MP Board · Class 9 · Science · SoundWhat is an echo? State the two necessary conditions required for a human ear to hear a distinct echo.

An echo is defined as the repetition of sound caused by the reflection of sound waves from a hard or distant obstacle back to the listener. \nThe two essential conditions required to hear a distinct echo are:

1. Time Interval: The time gap between the original sound and the reflected sound must be at least 0.1 seconds. This is because the sensation of sound persists in the human brain for about 0.1 seconds (persistence of hearing).
2. Minimum Distance: The minimum distance between the sound source and the reflecting obstacle must be at least 17.2 meters (assuming the speed of sound in air is $344\text{ m/s}$ at $22^\circ\text{C}$), allowing the reflected sound to take at least 0.1 seconds to travel back.