interfering structures or large amounts of vegeta-
a room of normal geometry in a fairly predictable
tion or woods that break the line of sight between a
manner, depending on room dimensions, distance
source and a receiver, normal outdoor sound prop-
from the source, and the amount and effectiveness
agation is fairly accurately predictable for long-
of sound absorption material in the room.
time averages. Variations can occur with wind and
a. Sound transmission through walls, floors,
large changes in thermal structure and with ex-
and ceilings. Sound energy is also transmitted by
tremes in air temperature and humidity. Even
the bounding walls and surfaces of the "source
these variations are calculable, but the long-time
room" to adjoining spaces (the "receiving rooms").
average conditions are the ones that determine the
The transmission loss of the walls and surfaces de-
typical sound levels received in a community,
termines the amount of escaping sound to these ad-
which in turn lead to judgments by the community
joining rooms. Chapter 5 of the N&V manual gives
on the relative acceptability or annoyance of that
details for calculating the indoor distribution of
noise. Large solid structures or heavy growths of
sound from the sound source (expressed either as
vegetation or woods that project well beyond the
PWL or SPL) into the room containing the source,
line of sight between the source and receiver area
and then to any adjoining room above, below, or
reduce the sound levels at the receiver positions.
beside the source room. Figures, tables, equations,
Chapter 6 of the N&V manual gives detailed infor-
and data forms in chapter 5 of the N&V manual
mation on all the significant factors that influence
provide the quantitative data and steps for eval-
outdoor sound propagation, and it is possible to cal-
uating indoor sound. The resulting sound level esti-
culate quite reliably the expected outdoor sound
mates are then compared with sound criteria se-
levels at any distance from a source for a wide
lected for the spaces to determine if the design
range of conditions that include distance, atmos-
goals will be met or if more or less acoustic treat-
pheric effects, terrain and vegetation effects, and
ment is warranted. Power plant equipment is tra-
solid barriers (such as hills, earth berms, walls,
ditionally noisy, and massive walls, floors, and ceil-
buildings, etc. ) Directivity of the source may also
ings are required to confine the noise.
be a factor that influences sound radiation; for ex-
b. Doors, windows, openings. Doors,
windows,
ample, chapter 7 data in the N&V manual and par-
and other openings must be considered so that they
agraph 28c in this manual indicate special direc-
do not permit excessive escape of noise. Paragraph
tivity effects of large intake and exhaust stacks of
54e of the N&V manual shows how to calculate
gas turbine engines. The calculated or measured
the effect of doors and windows on the transmis-
sound levels in a community location can then be
sion loss of a wall.
analyzed by the CNR (composite noise rating)
method of chapter 3 of the N&V manual to deter-
c. Control rooms. Control rooms or personnel
mine how the noise would be judged by the resi-
booths in the machinery rooms should be provided
dents and to decide if special noise control treat-
to ensure that work spaces and observation areas
ments should be applied. Some examples of outdoor
for personnel responsible for equipment operation
sound calculations are given in chapter 6 of the
are not noise-hazardous.
N&V manual.
d. Buffer zones. Building designs should incor-
porate buffer zones between the noisy equipment
27. Reciprocating engine noise data.
rooms and any nearby quiet work or rest areas (see
table 32 of N&V manual for the category 1 to 3
a. Data collection. Noise data have been collect-
areas that require very quiet acoustic background
ed and studied for more than 50 reciprocating die-
levels). Otherwise, massive and expensive con-
sel or natural-gas engines covering a power range
struction is required to provide adequate noise iso-
of 160 to 7200 hp (115 to 5150 kW). The speed
lation between adjoining noisy and quiet spaces.
range covered was 225 to 2600 rpm; the larger en-
gines run slower and the smaller engines run fast-
2-6. Outdoor sound propagation.
er. Cylinder configurations included in-line,
V-type, and radial, and the number of cylinders
An outdoor unenclosed diesel engine with a typical
ranged from 6 to 20. The engines were about equal-
ly divided between 2-cycle and 4-cycle operation;
ment can be heard at a distance of about 1 mile in a
about 20% of the engines were fueled by natural
quiet rural or suburban area under good sound
gas, while the remainder were diesel; many of the
propagation conditions. At closer distances, it can
smaller engines had naturally aspirated inlets but
be disturbing to neighbors. An inadequately muf-
most of the engines had turbocharged inlets. The
fled intake or discharge opening of a gas turbine
---
largest engines had cylinders with 15- to 21-in.
engine can also result in disturbing sound levels to
bores and 20- to 31-in. strokes. Fourteen different
neighbors at large distances. When there are no
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