The increase in jet engine thrust has been accompanied by an increase in the noise generated by the jet stream to such an extent that the associated noise pressures are now capable of exciting vibrations in an aircraft structure which are potentially dangerous from the fatigue aspect. Several examples of fatigue damage of this kind have appeared already on aircraft in this country and in the United States.

The noise generated by a gaseous jet arises principally from disturbances caused by high velocity gradients at the jet boundary. Classical theoretical work on noise generated in this way has been done by Professor Lighthill. He introduced the concept of an “acoustic quadropole” as the elementary sound generator, and showed mathematically that the total acoustical power radiated by a jet varies directly as the eighth power of the jet exit velocity and the second power of the jet exit diameter.