Hard
What is the minimum distance required between a sound source and a reflecting surface for a human ear to hear a distinct echo?
Explanation
Explanation
For an echo to be heard distinctly, the reflected sound must reach the ear at least 0.1 s after the original sound. Using speed of sound = 340 m/s, the minimum distance = (340 × 0.1)/2 = 17.2 m.
PRO TIP
Re-check your working by substituting the answer back into the original question.