Before discussing the noise associated with supersonic flight, it is pertinent to restate the sources of noise at subsonic speeds.As Lilley has pointed out, noise at subsonic speeds is due to turbulence, such as occurs in jets, wakes, boundary layers, and regions of separation, and to vortices and regions of shear flow. In all these types of flow viscosity plays a predominant part, and therefore at subsonic speeds the noise of an aircraft depends very much upon the viscosity of the fluid: in an inviscid fluid, an aircraft would make no noise, because, at subsonic speeds, the pressure changes induced by its motion would be too gradual to be audible.
