What are the conditions required to hear an echo?

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Time must be at least

0.1 s

or more.

The distance of the reflecting surface from the source must be at least

17 m

or more at

1 5^{\circ} c

The room must be small.

Both 1 and 2

answer is D.

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Detailed Solution

Time must be at least

0.1 s

or more and the distance of the reflecting surface from the source must be at least

17 m

or more at

1 5^{\circ} c

are the conditions required to hear an echo.
The distance between the sound source and the location where it is reflected must be greater than

17 m

, and the time between hearing the original sound and its echo must be greater than

0.1 s .

The human ear cannot distinguish between two successive sounds if the time difference between them is less than

0.1 \sec,

and the location from which it is being reflected, such as walls, mountains, or water bodies, must be presented to hear the echo.
Because the speed of sound in air is

340 m / s,

the distance travelled by the sound and its echo in

0.1 \sec

34 m

. The sound travels

17 m

from the source to the location where it will be reflected and

17 m

from that location to the ear.
Only the last two or three phases of the entire sound can be heard as an echo when the distance between hearing the actual sound and its reflection is multiple of