Two men are walking along a horizontal straight line in the same...

Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed $1.0m{s}^{- 1}$ and the man behind walks at a speed $2.0m{s}^{- 1}$ . A third man is 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 $330m{s}^{- 1} .$ 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 __________.