Mon Prof Augmenté
Measuring the Sea’s Depth with an Echo
Age 14 · Physics
A boat sends a short sound pulse down to the seabed and times how long the echo takes to come back. Since sound travels about 1500 metres per second in seawater, depth = speed × time ÷ 2, because the sound goes down and back up.
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In the picture
- Sonar transducer: sends out the sound pulse and listens for its return
- Outgoing pulse: sound travelling down through the water at about 1500 m/s
- Seabed: the hard surface that reflects the sound
- Echo: the reflected sound travelling back up the same distance
- Stopwatch: measures the total time for the trip down and back
Examples
- An echo of 2 seconds means the sound travelled 3000 m in total, so the sea is 1500 m deep.
- Car parking sensors use ultrasound echoes in air to measure the distance to a wall.
Key takeaways
- Distance = speed × time, and the echo time covers the path twice.
- Sound travels about 1500 m/s in seawater, over four times faster than in air.
Quiz: Measuring the Sea’s Depth with an Echo (Age 14)
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When working out the depth of the seabed from an echo, why do you divide speed × time by 2?
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A boat’s sonar pulse comes back 2 seconds after it was sent. About how deep is the seabed?
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How fast does sound travel in seawater, compared with air?
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