The echo (the sound reflection) and conditions necessary for hearing the echo
The echo is a repetition of sound produced due to its reflection, It can be used to tell how far away an object is, how fast the object is moving, and even its shape.
If you make a sound in a large empty room, you will hear a repetition of your sound, Also if you visit a large empty place containing hard flat surfaces and produce a sound, you will hear a repetition of your sound.
The conditions necessary for hearing the echo
The distance between the sound source and the reflecting surface must not be less than 17 metres where the time period between hearing the original sound and its echo should not be less than 0.1 of a second.
The human ear can not distinguish between two successive sounds if the period between them is less than 0.1 second, Wide and big reflecting surface must be presented to hear the echo such as the walls, the mountains or the water bodies.
The velocity of sound through the air is 340 m/sec, so the distance travelled by the sound and its echo in 0.1 sec = 34 metres, The distance travelled by the sound is 17 metres from the sound source to the reflecting surface and 17 metres from the reflecting surface to the ear (echo).
The echo can not be heard if the distance between the sound source and the reflecting surface is less than 17 metres because the time between hearing the main sound and its echo will be less than 0.1 of a second, and the human ear can not distinguish between the two successive sounds.
When the distance between hearing the main sound and the reflecting surface is multiplies of 17 metres (twice or three times), the echo is heard in the form of the last two or three phrases of the whole produced sound.