Two cars $X$ and $Y$ are approaching each other with velocities $36\mathrm{~km~h}^{-1}$ and $72\mathrm{~km~h}^{-1}$ 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 $340\mathrm{~ms}^{-1}$, the actual frequency of the whistle sound produced is $\_\_\_\_$ Hz.