doubles the PWL of the sound radiating to the east
The SPLs inside Engine Room No. 2 are approxi-
and a 3-dB increase is added at Item 7. Combining
mately those shown in figure 412. The PWL of the
all the factors, Item 13 of the analysis shows that
noise escaping through the unmuffled vent is calcu-
the vent will produce 2-dB excess indoor levels at
lated from equation 718 of the N&V manual. This
the housing in the 500-Hz band. When added to all
is given in Item 2 of figure 425, for the open vent
2
area of 40 ft. . A 3-ft.-long low-pressure-drop dissi-
other noise coming from the power plant, the total
pative muffler (data from table 310) is first
excess could be even larger. Thus, a better design
planned for the vent opening (Item 6 in fig. 4-25).
would be either a 5-ft.-long low-pressure-drop muf-
fler or a 3-ft.-long high-pressure-drop muffler or
The noise radiating from the open front of the muf-
fler has a small amount of directivity increase to-
some other acceptable combination available from a
ward the housing. If the opening could freely radi-
muffler supplier.
ate its sound in all horizontal directions, there
(c) Next, noise radiated from the exterior
would be no special directional effect, and normal
east wall of the building should be checked. Materi-
sound propagation would exist. However, the
al from paragraph 32a and equation 33 are in-
presence of the large-area east wall of the building
volved (LW = LP TL + 10 log A16). Figure 4-26
acts as a baffle that keeps one-half of the sound
summarizes the calculations of the PWL of the
from radiating to the west. Thus, the sound that
noise radiated externally by the east wall of En-
would have gone to the west (if the building were
gine Room No. 2.
not there), instead is reflected to the east. This
graph 32a, it should be determined that this cal-
Column 2 gives the SPL inside the Engine Room,
as taken from figure 412. Column 3 gives the TL
culated radiated PWL does not exceed the low-
frequency PWL of the sources inside the room.
of the exterior wall of the building, 10-in. -thick
This is done by comparing the Column 5 values
hollow-core concrete block, from N&V table 5-9.
with the sum of the engine casing PWLs of the
Column 4 represents the term (10 log A16), where
2
three engines in Engine Room No. 2 (from fig. 43
the area of the east wall is 30 x 40 = 1200 ft.
when the 40-ft. 2 area of the muffled vent opening is
and 44). This sum is shown in Column 6. It is clear
neglected). Column 5 is then the radiated PWL of
that the Column 5 values are less than the Column
equation 33 (Column 5 = Column 2 Column 3 +
6 values. The Column 5 PWL is next extrapolated
Column 4). In accordance with the caution of para-
to the base housing with the use of figure 427.
4-39
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