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【范围】
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2. INTRODUCTION: Acqui sition 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: (1) comparison of acoustical data
obtained from the same engi ne under various measurement conditions, (2) com-
pari son of the results obtained from node!s with those from an engine on a
test stand, (3) comparison of noise measurements made on the same engine
under static and in-flight conditions, (4) design of test facilities, (5)
standardization of techniques for "in situ" acoustical measurements, (6)
spectral decomposition to isolate the contribution of different sources to
the total noise, and (7) prediction of aircraft noi se on the basis of methods
which, generally, provide free-field data.
There is an increasing tendency to test full-scale engi ne 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 thi s AIR describe different methods for noi se
measurement in the field that conform with the state-of-the-art to a certain
extent. That is, each method has generally been systematical ly used hy at
least one organization and has been substantiated by data obtained from at
least one test site.
Si nee the international ly standardized 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 describes alternative measurement procedures for the overflight
case, together with simplified adjustments to obtain approximate free-field
sound pressure level s.
Each Appendix is dated, is subject to later modifications when additional
data become available, and provides an approach to a particular technique
presented by a member of the A-21 Committee.
The following section discusses certain practical aspects of ground
reflection effects.strRefField
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