Echoes are created by sound waves being reflected off of a large obstacle. Conditions have to be just right for the echo to be heard as a separate sound.
The original sound and the echo cannot be heard separately if the original sound is heard in 0.1 seconds, this time interval is known as the persistence of hearing. Echoes are heard separately when the reflected sound is heard after a time lapse of 0.1 seconds. The size of the object the sound wave is being reflected upon must be larger than the sound wave itself. Echoes must also be loud enough or intense enough to be heard, and the original sound must be very short.