At what speed should a source of sound move so that stationary observer finds the apparent frequency equal to half of the original frequency
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(d) Frequency is decreasing (becomes half), it means source is going away from the observes. In this case frequency observed by the observer is
$n' = n\left( {\frac{v}{{v + {v_s}}}} \right)$ ==> $\frac{n}{2} = n\,\left( {\frac{v}{{v + {v_S}}}} \right)$ ==>${v_S} = v$
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