How can a robot hear where the wall is?

Have you ever shouted hello in a tunnel or a big empty hall? A moment later, your own voice came bouncing back to you. Hello, hello! Maybe you have also seen a little robot car roll across the floor and stop right before it reaches a wall.

That robot has no eyes like yours, and it does not touch the wall first. So how can a robot hear where the wall is?

On the front of the robot sits a small speaker. The robot uses it to send out a quick squeak of ultrasound, which means a sound so high that your ears cannot hear it. That way the robot can squeak again and again without bothering anyone at home.

The ultrasound zooms forward through the air until it reaches the wall. The hard wall stops the sound and sends it bouncing back toward the robot, just like a ball you throw at a wall.

The sound that bounces back is an echo, like your voice in the tunnel. Right beside the speaker, the robot has a microphone that works like an ear. The microphone catches the echo the moment it comes home.

The robot also keeps track of the time, from the moment it squeaks until the moment the echo arrives. If the wall is very close, will the echo come back quickly or slowly, do you think?

The echo comes back very quickly, because the sound only has a short trip to make. When the wall is far away, the sound travels longer, so the echo takes more time. A short wait tells the robot the wall is near, and a long wait tells it the wall is far.

Each trip is faster than a blink, so the robot sends out ultrasound again and again, many times every second. That way it always knows where the wall is, even while it rolls along.

Ultrasound. The robot sends out a squeak of ultrasound and listens for the echo bouncing back off the wall, and a quick echo tells it the wall is close.

Next time you hear your own echo, you can smile and think of a robot listening its way gently around the room.