Echo

Introduction

Sound can reflect from hard surfaces such as mountains, cliffs, walls and long corridors. Sometimes, after producing a sound, we hear the same sound again after a short time. This repeated sound is called an echo.

Echo is one of the most common examples of reflection of sound.

What is an Echo?

An echo is the repetition of sound heard after it reflects from a distant hard surface and returns to the listener.

For example, if we shout near a mountain or cliff, our sound may reflect from the hard surface and come back to us. We then hear our own voice again after some time.

How is an Echo Produced?

Echo is produced due to the reflection of sound. The process can be understood in the following steps:

A person produces sound by shouting or clapping.
The sound travels through air and reaches a distant reflecting surface.
The hard surface reflects the sound back.
The reflected sound reaches the person’s ear after a short time.
If the time gap is enough, the reflected sound is heard separately as an echo.

Diagram: Formation of Echo

Explanation of the Diagram

In the diagram, the person produces a sound. The sound travels to the hard wall and reflects back. The reflected sound reaches the person again. If the reflected sound reaches after a sufficient time gap, it is heard as an echo.

Condition for Hearing an Echo

Echoes are not heard everywhere. In a small room, sound reflects from the walls, but the reflected sound reaches our ears too quickly. Therefore, our brain cannot hear it as a separate sound.

For a clear echo, the time gap between the original sound and the reflected sound should be at least:

\(0.1\,s\)

If the time gap is less than \(0.1\,s\), the two sounds are not clearly distinguished by our brain.

Minimum Distance for Echo

Let the speed of sound in air be \(340\,m\,s^{-1}\).

In \(0.1\,s\), sound travels:

\(\text{Distance} = \text{Speed} \times \text{Time}\)

\(\text{Distance} = 340 \times 0.1 = 34\,m\)

This \(34\,m\) is the total distance travelled by sound from the person to the reflecting surface and back to the person.

Therefore, the distance of the reflecting surface from the person should be half of \(34\,m\).

\(\text{Minimum distance} = \frac{34}{2} = 17\,m\)

Thus, for hearing a clear echo, the reflecting surface should be at a minimum distance of about \(17\,m\) from the listener.

Formula for Echo Distance

When sound travels to a wall and comes back, it covers twice the distance between the listener and the wall.

\(2d = vt\)

Therefore,

\(d = \frac{vt}{2}\)

where,

\(d\) = distance of the reflecting surface from the listener
\(v\) = speed of sound
\(t\) = time taken by sound to go and return

Example

A person claps in an empty corridor and hears an echo after \(0.5\,s\). If the speed of sound in air is \(340\,m\,s^{-1}\), find the distance of the wall from the person.

Given,

Speed of sound, \(v = 340\,m\,s^{-1}\)
Time, \(t = 0.5\,s\)

Using,

\(d = \frac{vt}{2}\)

\(d = \frac{340 \times 0.5}{2}\)

\(d = \frac{170}{2}\)

\(d = 85\,m\)

Therefore, the wall is \(85\,m\) away from the person.

Surfaces Suitable for Echo

Echoes are clearer when sound reflects from hard and smooth surfaces.

Type of Surface	Effect on Sound	Echo Formation
Hard and smooth surface	Reflects sound strongly	Clear echo may be heard
Soft surface	Absorbs sound	Echo is weak or not heard
Rough surface	Scatters sound in many directions	Echo is not clear

Examples of Echo

A shout near a mountain may return as an echo.
A clap in an empty long corridor may produce an echo.
Sound reflected from a distant cliff can produce an echo.
Large hard walls can reflect sound and produce echoes.

Why Echo is Not Heard in a Small Room

In a small room, the walls are very close. The reflected sound comes back in less than \(0.1\,s\).

Since the time gap is too small, our brain cannot separate the reflected sound from the original sound. Therefore, we do not hear a clear echo in a small room.

Difference Between Original Sound and Echo

Original Sound	Echo
The sound produced directly by the source	The reflected sound heard after some time
It reaches the ear first	It reaches the ear later
It does not require reflection	It is produced due to reflection of sound

Important Terms

1. Echo

The repetition of sound heard after reflection from a distant hard surface is called an echo.

2. Reflecting Surface

The surface from which sound reflects is called a reflecting surface.

3. Time Gap

The time difference between the original sound and the reflected sound is called the time gap.

4. Minimum Echo Distance

The minimum distance of the reflecting surface required to hear a clear echo is about \(17\,m\) in air when the speed of sound is taken as \(340\,m\,s^{-1}\).

Important Points

Echo is produced due to reflection of sound.
An echo is the repeated sound heard after reflection from a distant surface.
Echo can be heard near mountains, cliffs, large walls and long corridors.
The time gap between the original sound and reflected sound should be at least \(0.1\,s\) to hear a clear echo.
If the speed of sound is \(340\,m\,s^{-1}\), the minimum distance of the reflecting surface should be about \(17\,m\).
Hard and smooth surfaces produce stronger echoes.
Soft surfaces such as curtains absorb sound.
Rough surfaces scatter sound, so echoes are not heard clearly.
The formula for distance of the reflecting surface is \(d = \frac{vt}{2}\).

Conclusion

Echo is a repeated sound heard after reflection from a distant surface. It is produced when sound travels to a reflecting surface and comes back to the listener. For hearing a clear echo, the reflected sound should reach the ear at least \(0.1\,s\) after the original sound. In air, when the speed of sound is \(340\,m\,s^{-1}\), the reflecting surface should be at least about \(17\,m\) away from the listener. Echoes are clearer from hard and smooth surfaces.