Echo
Category : JEE Main & Advanced
An echo is simply the repetition of speaker's own voice caused by reflection at a distance surface e.g. a cliff. a row of building or any other extended surface. If there is a sound reflector at a distance d from source, then the time interval between original source and it's echo at the site of source will be
\[t=\frac{d}{v}+\frac{d}{v}=\frac{2d}{v}\]
As the persistence of hearing for human ear is 0.1 sec, therefore in order that an echo of short sound (e.g. clap or gun fire) will be heard if \[t>0.1\Rightarrow \frac{2d}{\upsilon }>0.1\Rightarrow d>\frac{\upsilon }{20}\]
If \[\upsilon =\] Speed of sound \[=340\,\,m/s\] then \[d>17\,m\].
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