Question

An echo is heard, if ____________ between the source and the reflector is atleast $17m$

• A. Time
• B. Distance
• C. Arc length
• D. None of these

Answer 1

The correct option is B Distance

Sound waves can be mirrored as can all waves. As a result of sound wave reflection from a large obstacle, the sound is heard which is named as an echo.

As the reflected sound gets merged with the original sound, echo is usually not heard. To hear an echo distinctly (as a separate sound) must be satisfied with certain conditions.

For about 0.1 seconds the sensation of any sound persists in our ear. That is known as hearing persistence.

When the echo is heard during this time span it is difficult to discern the original signal from its echo. So the most important condition for hearing an echo is that only after a lapse of at least 0.1 seconds after the original sound dies off, should the reflected sound reach the ear.

Since the sound speed is 340 m / s the sound traveling distance is 34 m in 0.1 seconds. The total distance between a sound source and the reflector is twice that. So, if the obstacle is at least 17 m away, then after 0.1 second, the reflected sound or the echo is clearly detected.

Hence, to hear a clear echo, the minimum distance between the reflecting surface and the observer is 17 m.