Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed $1.0\,m{s^{ - 1}}$ and the man behind walks at a speed $2.0\,m{s^{ - 1}}.$ A third man in standing at a height $12m$ above the same horizontal line such that all three men are in a vertical plane. The two walking men are blowing identical whistles which emit a sound of frequency $1430Hz$. The speed of sound in air is $330\,m{s^{ - 1}}.$ At the instant, when the moving men are $10m$ apart, the stationary man is equidistant from them. The frequency of beats in $Hz,$ heard by the stationary man at this instant, is $\underline{\hspace{2cm}}$.