A train S1, moving with a uniform velocity of 108km / h, approaches...

Speed of sound= $300\dfrac{m}{s}$
Calculate beat ${f}_{req}$

${f}_{b} = \left|{f}_{1} - {f}_{2}\right|$
Where, ${f}_{1}$ and ${f}_{2}$ are the apparent frequencies of train ${S}_{1}$ and ${S}_{2}$ observed by observer.
$∴$ Using Doppler's effect,
$⇒ {f}_{b} = 120\left[\left(\dfrac{330 + 10\cos 53 °}{330 - 30\cos 37 °}\right) - \left(\dfrac{330 + 10}{330}\right)\right] = 120\left[\dfrac{336}{306} - \dfrac{34}{33}\right] = 8.128Hz$