To distinguish a reflected sound (echo) from the original sound, the minimum time interval between the two must be:

The correct option is

0.1 s

Explanation

The phenomenon of hearing an echo is governed by the persistence of hearing. This refers to the retention of the sensation of sound in the human brain for a brief period after the source of the sound has ceased.

Analysis

For the human ear to distinguish a reflected sound (echo) as a separate entity from the original sound, the reflected wave must reach the ear only after the sensation of the original sound has faded. The persistence of hearing for the human ear is approximately 0.1 seconds (1/10th of a second).

• If the reflected sound arrives in less than 0.1 seconds, it overlaps with the original sound, and the brain perceives it as a single, prolonged sound (reverberation).
• If the time interval is 0.1 seconds or more, the brain processes the reflected sound as a distinct echo.

Key Takeaway: To hear a distinct echo, the minimum time interval between the original sound and the reflected sound must be 0.1 seconds.