Question

A sponge is kept at a distance of 19m from a source of sound.
(A)No Echo will be heard
(B)An Echo can be heard
(C)No sound will be heard
(D)None of the above is true

Answer 1

Hint
We will pull in the concept of least distance for an echo to be heard. Then we will see if the situation fulfills the requirements.

Complete step by step answer
We know, For the propagation of sound, there has to be some medium for it to flow. More precisely speaking, as sound is a longitudinal wave and we know that longitudinal waves are produced by disturbance in the particles of the environment.
Thus, clearly, for its propagation, there has to be a medium available for disturbance and in our situation we take air as the medium of propagation. Thus, this condition is satisfied.
Then, for an echo to be heard, the minimum distance between the obstacle and the observer must be equal to or more than$17.2m$ and here the distance is $19m$ . So this condition is also getting satisfied.
But, to produce an echo, the obstacle must be rigid and here the obstacle is a sponge which we cannot consider as rigid at all. Thus, this condition is not satisfied.
So, though the first two conditions are satisfied but if the third condition is not satisfied, there will be no echo. Hence, there is no use of thinking about its propagation and distance of hearing.
Hence, ‘No echo will be heard’ which is option (A).

Note
No doubt there will be propagation of sound as well as the distance of the observer from the obstacle is ideal for getting an echo, but if there is no production of an echo, then there is nothing to put our ears upon. The non-production of echo is due to the fact that a sponge is not a rigid surface.