TwiddleLab / Physics / Waves and Sound
Echoes: Sonar and Ultrasound
What you will learn
- Waves reflect: sound bounces back from a wall, a moth or the sea floor as an echo.
- The echo time covers the trip there and back, so distance = speed × time ÷ 2.
- Bats, ship sonar and ultrasound scanners all find things this way: echolocation.
How the lesson goes
Bats hunt in total darkness. How do they know a moth is exactly 3.4 metres away?
The bat clicks and listens for the echo.
Follow one click, out and back.
Move the moth until its echo takes exactly 30 ms.
In 0.03 s sound travels 343 × 0.03 = 10.3 m. That is the trip out and back, so the moth is half of it: 5.15 m.
Echoes, in one rule.
Waves reflect off things in their way. A reflected sound is an echo.
The echo time is for the trip there and back, so: distance = speed × time ÷ 2.
Echolocation: bats and dolphins click; ships use sonar (in sea water sound goes about 1480 m/s); doctors use ultrasound, sound too high for us to hear, to see inside the body.
Simplified: we treat each echo as coming straight back, and the speed as fixed. Real sea water changes speed a little with temperature and depth.
Game: Dark cave. Get the sonar boat through using as few pings as you can.
Each section has three channels. A rock closer than 10 m means a bump.
A ship pings the sea floor six times. Draw the floor.
Each ping gives one depth: 1480 m/s × the echo time ÷ 2. Join the depths and you have the shape of the floor.
Code: let a loop turn all the echo times into depths.
A loop runs its indented lines once for each item in the list. Each time, t is the next echo time.
You shout at a cliff and hear the echo 0.5 s later. How far away is the cliff?
The shout travels to the cliff and back in 0.5 s: 343 × 0.5 = 171.5 m. Half of that is 85.75 m.
Your sea floor against the real sonar map.
Previous lesson: Pitch and Loudness · Next lesson: Waves Add Up: Interference · Part of the course: Waves and Sound
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