text: The air blown comes out of a small opening.  At the opening the pressure drops suddenly,\n\nThe air in the small cylindrical vessel below the opening rushes to the reduced pressure spot.\n\nThe surrounding air also rushes to the spot.\n\nBut the equilibrium of pressure is attained only after some vibrations. (Just like a pendulum comes to stop after some vibrations.\n\nBut all these happen for a single burst of air blow.\n\nSound is the compressions and rarefactions of air.\n\nWhen we continuously blow air, the air is made to vibrate with  different frequencies  which depends upon the volume of air made to vibrate.\n\nThus the whistle sound is heard.