Acoustic sources within the "jet pipe" also contribute to the noise, mainly at lower speeds, which include combustion noise, and sounds produced by interactions of a turbulent stream with fans, compressors, and turbine systems.
The mixing initially occurs in an annular shear layer, which grows with the length of the nozzle.
The high-frequency components of the sound are mainly stationed close to the nozzle, where the dimensions of the turbulence eddies are small.
In supersonic or choked jets there are cells through which the flow continuously expands and contracts.
Screech is produced by a feedback mechanism in which a disturbance convecting in the shear layer generates sound as it traverses the standing system of shock waves in the jet.