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Showing 1 question in Physics for Class 9. Tap a card to reveal the answer.
PhysicsSoundmediumWhat is an echo, and what is the minimum distance needed to hear one clearly?
Reveal answer ↓
What it is
An echo is sound bouncing back off a surface and reaching you a moment later.
Answer
An echo is the sound heard again after it reflects off a hard surface and returns to the listener. To hear a distinct echo the reflected sound must reach the ear at least 0.1 second after the original, which needs the reflecting surface to be at least about 17.2 m away (since sound travels about 344 m/s in air).
distance = (speed x time) / 2 = (344 x 0.1) / 2 = 17.2 m
- •Echo = reflected sound heard again
- •Ear needs a 0.1 s gap to separate the two sounds
- •Minimum distance about 17.2 m (speed 344 m/s)
Why learn this
It's the principle behind SONAR, ultrasound scans and measuring ocean depth.
💡 Memory trick
Need a 0.1 s gap -> surface at least about 17 m away, or the echo merges with the sound.