Radiation of sound should not be confused with propagation of sound. Radiation is the process by which energy is transferred from one form, i.e. structural vibration, turbulent flow into a propagating acoustic wave. Succinctly, radiation of sound is caused by the transfer of momentum into the fluid medium.
Radiation From Structures
Radiation of sound from structures occurs most efficiently when there is a net volume velocity of the structure surface into the fluid. The process of radiation can be described using radiation modes. Radiation modes are the assumed forms of the volume velocity which couple in independent ways the radiated acoustic field.