shall size sheetmetal ductwork on the drawings to
compensate for the lining thickness.
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Flexible or rigid mineral fiber lining conforming to ASTM C 1071. Lining
shall not be less than 25 mm one inch thick.
2.3.4.2
Net Noise Reduction Values
Conform with the following:
Minimum Net Noise Reduction Values,
Sound Pressure Level dB
(Reference Sound Power at 10-12 Watts)
Octave Pass Band
2
3
4
5
6
7
Center-Frequency (Hz)
125
250
500
1000
2000
4000
Noise Reduction (dB)
11
16
19
30
40
32
2.3.4.3
Preformed Duct Liner
Preformed round duct liner designed for insertion in round ducts may be
used in the sizes commercially available. Provide duct liner sections with
slip-lap joints not less than 50 mm 2 inches wide. Make joints in
accordance with manufacturer's printed instructions. Furnish
fire-resistant adhesive to field-coated joints when recommended by the
manufacturer to prevent delamination or erosion at joints. Tubular
sections of duct liner shall fit the metal duct snugly and without gaps
between duct-liner sections.
2.3.4.4
Factory-Fabricated Sound-Attenuator Ducts
Sound-attenuator ducts may be provided in lieu of sound attenuators. Comply
with requirements specified herein for sound attenuators. Provide
double-walled duct and fitting of an outer zinc-coated metal pressure shell
with 25 mm one inch thick acoustical blanket insulation and an internal
perforated zinc-coated metal liner. Install sufficient length of run to
obtain the noise reduction value specified. Furnish certification from
manufacturer that the sound reduction values specified will be obtained
within the length of duct run provided. The internal perforated
zinc-coated metal-liner shall be not less than 24 gage, unless ribbed, then
not less than 28 gage for the duct liner and not less than 26 gage for the
fitting liner with perforations not larger than 2.40 mm 3/32 inch diameter.
Seal joints as specified in paragraph entitled "Round and Oval Ducts."
[Rigid molded fiber-glass inserts with the air side surface [PVC]
[neoprene]-coated, if complying with all requirements specified herein, may
be used in lieu of internal perforated zinc-coated metal liner.]
2.3.4.5
Sound Attenuators (Traps)
Provide factory-fabricated attenuators constructed of galvanized steel
sheets. Outer casing shall be not less than 22 gage. Acoustical fills
shall be mineral fiber conforming to ASTM C 1071. Air flow capacities
shall be as indicated. Pressure drops through attenuators shall not exceed
values indicated, or shall be not in excess of 15 percent of the total
external static pressure of the air handling system, whichever is less.
Acoustically test sound attenuators with metal duct inlet and outlet
sections while under rated air flow conditions. Noise reduction data shall
include effects of flanking paths and vibration transmission. Attenuators
SECTION 15810N
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