# Echo & Reverberation

## What is an Echo?

Echo is a phenomenon of hearing of successive reflected sound waves after hearing the direct sound wave. It is associated with reflection of sound from distant reflecting surface such as high cliffs, mountains, trees etc.

When a sound is generated, the original direct sound is first heard by an observer. The reflected sound reflecting from a distant surface takes more time to reach the observer and is heard later. The delay is directly proportional to the distance of the reflecting surface from the source and the observer.

This forms the basis of echo. Typical examples of echo are hearing reflecting sounds repeatedly by –

1. The bottom of a well.
2. By a tall building.
3. From cliffs and mountains etc.

A true echo is a single reflection of the sound source. Some animals use echo for location sensing and navigation purposes such as dolphins, whales and bats. Echoes may be desirable ( as in sonar ) or undesirable ( as in telephone systems ).

### Formation of Echo

Due to persistence of hearing, the human ear cannot distinguish two different sounds if the delay is less than \left ( \frac {1}{10} \right ) of a second.

Consider that an observer is firing a bullet who is ( x \ \text {m} ) from a reflecting surface such as a mountain. Then –

Distance traveled by the reflecting sound to reach the observer will be ( 2 x \ \text {m} ) . Also, the reflecting sound must reach the observer after \left ( \frac {1}{10} \right ) seconds.

The velocity of sound in dry air is approximately ( 344 \ \text {m-s}^{-1} ) at a temperature of ( 25 \degree C ) .

Therefore, minimum value of \quad 2 x = ( 344 \ \text {m-s}^{-1} ) \times \left ( \frac {1}{10} \right )

Or, \quad x = ( 17.2 \ \text {m} )

Therefore, the reflecting surface must be more than ( 17.2 \ \text {m} ) from the sound source and observer for echo to be perceived.

## Reverberation

When a sound is produced in a hall or auditorium, the produced sound waves suffer multiple reflections from different reflecting surfaces such as walls, ceiling, curtains, furniture and other materials present in the hall. This is called Reverberation of sound.

The intensity of sound we heard is the combined effect of direct sound and reflected sound waves from these surfaces. Due to this reason, the sound persist for some time in the hall or auditorium even after the source has stopped producing the sound. This phenomenon is called reverberation.

The persistent of audible sound after the source has ceased to emit sound is called reverberation.

### Reverberation Time

The time in which the original sound takes to fall in its intensity to one millionth i.e. 10^{-6} part of original intensity is called reverberation time.

Reverberation time depends upon the size of the hall, presence of sound absorbing materials etc. At a reflecting surface, a part of the sound is transmitted into the reflecting medium and remaining part is reflected to produce reverberating sound. Hence, the energy of original sound goes on decreasing after each reflection from the surface. This causes decrease in intensity of the reverberating sound.

For requirement of a good quality of sound available for the audiences in the hall, reverberation time should be optimum. It should neither be too long nor too short.

The long reverberation time is controlled by the following acoustic methods.

1. Covering of walls and doors of the hall with absorbent materials like asbestos, perforated card board etc.
2. Providing heavy curtains with folds.
3. Decorating the walls with pictures and maps.
4. By increasing the number of audience.
5. By making the floor sound absorbent.

### Difference between Echo & Reverberation

Echo and reverberation of sound both are the results of reflection of sound waves from hard surfaces but they have some basic differences as follows.

1. An echo is a single reflection of a sound wave from a distant reflecting surface in open area whereas reverberation is the reflection of sound waves created by the superposition of such echoes in an enclosed area.
2. Echo is a phenomenon of hearing of repeated sounds being reflected from a single reflecting surface whereas reverberation is the phenomenon of hearing multiple reflecting sounds reflected from multiple reflecting surface.
3. Echo can be heard when the reflecting surface is more than ( 17 \ \text {m} ) or ( 50 \ \text {feet} ) away from the source and observer. But reverberation can be heard even inside a hall.
4. Echoes can be used to determine the distance of a reflecting object such as a large building or mountain, depth of ocean etc. But reverberation does not allow enough travel time to use it for measuring distance.