1. What Is an Echo?
Echo is the repetition of a sound caused by the reflection of sound waves from a surface.
When a sound wave hits a large, hard surface like a wall, cliff or building, it bounces back and reaches the ear again after a short delay. This repeated sound is heard as an echo.
2. How an Echo Is Produced
When you produce a sound, the waves travel outward. If they strike a surface that does not absorb sound, they reflect back. If the reflected sound reaches your ear after a noticeable gap, you hear it as a separate sound.
2.1. Condition for Hearing an Echo
To hear an echo, the reflected sound must reach the ear at least 0.1 seconds after the original sound. This is the minimum time gap needed for the human ear to sense the two sounds as separate.
2.1.1. Why 0.1 Seconds?
The ear cannot distinguish two sounds that arrive within a very short interval. At around one-tenth of a second, the ear recognises the reflected sound as a separate one.
2.2. Distance Required for Echo
The distance required for an echo can be estimated using the speed of sound. In air, sound travels roughly 340 m/s. In 0.1 seconds, it covers:
\( 340 \times 0.1 = 34 \text{ metres} \)
Since the sound has to travel to the wall and back, the minimum distance of the reflecting surface should be about 17 metres.
3. Examples of Echo
- Shouting in a large empty hall or tunnel and hearing your voice return.
- Calling out in a valley or near a mountain cliff.
- Hearing sounds bounce back near tall buildings or inside large chambers.
4. Why Some Places Do Not Produce Echoes
Soft materials such as curtains, carpets, and padded walls absorb sound instead of reflecting it. This prevents echoes from forming.
4.1. Everyday Observation
A fully furnished room rarely produces an echo, but an empty room often does because there are fewer materials to absorb sound.