ENCLOSURE INTERIOR NOISE
Introduction. This section deals with the interior noise of hush-
houses and jet engine test cells. The data reported were obtained either by
the performance evaluation of completed full-scale facilities or by model-
scale experimental studies. Many key acoustical results of checkout
measurements and model studies are included. The structure of aircraft during
ground runup in hush-houses or that of engines during out-of-airframe tests in
a jet engine test cell may experience sound and sound-induced vibration that
differs from that obtained when the test is run outdoors.
certain parts of aircraft are frequently exposed to substantially
higher noise levels than those encountered during ground runup
outdoors. This occurs when aircraft are taking off pairwise on
the same runway and when they are parked on the deck of an
aircraft carrier during the takeoff of other aircraft.
Enclosure Interior Noise Sources. The sources of enclosure interior
noise are the engine intake and the engine exhaust. While all the engine
intake noise enters the enclosure, only a part of the engine exhaust noise
"spills" into the enclosure. The larger the distance between the engine
exhaust plane from the augmenter entrance, X+N,, and the smaller the
equivalent diameter of the augmenter, D+A,, the larger portion of the engine
exhaust noise reaches the enclosure. The sound field inside of the enclosure
is made up from the direct sound radiated from the engine and from the
reflections of the direct sound from the enclosure interior surfaces.
The enclosure interior noise is of concern because of:
a) Sound induced vibrations of the aircraft, engine components and
the structure of the enclosure
Its potential impact on the hearing of operating personnel
to the outside and through the viewing window to the control room.
The interior noise data obtained in full-scale test facilities are
compiled in Table 7. The objectives and key results of model studies are
presented in Tables 8A through 8C.
Enclosure Interior Noise in Full-Scale Test Facilities. The A-
weighted interior noise level obtained at standard interior microphone
positions is presented in the right columm in Table 7. The location of the
standard interior microphone positions for the different facilities is shown
in Table 9.
Typical Interior Noise Level Spectra. Figure 25 shows the
1/3-octave band spectrum of the interior noise measured in the Miramar No. 2
hush-house at Standard Interior Microphone Position No. 3 obtained while the
port engine of the F-4 and F-14A aircraft was operating at maximum
afterburner. Although the F-4 aircraft has an engine of lower sound power
output than that of the F-14A aircraft, it produces substantially higher