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26. For hearing an echo, minimum distance of the obstaclebe(Speed offrom the source of sound must besound 344 ms )(A) 16.2 m(C) 172 m(B) 17.2 m(D) 17.2 cnm

Answer»

As the sensation of sound persists in our brain for about 0.1 s, to hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least (344 m/s) ×0.1 s = 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance. I.e., 34.4/2 = 17.2 m



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