Question

To hear a clear echo, what should be the minimum distance between the reflecting surface and the observer?

• A. $17\mathrm{feet}$
• B. $17\mathrm{meters}$
• C. $170\mathrm{feet}$
• D. $170\mathrm{meters}$

Answer 1

The correct option is B

$17\mathrm{meters}$

Sound waves:

1. Sound waves, like so many waves, could be reflected. Sound waves are reflected by the big obstructions. An echo is a tone that is heard as a consequence of the reflection of such a sound wave from either a huge obstruction. Normally, an echo isn't really heard because the reflected sound merges with the originating sound. Numerous circumstances must be met in order to hear an echo clearly.
2. The experience of any sound lasts roughly $0.1\mathrm{second}$ within the human ear. This would be referred to as hearing perseverance. This same original sound and its echoes cannot be separated if the echo is received or experienced within this time span. The most critical need for perceiving an echo is that the reflected noise reaches the ear at least $0.1\mathrm{seconds}$ just after originating sound has died off.
3. Considering sound travels at a speed of $340\mathrm{meters}\mathrm{per}\mathrm{second}$, the distance traveled by sound in $0.1\mathrm{seconds}$ is $34\mathrm{meters}$. This is twice the length between a sound coming and a reflector. So, if somehow the obstruction is at least $17\mathrm{meters}$ away, the reflected sound might well be heard repeatedly after $0.1\mathrm{seconds}$.

Therefore, option B ($17\mathrm{meters}$) is the correct choice.