Resonance is a phenomenon in which when natural frequency of a body matches with the external frequency, the body starts vibrating with greater amplitude. Such resonance phenomenon is observed during earth quakes when natural frequency of building matches the frequency of the shock resulting in collapse of buildings. An engineer decides to bring down a wall using sound waves. He realizes that the wall has a resonance frequency of 600Hz. But he finds that the devices he has, can not generate that frequency. He further calculates and finds that if the source is attached to a car and if it travels with a speed of 30m/s towards the wall then it can generate the resonant frequency (velocity of sound in air is 346 m/s)

(i) What is the frequency emitted by the source?