The sound that we hear leaves an impression on our mind for 1/10th of a second. We are, therefore able to distinguish between two sounds clearly, only if the time interval of sound waves reaching our ear is greater than 1/10th of a second. We also know that sound travels at a speed of 344 m/s at 20°C. Therefore, the distance travelled by the sound in 1/10th second is 34.4 m. If the observer is situated at a distance of 17.2 m from the reflecting surface, then the distance travelled by the sound from the observer to reflecting surface and then from reflecting surface to the observer is 34.4 m. This reflected sound can be distinctly heard by the observer because it takes the second to reach the observer. 10
Therefore, the minimum distance of the observer from the reflecting surface is 17.2 m.