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What is an echo, and when can we hear one?

Class 9 ScienceNCERT chapter: Soundइसे हिंदी में पढ़ें

Exam-ready answer

An echo is the repetition of a sound caused by the reflection of sound waves from a hard surface. We can hear an echo only if the reflected sound reaches our ears at least 0.1 second after the original sound. This requires a minimum distance of about 17.2 m from the reflecting surface in air at room temperature.

Explanation

When you shout in front of a large building, a cliff, or a hill, the sound waves travel through the air and strike the hard surface. The surface reflects the sound waves back towards you, just like a mirror reflects light. This reflected sound, when heard separately from the original sound, is called an echo.

Our ears can distinguish two sounds only if there is a time gap of at least 0.1 second between them. If the reflected sound reaches us sooner than 0.1 s, it merges with the original sound and we cannot hear a separate echo. This is why echoes are not heard in small rooms; the reflected sound returns too quickly.

The minimum distance needed to hear an echo in air at room temperature can be calculated using speed = distance / time. Taking the speed of sound in air as about 344 m/s and time as 0.1 s, the total distance travelled by sound is 344 × 0.1 = 34.4 m. Since sound travels to the surface and back, the minimum distance to the reflecting surface is half of this, i.e., about 17.2 m.

Points to remember

Echo: repetition of sound due to reflection from a hard surface (e.g., cliff, wall, metal sheet).
Condition to hear an echo: time gap between original and reflected sound must be at least 0.1 s.
Minimum distance for echo in air at room temperature: about 17.2 m (using speed of sound ≈ 344 m/s).
Formula used: 2d = v × t, where d is distance to reflecting surface, v is speed of sound, t is time.
Echo is not heard in small rooms because the reflected sound reaches too quickly and mixes with the original sound.

Exam tip

While writing the answer, always state the 0.1 s condition and, if asked for minimum distance, show the calculation using speed = distance/time. Mention the speed of sound in air as 344 m/s (or 340 m/s) and remember to divide the total distance by 2.

Questions students also ask

Why can't we hear an echo in a small room?

In a small room, the distance to the walls is less than 17.2 m, so the reflected sound reaches our ears in less than 0.1 s. It merges with the original sound, and we cannot distinguish it as a separate echo.

What is the minimum distance to hear an echo in air?

The minimum distance from the reflecting surface to hear a distinct echo in air at room temperature is about 17.2 m. This is because sound must travel to the surface and back in at least 0.1 s.

Does echo occur in water?

Yes, echo can occur in water because sound reflects from surfaces like the sea bed or rocks. However, the minimum distance may differ because the speed of sound in water is much higher (about 1500 m/s).

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