The driver of a car travelling with a speed ' \(\mathrm{V}_1\) ' \(\mathrm{m} / \mathrm{s}\) towards a wall sounds a siren of frequency ' \(n\) ' Hz. If the velocity of sound in air is ' \(V\) ' \(\mathrm{m} / \mathrm{s}\), then the frequency of the sound reflected from the wall and as heard by the driver in Hz is

For the source moving towards the wall,
\(\therefore \quad \mathrm{n}_1=\mathrm{n}\left[\frac{\mathrm{~V}}{\mathrm{~V}-\mathrm{V}_1}\right]\)
For the reflected sound waves, the driver acts as an observer moving towards the wall
\(\begin{array}{ll}
\therefore & \mathrm{n}_2=\mathrm{n}_1\left[\frac{\mathrm{~V}+\mathrm{V}_1}{\mathrm{~V}}\right] \\
\therefore & \mathrm{n}_2=\left[\frac{\mathrm{V}+\mathrm{V}_1}{\mathrm{~V}}\right] \times\left[\frac{\mathrm{V}}{\mathrm{~V}-\mathrm{V}_1}\right] \times \mathrm{n} \\
\therefore & \mathrm{n}_2=\mathrm{n}\left[\frac{\mathrm{~V}+\mathrm{V}_1}{\mathrm{~V}-\mathrm{V}_1}\right]
\end{array}\)