Acquisition of free-field data is of practical significance in the field of aeronautical acoustics. The need for free-field data includes (but is not restricted to) the following: comparison of acoustical data obtained from the same engine under various measurement conditions; comparison of the results obtained from models with those from an engine on a test stand; comparison of noise measurements made on the same engine under static and in-flight conditions; design of test facilities; standardization of techniques for in situ acoustical measurements; spectral decomposition to isolate the contribution of different sources to the total noise; and prediction of aircraft noise on the basis of methods which, generally, provide free-field data. There is an increasing tendency to test full-scale engine components and scale models in anechoic test facilities that provide free-field conditions. This AIR complements this work by identifying those methods in current use which provide free-field acoustic data for measurements on engines under static conditions in the presence of a ground surface. Separate Appendices to this AIR describe different methods for noise measurement in the field that conform with the state-of-the-art to a certain extent. That is, each method has generally been systematically used by at least one organization and has been substantiated by data obtained from at least one test site.