LEARNATHON
III

Competition for grade 6 to 10 students! Learn, solve tests and earn prizes!

The sound sensation lasts about $$0.1$$ seconds in our brain. As a result, the time interval between the original sound and the reflected sound must be at least $$0.1$$ $$s$$ to hear a distinct echo. Assume that the speed of sound is $$340$$ $m/s$ at $$25$$ $$° C$$. The sound must travel to the obstacle and then reflect back to the listener's ear after $$0.1$$ $$s$$. Sound should travel a total distance of at least $$340$$ $m/s$ $×0.1\phantom{\rule{0.147em}{0ex}}s\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}34\phantom{\rule{0.147em}{0ex}}m$ from its source to the reflecting surface and back.
As a result, in order to hear distinct echoes, the minimum distance between the obstacle and the source of sound must be half of this distance, or $$17$$ $$m$$. The temperature of the air will affect this distance.