noise reduction of 10 dB (from N&V table 64). As
The N&V table 34 or figure 34 provides a means
one alternative, the base housing can be designed
of determining the correction number for the back-
and constructed to have higher TL walls and closed
ground noise in the area. If background noise
windows facing the power plant. This would reduce
measurements can be made at the existing base,
indoor SPLs but would not change the outdoor
N&V figure 34 should be used; otherwise the
SPLs. If possible, other large buildings on the base
background noise correction may be estimated by
could be used to shield the housing area from the
selecting the most nearly applicable conditions of
power plant. Two feasible alternatives could be ap-
N&V table 34. For this sample problem, a back-
plied at the power plant. In one, special large-
ground noise correction of +1 is used. N&V table
volume, low-pressure-drop mufflers could be used,
35 is then used to determine other correction
either singly or in series, in the exhaust lines from
numbers applicable to the problem. The following
the engines to provide greater insertion loss than is
corrections are here assumed:
quoted in table 32 for the rather conventional
Correction for temporal or seasonal factors
o
grades of mufflers. Such mufflers have been used
Day and night
o
successfully with large engines located as close as
Summer and winter
o
600 to 800 ft. from residential areas. As another al-
"On" full time
ternative, an outdoor L-shaped barrier wall ex-
Correction for character of noise
o
tending above the top of the exhaust pipe openings
No unusual sounds
for the engines in Engine Room No. 1 could be
Correction for previous exposure
built above the second-floor Mechanical Equipment
Some previous exposure and good
o
Room and the south wall of the Engine Room to
community relations
give a beneficial amount of noise reduction for the
Background noise correction
exhaust of the three 3500-hp engines. The exhaust
+1
From discussion above
mufflers for the two 1600-hp engines could be
specified and purchased to have a larger amount of
+1
Total corrections
insertion loss than assumed in the figure 44 analy-
The CNR (composite noise rating) is then e + 1 =
sis. The 900-hp engine is the quietest one of the en-
F. The N&V figure 35 is used to estimate the ex-
tire group and may or may not need additional
pected community response, where base personnel
muffling, depending on the success of the other
are assumed to be the equivalent of "average resi-
pursuits.
(10) Other engine noise to on-base housing.
dents. " A CNR value of F indicates a strong reac-
(a) Turbocharger inlet noise for the three
tion against the noise for the conditions assumed
here. A noise reduction of about 10 dB would bring
outdoor inlets of the 3500-hp engines should be
the reaction down to "sporadic complaints, " which
checked for meeting the desired indoor and outdoor
might be considered a reasonable condition. CNR
levels of the base housing. The PWLs of the un-
values of C or D are often encountered in nonmili-
muffled inlet of one such engine is given in Item 16
tary situations.
of figure 42. These levels should be increased by 5
(d) On the basis of both indoor and outdoor
dB (for three engines), then extrapolated to the
power plant noise at the base housing, the above
1200-ft. distance. The inlet openings are partially
analyses strongly suggest the need for a 5- to
shielded by the power plant building, and the bar-
10-dB reduction of noise, with principal emphasis
rier effect of the building can be estimated. Ab-
on noise control in the 125- to 1000-HZ frequency
sorbent duct lining in the air inlet ducts or dissipa-
range.
tive mufflers at the intake to the air cleaners can
(e) Several possibilities exist for reduction
be very effective at reducing the high-frequency
of the excess noise. If the base has a large land
tonal sounds of the turbochargers.
(b) Sound from Engine Room No. 2 can es-
area and is not yet constructed, the power plant
and the housing area can be moved farther apart.
cape from the open vent on the east wall of this
An increase in distance from 1200 ft. to 2000 ft.
room and travel directly to the housing area. Fig-
would give a 250-Hz noise reduction of 5 dB, and an
ure 423 shows the principal steps in the analysis
increase in distance to 3000 ft. would give a 250-Hz
of this part of the problem.
4-37
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