Editing Jet engine (section)

===Noise===
The noise emitted by a jet engine has many sources. These include, in the case of gas turbine engines, the fan, compressor, combustor, turbine and propelling jet/s.<ref>"Softly, softly towards the quiet jet" Michael J. T. Smith New Scientist 19 February 1970 p. 350</ref>

The propelling jet produces jet noise which is caused by the violent mixing action of the high speed jet with the surrounding air. In the subsonic case the noise is produced by eddies and in the supersonic case by [[Mach wave]]s.<ref>"Silencing the sources of jet noise" Dr David Crighton New Scientist 27 July 1972 p. 185</ref> The sound power radiated from a jet varies with the jet velocity raised to the eighth power for velocities up to {{cvt|2000|ft/s|-2|disp=flip}} and varies with the velocity cubed above {{cvt|2000|ft/s|-2|disp=flip}}.<ref>"Noise" I.C. Cheeseman Flight International 16 April 1970 p. 639</ref> Thus, the lower speed exhaust jets emitted from engines such as high bypass turbofans are the quietest, whereas the fastest jets, such as rockets, turbojets, and ramjets, are the loudest. For commercial jet aircraft the jet noise has reduced from the turbojet through bypass engines to turbofans as a result of a progressive reduction in propelling jet velocities. For example, the JT8D, a bypass engine, has a jet velocity of {{cvt|1450|ft/s|-2|disp=flip}} whereas the JT9D, a turbofan, has jet velocities of {{cvt|885|ft/s|-2|disp=flip}} (cold) and {{cvt|1190|ft/s|-2|disp=flip}}(hot).<ref>"The Aircraft Gas Turbine Engine and its operation" United Technologies Pratt & Whitney Part No. P&W 182408 December 1982 Sea level static internal pressures and temperatures pp. 219–220</ref>

The advent of the turbofan replaced the very distinctive jet noise with another sound known as "buzz saw" noise. The origin is the shockwaves originating at the supersonic fan blade tip at takeoff thrust.<ref>'Quietening a Quiet Engine – The RB211 Demonstrator Programme" M.J.T. Smith SAE paper 760897 "Intake Noise Suppression" p. 5</ref>