Packaged commercial duct mufflers (table 3-10)
(a) In Item 10, Selection A of figure4-16, it
is found that the planned 30% area of l/2-in. con-
adapted to the input of the Roots Blowers would
ventional plate glass and 70 percent wall area of
reduce noise levels in Engine Room No. 2 and
would benefit all nearby work spaces. Possibly the
10-in. hollow-core concrete block will fail to meet
engine manufacturer or a muffler manufacturer al-
the 55-dB SIL requirement by 9, 6, and 2 dB in the
ready has a retrofit attachment for reducing the
500-, 1000-, and 2000-Hz octave bands. The TLC of
blower noise. It is cautioned that the intake muf-
this combination is calculated in accordance with
fler must have a large enough open area to allow
paragraph 54e of the N&V manual, using N&V ta-
free flow of adequate air to the engine. The analy-
bles 5-9 and 514 for the TL of the concrete block
sis is not reworked here to accommodate this modi-
and glass portions of the wall. In Selection B, the
fication, but this situation illustrates that noise
glass area is reduced to 20 percent of the total wall
control can come in different forms. The remainder
are, and a double glass window is assumed (two
of the analysis is carried out without the benefit of
sheets of l/4-in. glass with a 6-in. air space; N&V
the Roots Blower muffler, but such a muffler would
table 515). This represents an improvement but is
reduce several building design problems.
still weak in the 500-Hz band. Selection C shows
that a special laminated safety plate glass (footnote
(7) Noise levels to the offices.
4 in N&V table 514) containing a viscoelastic
(a) The SPLs in Engine Room No. 2 are giv-
damping layer between the glass sheets will do as
well as the double glass window. Although the spe-
en at the bottom of figure 412 for the region be-
cial glass is more expensive, it will probably be less
side the office corridor wall. The noise criterion for
expensive than the special mounting required for
each of the offices on the other side of the corridor
the double glass window. Thus, Selection C is
is NC40, with the doors closed. The partitions be-
favored.
tween adjoining offices and the partitions between
(b) Figure 4-17 carries out the same type of
the corridor and the offices are made of standard
analysis for noise from Engine Room No. 2. Here,
gypsurm. board and stud construction. The acoustic
however, use of the 20 percent area window made
tile ceilmgs of the offices and the corridor have an
of the special laminated and damped safety glass
NRC value in the range of 0.65 to 0.75. The Room
fails to achieve the 55-dB SIL by 8, 4, and 3 dB in
Constants for the corridor and for a typical office
the three speech frequency bands. This is a serious
are estimated in figures 418 and 419. For this
deficiency, and it suggests that bold measures
particular geometry, the lobby-like space to the
must be considered. Selection B is made up of a 20
left of the corridor is included in the corridor since
percent area special double glass window of the
it will influence the sound levels entering the left
damped laminated glass (1/2-in. glass, 2-in. air
wall of the left office. If this were a very critical
space, 3/8-in. glass) set in a wall of 10-in. solid con-
problem, the Room Constant of the corridor alone
crete block. Even this wall arrangement still has a
would be calculated and used with the sound trans-
3-dB deficiency at 500 Hz, but it would be recom-
mission path from the Engine Room to the corridor
mended as a slightly marginal solution.
and then to the office; and the Room Constant of
(c) A more beneficial approach is to go back
the lobby space alone would be calculated and used
to figures 44 and 412 and observe that the Roots
with the sound transmission from the Engine Room
Blowers on the 1600-hp engines are the major
to the lobby space and then to the left-side office
causes of the 500-Hz and 1000-HZ sound levels.
through its left-side partition.
4-25
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