Two cars $$X$$ and $$Y$$ are approaching each other with velocities 36 km h$$^{-1}$$ and 72 km h$$^{-1}$$ respectively. The frequency of a whistle sound as emitted by a passenger in car $$X$$, heard by the passenger in car $$Y$$ is 1320 Hz. If the velocity of sound in air is 340 ms$$^{-1}$$, the actual frequency of the whistle sound produced is _________ Hz.