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