be provided elsewhere as indicated. The sound attenuators shall be
factory fabricated and shall be tested by an independent laboratory for
sound and performance characteristics. Net sound reduction shall be as
indicated. Maximum permissible pressure drop shall not exceed 157 Pa
0.63 inch water gauge. Construct traps to be airtight when operating
under an internal static pressure of 2.5 kPa 10 inch water gauge.
Air-side surface shall be capable of withstanding air velocity of 50
m/s 10,000 fpm. The Contractor shall certify that the sound reduction
values specified will be obtained after the equipment is installed in
the system and coordinated with the sound information of the system fan
to be provided. Sound absorbing material shall conform to ASTM C 1071,
Type I or II. Sound absorbing material shall meet the fire hazard
rating requirements for insulation specified in Section 15080 THERMAL
INSULATION FOR MECHANICAL SYSTEMS. For connection to ductwork, provide
a duct transition section. Factory fabricated double-walled internally
insulated spiral lock seam and round duct and fittings designed for
high pressure air system may be provided in lieu of factory fabricated
sound attenuators, and shall comply with requirements specified for
factory fabricated sound attenuators. Construct the double-walled duct
and fittings from an outer metal pressure shell of zinc-coated steel
sheet, 25 mm 1 inch thick acoustical blanket insulation, and an
internal perforated zinc-coated metal liner. Provide a sufficient
length of run to obtain the noise reduction coefficient specified. The
Contractor shall certify that the sound reduction value specified will
be obtained within the length of duct run provided. The outer sheet
metal of the double-walled duct shall have welded, or spiral lock,
seams to prevent water vapor penetration. The outer sheet of the duct
and fittings shall conform to the metal thickness of high-pressure
spiral and round ducts and fittings shown in SMACNA HVAC Duct Const Stds.
The acoustical insulation shall have a thermal conductivity "k" of not
more than 0.0389 W/m-K 0.27 Btu/inch/square foot/hour/degree F at 24
degrees C 75 degrees F mean temperature. The internal perforated
zinc-coated metal liner shall be not less than 0.7 mm 24 gauge with
perforations not larger than 6.35 mm 1/4 inch in diameter providing a
net open area not less than 10 percent of the surface.
b. For system with total pressure of 1000 Pa 4 Inch Water Gauge and
Lower: Use sound attenuators only where indicated. Factory fabricated
sound attenuators shall be constructed of galvanized steel sheets.
Outer casing shall be not less than 0.85 mm 22 gauge. Acoustical fill
shall be fibrous glass. Net sound reduction shall be as indicated.
Values shall be obtained on a test unit not less than 600 by 600 mm 24
by 24 inches outside dimensions made by a certified nationally
recognized independent acoustical laboratory. Air flow capacity shall
be as indicated or required. Pressure drop through the attenuator
shall not exceed the value indicated, or shall not be in excess of 15
percent of the total external static pressure of the air handling
system, whichever is less. Sound attenuators shall be acoustically
tested with metal duct inlet and outlet sections while under the rated
air flow conditions. Noise reduction data shall include the effects of
flanking paths and vibration transmission. Construct sound attenuators
to be airtight when operating at the internal static pressure indicated
or specified for the duct system, but in no case less than 500 Pa 2
inch water gauge.
c. For acoustical duct liner: Use fibrous glass designed exclusively
for lining ductwork and conforming to the requirements of ASTM C 1071,
Type I and II. Liner composition may be uniform density, graduated
density, or dual density, as standard with the manufacturer. Lining
SECTION 15895
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