Question

Why does sound get reflect only after a certain distance ?that is 17m this point are also called eco point .

Why does eco (sound reflect)after 17m? Not in short distance?

Answer 1

it is a misconception that sound get reflected only after 17m.It also get reflected in lesser distances,but you can hear eco only at a distance of 17 m,because whatever sound you heard (will be in your memory for 0.1 seconds(this is called perception of sound).when you make sound in front of a wall which is less than 17 m,then the reflected sound reaches your ear in less than 0.1 seconds(This will be proved in the later half of this solution)so at that time the original sound is still in your ear and the reflected sound is also there in your ear.so this produces a prolonged vibration.you will hear only a single but prolonged sound.This is called reverberation.
but when the distance increases beyond 17 m the reflected sound has to travel more distance to get back to your ear,so the sound reaches your ear after 0.1 seconds.By that time the sound you ahear through perception will be vanished,and you hear a new sound which is the reflection of the original sound This is called echo.
Now let us check how this 17 m came from,
we know that we hear echo if and only if the sound reflect back in more than 0.1 second.
the velocity of sound is equal to 340 m/s in air
we know speed=distance/time
time=distance/speed
distance=speed *time
this distance is the distance travelled by the sound wave,which will be double the distance between wall and the sound source(because sound wave has to go to and fro between the wall and the listener)
giving time=0.1 s and speed=340 m/s
we get 2*distance between the wall and listener=0.1*340=34m
so disttance between wall and listener=34/2=17m
so if the wall(or reflecting surface)is less than 17 m we hear reverberation else we hear echo
Thank you