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 task. The task is even more complicated because the directivity and shielding
effects for each particular source-path combination usually depends on
Due to the complexity of the problem, sufficiently accurate
prediction of the far-field noise is possible only if carried out on the basis
of appropriate scaling of measured noise data obtained during the field
checkout of completed test cells and hush-houses of similar construction,
whereby the scaling is aided by the results of systematic scale model studies
and by theoretical considerations.
12.2.8
Effect of Geometry Change on Noise. The acoustical data presented
in Sections 11 and 12, and in Acoustic Report on the 1/15-Scale Hot/Cold-Flow
Model Tests of Forcing Cone Augmenter Pickup for Hush-Houses and Test Cells
[17]; 1/15-Scale Model Testing of Dry-Cooled Jet Engine Noise Suppresors Using
Hot Jet Simulating the TF-30-P-412 Fan Jet Engine [18]; Noise Levels of NAS
Lemoore Cell #1 [20]; Letter Report on the Acoustical Performance Checkout of
the NAS Dallas Jet Engine Test Cell [24]; and Noise Levels from the Operation
of the J79-GE-80 Engine in the NAS Dallas, Texas, Air-Cooled Round Stepped
Augmenter Test Cell [25]; and References [1, 3, 9, 21, 22, and 23], and Noise
Levels of NARF, North Island Test Cell No. 20, R.E. Glass [19] can serve as a
base for predicting exterior and interior noise of new facilities that have
different geometry and utilize different engines than previously used. Based
on the experiences that small changes in geometry or operating parameters
sometimes can result in substantial changes in noise, scaling of data is not a
simple matter.
12.3
External Noise of Full-Scale Test Facilities. The external noise of
hush-house and jet engine test cells of the U. S. Navy is evaluated at seven
standard microphone positions equally spaced (i.e., 30 deg. apart) on a
250-ft (76.2-m) radius half-circle (experience shows that the polar plot is
practically symmetrical around the axis of the facilities. Consequently, a
360 deg. coverage is not necessarily centered at the engine exhaust. The
first far-field microphone position (0 deg.) is in the front and seventh
(180 deg.) behind the exhaust stack.
The A-weighted sound pressure level at these standard 250-ft
positions is compiled in Table 6. This table includes far field noise data
obtained for four hush-houses and three test cells. It contains 231 data
points obtained for the A-4, A-6, F-4, F-14, F-18, and S-3 naval aircraft and
for the J79-GE-8D, F-404, TF41-A2B, J57-P10, and TF30-P408 engines operating
in military and maximum afterburner setting.
Figure 30 shows the 1/3-octave band spectrum of the far-field noise
obtained at the Miramar No. 2 hush-house at front (0 deg.) and aft (180 deg.)
location at 250 ft when the port engine of the F-4 aircraft was operating at
max A/B. References [1, 9], and [20 to 25], and Noise Levels of the NARF
Alameda Test Cell No. 15 [26], contain 1/3-octave band spectra obtained at
all far-field positions for the test facilities for which A-weighted levels
are listed in Table 6.
12.4
External Noise Studies Utilizing Scale Models. Most of the model
studies undertaken dealt with the split of sound power between the enclosure
and the augmenter entrance and with the sound-power-based attenuation of
various augmenter configurations [3, 17].
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