Question

A source and an observer are moving towards each other with a speed equal to $v/2$, where $v$ is the speed of sound. The source is emitting sound of frequency $n$. The frequency heard by one observer will be

• A. zero
• B. $n$
• C. $\frac {n}{3}$
• D. $3n$

Answer 1

Answer: (D)

$3n$

Answer 2

When the source and ouserver are approaching towards each other then apparent frequency heard by the observer is $n'=\left(\frac{v+v_{0}}{v-v_{0}}\right) n$ $=\frac{\left(v+\frac{v}{2}\right)}{\left(v+\frac{v}{2}\right)} n$ $n'=\left(\frac{3}{2} \times \frac{2}{1}\right) n=3 n$