Full-Scale Investigation of Several Jet-Engine Noise-Reduction Nozzles Page: 2 of 46
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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
NASA TechnTcalLb
TECHNICAL NOTE 3974ASA Tecncl
3 1176 01425 9080
FULL-SCALE INVESTIGATION OF SEVERAL JET-ENGINE
NOISE-REDUCTION NOZZLES
By Willard D. Coles and Edmund E. Callaghan
SUMMARY
A number of noise-suppression nozzles were tested on full-scale
engines. In general, these nozzles achieved noise reduction by the mix-
ing interference of adjacent jets, that is, by using multiple-slot noz-
zles. Several of the nozzles achieved reductions in sound power of
about 5 decibels (nearly 70 percent) with small thrust losses (approx.
1 percent).
The maximum sound-pressure level was reduced by as much as 18
decibels in particular frequency bands. Some of the nozzles showed con-
siderable spatial asymmetry, that is, the sound field was not rotational-
ly symmetrical.
A method of calculating the limiting frequency affected by such
nozzles is presented. Furthermore, data are shown which appear to in-
dicate that further reductions in sound power will not be easily achieved
from nozzles using mixing interference as a means of noise suppression.
INTRODUCTION
The normal development of the jet engine has produced sizable in-
creases in thrust and, also, unfortunately, jet-engine noise. In fact,
current jet engines are truly awesome noise producers. There are several
approaches to reducing the jet-engine noise heard by the observer, that
is, the public. The takeoff and climb-out pattern of the aircraft (ref.
1) can be adjusted to cause the least annoyance, or the engine itself
can be made quieter.
It is well established that the principal source of jet-engine noise
arises from the turbulent mixing of the jet with the surrounding atmos-
phere (ref. 2). The noise generated by this process is a function of the
product of the eighth power of the jet velocity and the jet area (refs.
2 and 3). Consequently, reductions in jet velocity will greatly reduce
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Coles, Willard D. & Callaghan, Edmund E. Full-Scale Investigation of Several Jet-Engine Noise-Reduction Nozzles, report, April 1957; (https://digital.library.unt.edu/ark:/67531/metadc56345/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.