A person rings a metallic bell near a strong concrete wall. He hears the echo after 0. 3.s. If the sound moves with a speed of 340 m/s, how far is the wall from him?

To calculate the distance of the wall, one must understand that an echo involves the sound traveling from the source to the reflector and back again. This means the total distance covered by the sound wave is twice the actual distance (2d) between the person and the wall. Speed is defined as the total distance covered divided by the total time taken [1]. Using the formula Speed (v) = 2d / t, we can rearrange it to find distance: d = (v × t) / 2. Given the speed of sound (v) is 340 m/s and the time (t) for the echo is 0.3 seconds, the calculation is (340 × 0.3) / 2. This equals 102 / 2, resulting in a distance of 51 meters. This satisfies the physical requirement that the reflector must be at a minimum distance (approximately 17m) to hear a distinct echo.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 8: Measurement of Time and Motion > 8.3 Speed > p. 113