Question

State whether the given statement is True or False :

A girl is sitting in the middle of a park of dimension $12m\times 12m.$ On the left side of it there is a building adjoining the park and on right side of the park, there is a road adjoining the park. A sound is produced on the road by a cracker. The girl will be able to hear the echo of this sound.

• A. True
• B. False

Answer 1

The correct option is B False

Like all waves, sound waves can be reflected. Sound waves suffer reflection from the large obstacles. As a result of reflection of sound wave from a large obstacle, the sound is heard which is named as an echo. Ordinarily echo is not heard as the reflected sound gets merged with the original sound. Certain conditions have to be satisfied to hear an echo distinctly (as a separate sound).
The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off. As the speed of sound is 340 m/s, the distance traveled by sound in 0.1 second is 34 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly.
In this case, the girl is seated in the middle of the park. So, the sound travels through a distance of 6 m up to the girl and then another 6 m up to the building. The total distance is just 12 m which is less than the minimum distance required for the sound to echo. Hence, the echo cannot be heard in this case.