Echo: Reflection of Sound for Class 9 Students

An echo is the sound that we hear again after it is reflected from a surface. Have you ever shouted near a mountain or a tall building and heard your voice come back? That repeated sound is called an echo.

Echo is important because it helps us learn how sound reflects and travels. This article explains what an echo is and the conditions needed to hear it clearly.

Table of Contents

• What is Echo?
• Conditions to Hear an Echo
• Minimum Distance for Echo
• Multiple Echoes

What is Echo?

When we shout or clap near a large reflecting surface, such as a tall building or a mountain, we hear the same sound again after a short time. This reflected sound is called an echo.

Now, you may ask, why don’t we always hear an echo?

The reason is that our brain can hold a sound for about 0.1 seconds. If the reflected sound reaches our ears within this time, it mixes with the original sound and is not heard separately.

So, to hear a clear echo, the reflected sound must reach our ears after at least 0.1 seconds.

Conditions to Hear an Echo

To hear a clear echo, certain conditions must be satisfied.

• The reflected sound should reach the ear after at least 0.1 seconds
• The reflecting surface should be large and hard
• The distance between the source and the surface should be enough

So, in simple words, an echo is heard only when there is enough time gap between the original and reflected sound.

Now, there’s an interesting question: how far should the obstacle be to hear an echo? Let’s find out.

Minimum Distance for Echo

The speed of sound in air at 22°C is 344 m/s.

For an echo to be heard, sound must travel to the obstacle and come back in 0.1 seconds. So, 

Total distance covered by sound = Speed × Time = 344 × 0.1 = 34.4 m

This distance includes both forward and return paths.

Therefore, the minimum distance between the source and the reflecting surface is: 34.4 ÷ 2 = 17.2 m

So, the surface should be at least 17.2 metres away to hear a clear echo.

Multiple Echoes

Sometimes, we hear more than one echo. This happens due to multiple reflections of sound.

Have you ever noticed the rolling sound of thunder?

This happens because sound waves reflect again and again from surfaces like clouds and the ground. These repeated reflections produce multiple echoes.

In this article, we learned that an echo is the reflection of sound heard after a delay. We also understood that a minimum distance of about 17.2 m is required to hear a clear echo, and multiple reflections can produce more than one echo.