A person carrying a whistle emitting continuously a note of $272 Hz$ is running towards a reflecting surface with a speed of $18\, km/hour. $ The speed of sound in air is $345m{s^{ - 1}}$. The number of beats heard by him is
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(c) According the concept of sound image
$n' = \frac{{v + {v_{{\rm{person}}}}}}{{v - {v_{{\rm{person}}}}}}.272 = \frac{{345 + 5}}{{345 - 5}} \times 272 = 280\,Hz$
$\Delta n = $ Number of beats $ =280 -272 = 8 Hz$
$n' = \frac{{v + {v_{{\rm{person}}}}}}{{v - {v_{{\rm{person}}}}}}.272 = \frac{{345 + 5}}{{345 - 5}} \times 272 = 280\,Hz$
$\Delta n = $ Number of beats $ =280 -272 = 8 Hz$
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