An ambulance siren emits a sound of frequency 1200 Hz . The speed of sound in air is $330 \mathrm{m} \mathrm{s}^{-1}$. The ambulance moves towards a stationary observer at a constant speed of $40 \mathrm{m} \mathrm{s}^{-1}$. What is the frequency heard by the observer?

An ambulance siren emits a sound of frequency 1200 Hz . The speed of sound in air is $330 \mathrm{m} \mathrm{s}^{-1}$. The ambulance moves towards a stationary observer at a constant speed of $40 \mathrm{m} \mathrm{s}^{-1}$. What is the frequency heard by the observer?