MP Board · Class 9 · Science · SoundWhat is an echo? Calculate the minimum distance required between the source of sound and the reflecting obstacle to hear a distinct echo in air at $22^\circ\text{C}$.

An echo is defined as the repetition of sound caused by the reflection of sound waves from a distant obstacle or hard surface. When sound is produced, it travels through the medium, hits a reflecting surface, and returns to the listener's ears. \nTo hear a distinct echo, the time interval between the original sound and the reflected sound must be at least $0.1\text{ seconds}$, because the persistence of hearing for human ears is about $0.1\text{ seconds}$. \nGiven the speed of sound in air at $22^\circ\text{C}$ is $344\text{ m/s}$: $$\text{Total distance travelled by sound} = \text{Speed} \times \text{Time} = 344\text{ m/s} \times 0.1\text{ s} = 34.4\text{ m}$$ \nSince sound travels to the reflecting barrier and back, the minimum distance between the sound source and the reflecting surface must be half of the total distance: $$\text{Minimum distance} = \frac{34.4\text{ m}}{2} = 17.2\text{ m}$$ \nTherefore, the obstacle must be located at a minimum distance of $17.2\text{ metres}$ from the sound source to produce a clear echo.