# RELATION BETWEEN SPEED OF SOUND, TIME OF HERING ECHO-AND DISTANCE OF REFLECTING BODY

## Sound of Class 9

If t is the time at which an echo is heard, d is the distance between the source of sound and the reflecting body and v is the speed of sound. The total distance traveled by the sound is 2d.

Speed of sound,

or

## CALCULATION OF MINIMUM DISTANCE OF HEARING ECHO:

d is minimum distance required for hearing an echo when persistence of hearing is second. The velocity of sound (at room temperature) is 330 m/s.

So,

11 metre is the minimum distance of hearing echo.

### CONDITIONS FOR FORMATION OF AN ECHO:

• The minimum distance between the source of sound and the reflecting body should be 11 metres.
• The wavelength of the sound should be less than the height of the reflecting body.
• The intensity of sound should be sufficient so that it can be heard after reflection.

### REVERBERATION:

When a sharp sound is made in a hall, the listener cannot hear it as such. It is found to get prolonged. The intensity of sound first reaches a maximum and then falls till it becomes inaudible. It is interesting to note that a sound wave suffers 300 reflections in a room of ordinary size before becoming inaudible.

Thus, the phenomenon of persistence or prolongation of audible sound after the source has stopped emitting sound is called reverberation. The time for which reverberation persists until it becomes inaudible is called reverberation time.

### How is reverberation reduced?

Since reverberation is due to repeated reflections of sound waves from the ceiling, floor, walls etc. of a hall or an auditorium, we can reduce it by increasing the absorption of sound energy. To achieve this :

• The walls are covered with some sound absorbing material like felt, fiberboard, glass wool etc. or by heavy curtains with folds.
• The floor is carpeted.
• The furniture is upholstered.
• False ceiling of a suitable sound absorbing material is used.

A certain amount of reverberation is always desirable. This would enrich the music played or speech delivered and makes it more pleasant. So the real job is to reduce reverberation to the right amount and not to completely eliminate it.