits a hard, flat surface — a cliff, a wall, the side of a building — it reflects, bouncing back the way it came, just as a ball bounces off a wall. If the surface is far enough away, the reflected sound arrives back at your ears a noticeable moment after your shout, and you hear it as a separate sound: an echo.

Echoes are reflections of sound, in the same way that your face in a mirror is a reflection of light. If you have studied how light bounces, you will recognise the pattern — sound obeys similar rules. You can revisit the basics of sound waves in sound and how we hear.

Why you don't always hear an echo

You hear a clear echo only when there is a clear time gap between the original sound and its reflection. Your brain needs the reflection to arrive roughly a tenth of a second or more after the original to tell them apart.

Sound travels at about 340 metres every second in air. In a tenth of a second it covers about 34 metres. Because the sound must travel to the wall and back, the wall needs to be at least about 17 metres away for you to notice a separate echo.

That is why a big empty sports hall or a tall canyon gives crisp echoes, but your bedroom does not. In a small room the reflections still happen, but they come back so fast they blend into the original sound. We call that blurry mix reverberation — the slight ringing you hear in a tiled bathroom. Soft things like carpets, curtains, and sofas absorb sound and kill echoes, which is why a cinema is lined with thick padded walls.

Worked example: measuring distance with an echo

Because we know the speed of sound, an echo becomes a measuring tool. Here is the key idea: the sound travels there and back, so the total distance is twice the gap between you and the surface.

distance = speed × time

Problem. You stand in front of a tall cliff, clap your hands, and hear the echo 2 seconds later. How far away is the cliff? (Speed of sound = 340 m/s.)

Total distance the sound travelled = speed × time Total distance = 340 m/s × 2 s = 680 m But that 680 m is the round trip — to the cliff and back. Distance to the cliff = 680 ÷ 2 = 340 m

So the cliff is 340 metres away. This "send a pulse, time the echo, halve it" method is the heart of every echo-ranging device on Earth.

Ultrasound: sound we cannot hear

Humans can hear sounds with frequencies between roughly 20 Hz (very low rumbles) and 20,000 Hz (very high whistles). Frequency means how many wave wobbles happen each second, and it sets the pitch of a sound.

Ultrasound is simply sound with a frequency above 20,000 Hz — too high-pitched for the human ear to detect. We cannot hear it, but it is real sound, and it behaves like any other sound: it travels, reflects, and makes echoes. Many animals hear and use it easily.

Ultrasound is especially useful because its short waves bounce off small objects and can be aimed in a tight, focused beam — perfect for "seeing" with sound.

Echoes at work in the world

Sonar (ships and submarines). A ship sends a pulse of sound or ultrasound straight down and times the echo from the seabed. Using distance = speed × time, halved for the round trip, it works out the depth of the ocean. Submarines use sonar to detect other vessels in water too dark and deep for any light.

Echolocation (bats and dolphins). A bat squeaks out rapid bursts of ultrasound and listens for the echoes bouncing off moths and walls. From the timing and direction of each echo, its brain builds a "sound picture" of the night around it — so it can catch an insect in total darkness. Dolphins do the same thing underwater to find fish.

Medical ultrasound scans. A doctor presses a small device called a probe against the skin. It sends gentle ultrasound pulses into the body and listens to the echoes bouncing off different tissues. A computer turns those echoes into a moving picture — which is how parents get the first photo of a baby before it is born. Because ultrasound is just sound, it does not use radiation, making it very safe.

Cleaning and more. Powerful ultrasound shaken through liquid can scrub the dirt off jewellery and surgical tools, and even break up painful kidney stones inside the body — all by vibrating things very fast.

Try it yourself! 🧪

Experiment 1 — Find an echo. Go somewhere with a large, hard, flat surface a long way off — a big brick wall across a playground, a railway underpass, or the side of a tall building (at least about 20 metres away, the further the better). Stand facing it and give one short, sharp clap or shout. Listen carefully for the sound to come back. Try clapping in a steady rhythm and you may be able to "play along" with your own echoes. Then go into a carpeted room and try the same clap — notice how the echo vanishes because soft surfaces soak up the sound.

Experiment 2 — Estimate a distance (with a stopwatch). Stand a good, known distance from that big wall (pace it out — roughly 50 to 100 metres works well). Clap once and start a stopwatch at the exact moment of the clap, stopping it the instant you hear the echo. Repeat several times and take an average, since the times are very short. Now use the maths: distance to wall = (340 × time) ÷ 2. Compare your answer with the distance you paced out. You have just used sound to measure the world — the very same method as a submarine's sonar.

(Stay in a safe, open area away from traffic, and never shout near someone's ear — loud sound up close can hurt hearing.)

From a clap in a canyon to a baby's first photo, it is all the same simple physics: send out sound, wait for the echo, and let the timing tell you what is out there.