siren, $f=800 \mathrm{Hz}$

Speed of observer, $u=2 \mathrm{m} / \mathrm{s}$

Velocity of sound, $v=320 \mathrm{m} / \mathrm{s}$

No. of beats heard per second $=?$

No. of extra waves received by the observer

per second $=\pm 4 \lambda$

$\therefore \quad$ $Na$ of $beats$ $/$ $sec$

${=\frac{2}{\lambda}-\left(-\frac{2}{\lambda}\right)=\frac{4}{\lambda}}$

${=\frac{2 \times 2}{\frac{320}{800}}\left(\because \lambda=\frac{V}{f}\right)}$

$ = \frac{{2 \times 2 \times 800}}{{320}} = 10$