 investig nois produc subson air jet investig nois produc subson air jet investig theoret predict lighthil aerodynam sound measur sound field 1 air jet issu long pipe measur wide frequenc band 30 10 000 cycl on-third octav band frequenc rang mach number pipe orific vari 0 3 1 0 depend appar posit nois sourc frequenc jet speed investig frequenc sourc situat farther jet orific higher jet speed lower frequenc sourc appear farther downstream higher frequenc consist associ larger eddi direct characterist sound field frequenc jet speed illustr scale diagram show line constant sound intens sound field analyz term move quadrupol sourc lighthil's theori good agreement appar spread sourc low frequenc doppler low frequenc rel frequenc maximum power output radiat three mutual orthogon longitudin quadrupol convect sound field monopol sourc higher frequenc sound field later longitudin quadrupol predomin 