Application to jet noise calculation
Digital aeroacoustics: sound sources and their radiation

As mentioned at the start of paragraph 2, broadband acoustic radiation from a flow in a quiescent medium was a thorny problem in the 1950s. Indeed, the fast jet of the first (single-flow) turbojet engines, which were beginning to supplant propeller propulsion in transport aircraft, generated extremely high noise levels. The noise emitted could not be associated with flow fluctuations (monopoles) or forces on solid surfaces (dipoles). To Lighthill's credit, he showed that it was due to quadrupole terms associated with jet turbulence ...