The conversion of free jet facility data into equivalent flyover results is discussed. The essential problem is to “calibrate out” the acoustic influence of the outer free jet shear layer on the measurements, since this is absent in the flight case. Results are presented which illustrate the differences between current simplified models (vortex sheet and geometric acoustics), and a more complete model based on the Lilley equation. Finally, the use of geometric acoustics for facility-to-flight data conversion is discussed.