What it does
An echo is sound bouncing back from a wall, a cliff or the sea bed. Because the sound goes there and back, the distance to the reflector is
distance = speed × time ÷ 2.
In Echo off a wall, set how far away the wall is and the air temperature. The page works out the speed of sound, the time for the echo, and whether you would hear it as a separate sound. Your ear keeps a sound for about 0.1 s, so the echo must arrive later than that: the wall has to be at least about 17 m away.
In SONAR at sea, a ship sends a pulse of ultrasound down to the sea bed. Set the depth to get the echo time, or type an echo time you measured to get the depth.
It follows the Class 9 science chapter Sound Waves: Characteristics and Applications (echo, SONAR). How the air vibrates in a sound wave is in sound wave visualiser, and how hospitals use echoes of ultrasound is in ultrasound calculator.
How to use it
- Pick Echo off a wall or SONAR at sea.
- Move the sliders, or type values.
- Press Send a sound to watch the pulse go out (blue) and come back (red).
- Read the answer and the working.
A worked example
At 22 °C sound travels at about 331.3 × √(1 + 22/273.15) ≈ 344.4 m/s. With the wall 100 m away, the sound travels 200 m, which takes 200 ÷ 344.4 ≈ 0.581 s. That is more than 0.1 s, so you hear a clear echo. The shortest distance for an echo at this temperature is 344.4 × 0.1 ÷ 2 ≈ 17.2 m.
At sea, type an echo time of 4 s: the pulse travels 1 500 × 4 = 6 000 m, so the sea is 3 000 m deep.
Tips
- Forgetting to halve is the most common mistake.
- Warm air carries sound faster, so echoes come back a little sooner on hot days.
- Bats and dolphins find their way with the same idea.
FAQ
Why ultrasound for SONAR? Short, high-frequency waves make narrow beams that go far in water and give sharp echoes from small things.
Is this the NCERT book? No. It covers the same topic with fibub’s own examples. fibub is not part of NCERT.
Where the numbers come from
- Speed of sound in dry air from the ideal-gas relation v = 331.3·√(1 + T/273.15) m/s; 1 500 m/s for sea water is the usual school value (real sea water is about 1 450 to 1 550 m/s). Persistence of hearing about 0.1 s, as used in school physics.
- Checked against hand calculation and Python: 331.3 m/s at 0 °C, 343.2 m/s at 20 °C; 100 m at 22 °C → 0.581 s; minimum echo distance at 22 °C → 17.2 m.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.