One investigation  also dealt with the direct comparison of the
sound pressure level at the scaled far-field microphone positions obtained for
the bare model jet and those obtained at the same positions for the model
exhaust system, respectively.
For Figure 31, the results of a model-scale investigation show how
the axial distance of the jet exhaust from the augmenter entrance, .X-N,
influences the sound power that enters the augmenter. The larger the axial
distance, the smaller is the sound power that enters the augmenter at mid and
high frequencies. At low frequencies, where the noise source is within the
augmenter, the axial distance has little influence on the sound jet power that
enters the augmenter.
In Figure 32, the results of a model-scale investigation show how
the particular position of a 12-in. (304.56 mm) long (15 ft (4.57 m) at full-
scale) lined augmenter segment with a 60-in. (1523 mm) (75 ft (23 m) at
full-scale) hard-walled augmenter influences the power-based insertion loss.
References [3, 17, and 18] contain results of scale-model acoustical
studies for a variety of model-scale engines, exhaust system configurations,
and specific acoustical treatments.