A sound persists in our ear for 0.1 s. At temperature 22℃ in air and speed of sound 344 ms^-1, the total distance covered by sound between the point of generation and reflecting surface should be at least (344 m/s) x 0.1 s = 34.4 m. Thus, the minimum distance between point of generation and reflecting surface should be half of it i.e. 17.2 m. So, we cannot hear echo in a smaller room of 10 m.