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Curious Kids: what makes an echo?


Noel Hanna is a member of the Australian Acoustical Society, Acoustical Society of America, Australian Institute of Physics and the Teachers' Guild of New South Wales. He holds the role of Leading Education Professional (Physics) with UNSW Global, a wholly owned subsidiary of UNSW Sydney.

That’s a tricky question. The simplest way to answer is to say that an echo is a sound that later comes back to where it came from.

Before we get into what makes an echo, we need to have a think about sound.

What is a sound?

What we call “sound” is really just the air in our ears moving back and forth.

The air can move fast or slow. We can hear air moving back and forth between 20 and 20,000 times per second. That’s really, really fast! (For the grownups reading right now, human hearing is from about 20 - 20,000 Hertz, which means repetitions per second).

But did you know that there are faster and slower air movements that can be heard by other animals, but not people?

Where does sound come from?

If we hear the air moving in our ears, where did that moving air come from?

A sound can come from anything that vibrates or moves back and forth.

It could start with the moving string of a guitar or the vocal folds in your voice box that move when you speak or sing.

Once the air starts to move, it travels in all directions until it finds something to stop it.

Read more: Curious Kids: Why are we ticklish?

Bounce back!

When sound travelling in air (we call this a sound wave) hits a hard flat surface, like a tiled bathroom wall, most of it bounces back. Maybe this is why people like to sing in the shower.

This is a drawing of how sound bounces back. gritsalak karalak/shutterstock

But to get a really good echo, that sounds the same as the original sound, we need a very big bathroom, or another very big, hard-walled place – like a valley or a canyon!

Here’s a video of a man playing trumpet in