Like all waves, sound waves can be reflected. Sound waves suffer reflection from the large obstacles. As a result of reflection of sound wave from a large obstacle, the sound is heard which is named as an echo. Ordinarily echo is not heard as the reflected sound gets merged with the original sound. Certain conditions have to be satisfied to hear an echo distinctly (as a separate sound).
The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off. As the speed of sound is 340 m/s, the distance travelled by sound in 0.1 second is 34 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 17 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly.
Further, for reflection of any wave to take place, the size of the reflector should be large compared to the wavelength of the sound, which for ordinary sound is of the order of 1 metre. A large building, a mountain side, large rock formation etc. are good reflectors of sound for producing an echo. Also, for the reflected sound to be heard, it must have enough intensity or loudness. Moreover, if the echo is to be distinguished from the original sound the two should not mix or overlap. For this, the original sound should be of very short duration, like a clap or shout.
So, following conditions could be listed for formation of echo: 
- The size of the obstacle/reflector must be large compared to the wavelength of the incident sound (for reflection of sound to take place).
- The distance between the source of sound and the reflector should be at least 17 m (so that the echo is heard distinctly after the original sound is over).
- The intensity or loudness of the sound should be sufficient for the reflected sound reaching the ear to be audible.
The original sound should be of short duration.
Example 1:
A man stands 250 m from a wall and hears the echo of his gunshot after 1.5 s. Calculate the velocity of sound under the circumstances.
Suggested answer :
Distance travelled by sound = 2 x 250 m
= 500 m
Velocity of sound = 
= 333.3 m/s
Example 2:A man stands at a certain distance from a wall and sets up a simple pendulum which makes three vibrations per second. He claps his hands and hears the echo exactly after five complete vibrations of the pendulum. If the velocity of sound in air is 350 m/s, calculate the distance between the man and the wall.
Suggested answer :
3 vibrations - 1s
5 vibrations - 
distance = velocity x time
2d = 340 x 
= 283.3 m
Advantages and Disadvantages of Echoes
Echoes can be useful or a nuisance. In a concert hall, echoes can ruin a performance if the walls and ceiling are not properly designed. If the walls are too hard, or too flat, they make good reflecting surfaces for the sound waves.
Echoes can be used to give vital information. A sonar (Sonar stands for sound navigation ranging) device sends out high frequency sound waves from a ship to find out how close the vessel is to the sea bed. An ultrasound scanner, particularly known for giving images of the unborn baby, works in roughly the same way.Bats use echoes to navigate as they fly in the night. This works on the same principle as sonar and ultrasound scanner. The bat sends out tiny, high pitched squeaks, which bounce off the objects in the bat's flight path. The echoes reach the bat, which then adjusts its course to avoid the obstacles. Many bats have very large ears to catch as much of the reflected sound as possible.
When animals such as bats and dolphins use echoes, it is called echo location. They use it to find their way around or to locate prey. Echo location describes the way of how some animals detect the size and position of objects around them.
