Item 2 is the algebraic sum of Items 3 and 4. Thus,
The NC20 acceptable indoor sound levels are first
inserted in Items 11 and 12. If the criterion levels
Item 2 represents the total PWL of the power
are met, the Item 10 values will be the same as the
plant sound that would just produce an NC20 in-
Item 12 values, so they are repeated in Item 10.
door criterion at the hospital--after traveling 1600
Item 9 shows the noise reduction of outdoor noise
ft., passing through the wooded area, and entering
coming indoors through the wall, which most near-
the hospital through the type C wall structure.
ly resembles wall type C of the N&V table 6-7.
This is--called the PWL criterion. In a real-life situ-
The allowable outdoor noise levels (Item 8) are
ation, the outdoor SPLs at the hospital (Item 8 of
figure 432) probably would not be acceptable to
then the algebraic sum of Items 9 and 10. In travel-
ing to the hospital, the sound encounters the
residential neighbors. Further, the NC20 criteri-
on levels inside the hospital would not be achieved
wooded area evaluated figures 430 and 431. For
a conservative estimate (lower value) of the inser-
inside residences, at the same distances, that have
their windows open much of the time. Thus, the
tion loss of the woods, the winter insertion loss
from figure 4-31 is used in Item 5 of figure 4-32.
problem developed here is based only on the condi-
Item 4 of figure 432 is the algebraic sum of Items
tions as defined.
5 and 8. This "tentative outdoor SPL" would have
c. PWL of engine sources. The three principal
been the same as the Item 8 values if there had
sources of a gas turbine engine are calculated with
been no woods. Item 3 is the distance term (N&V
the use of DD Form 2305. The calculation is carried
table 64 for standard-day sound propagation con-
out for this 15-MW engine in figure 433.
ditions) for the 1600-ft. distance (Item 1). Finally,