We can experience it in an industrial warehouse, on a street or, as usual, during a mountain excursion. Be that as it may, the time it takes to hear it will obviously depend on the distance at which the obstacle that reflects our cry is located. The further away it is, the longer the sound will need to travel the round trip distance and, therefore, the longer it will also take to hear that “Hello!”
Now, there is a minimum distance that we must find ourselves in order for the echo to occur. It has nothing to do with the physics of sound, but with the physiology of our ear: we are unable to distinguish two sounds if the difference in arrival time between one and the other is not greater than 50 milliseconds. More specifically, the brain needs a time interval of 0.1 seconds to discern between two musical sounds and 0.067 if it is two dry sounds, such as a word or a clap. This means that we will not hear an echo if the distance we are from the object is less than 17 meters, because in this case we only perceive an annoying reverberation, the nightmare of those who design auditoriums and concert halls.
On the other hand, the duration of an echo, that is, the time that we are going to be able to hear it, depends on two variables: the number of reflections that are produced and the natural decay of the sound intensity. If there are different obstacles located at different distances, we will be able to hear several echoes and even echoes of the echo. Of course, we must have powerful lungs, because the intensity of the sound drops six decibels if we double the distance: an echo against a mountain located at 800 meters is six decibels weaker than if it were at 400 meters.
