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AIR1672B - Practical Methods to Obtain Free-Field Sound Pressure Levels From Acoustical Measurements Over Ground Surfaces

Document Number: AIR1672B
Revision Number: B
Practical Methods to Obtain Free-Field Sound Pressure Levels From Acoustical Measurements Over Ground Surfaces
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 test site. Since the internationally stanardized methods of obtaining flight test noise levels suffer from the same ground reflection interference effects as measurements above the ground plane in the vicinity of a test stand, one appendix desribes alternative measurement procedures for the overflight case, together with simplified adjustments to obtains approxiamte free-field sound pressure levels. Each Appendix is dates, is subject to alter modifications when additional data become available, and provides an approach to a particular technique presented bya member of the A-21 Committee.
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