In which one of the following locations can a person hear the echo of a sound?

*A. * Less than 16.6 meters in front of the reflecting surface

*B. * At least 33.2 meters behind the reflecting surface

*C. * Less than 33.2 meters behind the reflecting surface

*D. * At least 16.6 meters in front of the reflecting surface

If you are at least roughly 16.6 meters or so from the

reflecting surface, then you can hear the echo. If you're

closer than that, there's still an echo, but the delay between

the original sound and the echo is so short that your ear/brain

mechanism can't separate them.

The speed of sound in typical air is around 340 meters per

second. So somewhere around 16.5 - 17 meters of physical

separation results in 1/10 of a second delay until the echo.

That's considered to be the minimum perceptible delay for

an echo.