Theory:

An echo is a sound that is reproduced due to the original sound being reflected off from various rigid surfaces such as walls, ceilings, mountain surfaces, and so on.
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Shouting near a mountain
 
You can hear the same sound if you shout or clap near a mountain or a reflecting surface, such as a building. The sound you hear is known as an echo. It is because of sound reflection. In a small room, there is no possibility of echo. This situation does not mean that sound bounces around in a small space. This is due to the fact that smaller rooms do not meet the basic requirements for hearing an echo.
  
Conditions necessary for hearing the echo:
  • Human ears have a \(0.1\) \(-\) \(second\) persistence of hearing. This means that if the time interval between two sounds is at least \(0.1\) \(s\), you can hear both of them. As a result, the time gap between the original sound and the echo must be at least \(0.1\) \(seconds\).
  • The above criterion can be met if the distance between the source of sound and the reflecting surface obeys the following equation:
Velocity=DistancetravelledbysoundTimetakenv=2dtd=vt2
since, t=0.1second,thend=v×0.12d=v20
As a result, the minimum distance required to hear an echo is 1/20th part of the magnitude of sound velocity in air. If you consider the velocity of sound as \(344\) m/ss, the minimum distance required to hear an echo is \(17.2\) \(m\).
 
Application of echoes:
  • Some animals use sound signals to communicate and locate objects over long distances.
  • Obstetric ultrasonography, which creates real-time visual images of the developing embryo or foetus in the mother's uterus, uses the echo principle. Because it does not use any harmful radiation, this is a safe testing tool.
  • In any medium, echo is used to determine the velocity of sound waves.