# When does a sound echo not disturb its listener

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1. Apr 19, 2017

### randomgamernerd

1. The problem statement, all variables and given/known data:

At a prayer meeting the disciples sing JAI RAM .The sound amplified by a loudspeaker comes back after reflection from a building at a distance of 80 m from the meeting. What maximum time interval can be kept between one JAI-RAM and the next JAI-RAM so that the echo does not disturb a listener sitting in the meeting. Speed of sound in air is 320 m s-1.

2. Relevant equations: s/t =v

3. The attempt at a solution:
I did a blind guess and worked out the problem as follows:
Time taken to go to the wall = Time taken to come back = 80/320 s
⇒ Time difference = 2 x 80/320 = 1/2s.
Answer matches with the one given in the text.
I just want to know significance of the line "echo does not disturb a listener"

2. Apr 19, 2017

### PeterO

With that time difference, the echo will be heard at exactly the same time as the next JAI-RAM, so the listener may well be unaware of the echo at all - and thus "the echo does not disturb the listener".

3. Apr 19, 2017

### randomgamernerd

oh, ok...thanks..

4. Apr 20, 2017

### PeterO

A similar method can be used to estimate the speed of sound.
If you were to clap while standing 80m from a wall, the echo would return in approximately 0.5 seconds.
You can improve the accuracy of your timing, by clapping in rhythm with the echo (which you will find hard to hear when you clap exactly in time with it, but it is easy to hear of the echo arrives a tiny bit early / late, so you acn adjust your rate of clapping until it is just right).
While clapping in time, you have someone else time, say, 20 claps - and they might get a time of 9.85 seconds - showing that it was a tiny bit less than 0.5 seconds for a single sound to travel there and back.
You are thus in a position to make quite a reasonable calculation of the speed of sound, on that day, at that air temperature and that air pressure.
It wouldn't matter, to the method, if the wall was actually 77.45m away - it just makes the arithmetic a little more complicated.

5. Apr 20, 2017

### randomgamernerd

whoa...thAnks for enlightening me.