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National Advisory Committee for Aeronautics, Technical Notes - Experimental Studies of Noise from Subsonic Jets in Still Air

Experimental studies, which were conducted on the ground in still
air, were made to evaluate some of the effects of.parameters such as
Jet velocity, density, and turbulence level, as well as Jet size, on
the noise generated by subsonic Jets. Most of the tests were conducted
with simple model Jets so that flow conditions could be closely cOn-
trolled, and the results are compared with data obtained with a turboJet
engine. The noise intensity was found to increase considerably with
increases in exit velocity and turbulence level and by a lesser amount
with increases in Jet size and exit gas density, with the highest levels
being generally observed downstream of the orifice and near the Jet
boundary. The‘Jet-noise spectrum was found to be a function of Jet size
and observer's position; the spectrums having a relatively large low—
frequency content are associated with the larger Jet sizes and'locations
close to the Jet axis.

The noise generated by a turboJet engine is shown to be closely
related to that generated by simple model Jets and an empirical rela—
tion is given to allow the extrapolation of available Jet—noise data to
other Operating conditions.

The Jet—noise problem has recently attracted much attention because
of the increased use of Jet engines for aircraft propulsion. The problem
is of special concern in areas where people are frequently exposed to the
noise either during static engine testing or low-altitude flight, as in
take-off or landing. Though many studies of the problem have been made
by various agencies and aircraft companies, the resulting information is' '
difficult to correlate because of the use of many types of instrumentation
which differ in regard to sensitivity, frequency response, and the manner
in which data are recorded. The corresponding test conditions for experi-
mental studies may also vary widely and, in the event that only a few data
points are recorded, the directional properties_of the noise source may
not be adequately accounted for. Hence, in many cases the information
obtained is of a specific nature and has only limited use in estimating
the noise from other engines or for other operating conditions of a
given engine.

In general, Studies of Jet noise, of which references 1, 2, and 3
are typical, have shown that continuous—type Jet engines are prolific
generators of a random—noise spectrum which_includes essentially all
frequencies from the subaudible to the ultrasonic. The intensities in
some parts of the spectrum are of such magnitude as to produce adverse
physiological effects on man. The noise field is directibnal, with the
bulk of the Sound energy radiated to the rear of the engine.