Handbook of air conditioning and refrigeration / Shan K

19.32 CHAPTER NINETEEN
Structure-Borne Noise
REFERENCES
widely used for rooftop units. A rectangular return duct may be used if its duct wall is made of
16-gauge (1.6-mm thickness) galvanized sheet metal and breakout noise is not critical. Adopt a
duct velocity of 1500 fpm (7.5 m/s) or less. Use 2-in. (50-mm) thick inner lining in the return
side (including ductwork) of the rooftop unit. Use tees with a minimum leg length of 3 to 5 equivalent
diameters to increase end reflection loss.
● For ducted return air systems, the sound power level at the supply or return fan intake should be
attenuated by the return duct liners, return side silencer, and room effect. For a rooftop unit with a
collective return grille, a sound-attenuating plenum, as shown in Fig. 19.3, is often cost-effective
if space is available.
● For an open fan room (Fig. 16.7), return air is often transferred to the open fan room from the return
ceiling plenum through a transfer duct or a transfer wall passage. Both require an inner lining
2 in. (50 mm) thick with a perforated metal sheet facing. Air velocity in the return transfer duct or
passage should be 1000 fpm (5 m/s) or less.
Structure-borne noise can be controlled by using suitable vibration isolators. Special curb-mounting
bases should be used to provide better sound and vibration control. Sound structural support should
be provided for rooftop packaged units. Borzym (1991) says that the best place for the rooftop
packaged unit is directly over a column or straddling a major beam close to a column. Good locations
are those near the intersection of beam with a heavy end close to a column, straddling a beam,
or close to a beam. The rooftop packaged unit should be mounted on a spring-isolated roof curb and
should float on the springs. The roof below the rooftop packaged unit should not vibrate.
The roof directly under the rooftop packaged unit within the roof curb should contain a good
acoustic barrier to prevent sound transmission through the bottom panel of the packaged unit, the
roof, and the ceiling plenum. A typical sound barrier is made of semirigid fiberglass insulation, at
5 least 4 in. (100 mm) thick, covered by two layers of �8-in.
(16-mm) gypsum board. The staggered
gypsum boards are used to seal the joints and seams airtight.
ASHRAE, ASHRAE Handbook 1991, HVAC Applications, ASHRAE Inc., Atlanta, GA, 1991.
ASHRAE, ASHRAE Handbook 1995, HVAC Applications, Atlanta, 1995.
ASHRAE, ASHRAE Handbook 1999, HVAC Applications, Atlanta, 1999.
Beatty, J., Discharge Duct Configurations to Control Rooftop Sound, Heating/Piping/Air Conditioning, July
1987, pp. 53–58.
Blazier, W. E., Noise Rating of Variable-Air-Volume Terminal Devices, ASHRAE Transactions, 1981, Part I,
pp. 140–152.
Bodley, J. D., An Analysis of Acoustically Lined Duct and Fittings and the State of the Art of Their Use,
ASHRAE Transactions, 1981, Part I, pp. 658–671.
Borzym, J. X., Acoustical Design Guidelines for Location of Packaged Rooftop Air Conditioners, ASHRAE
Transactions, 1991, Part I, pp. 437–441.
Cummings, A., Acoustic Noise Transmission through Duct Walls, ASHRAE Transactions, 1985, Part II A,
pp. 48–61.
Ebbing, C., and Waeldner, W. J., Industry Standard 885: An Overview, Estimating Space Sound Levels for Air
Terminal Devices, ASHRAE Transactions, 1989, Part I, pp. 529–533.
Ebbing, C. E., Fragnito, D., and Inglis, S., Control of Low Frequency Duct-Generated Noise in Building Air
Distribution Systems, ASHRAE Transactions, 1978, Part II, pp. 191–203.
Gelin, L. J., Active Noise Control: A Tutorial for HVAC Designers, ASHRAE Journal, no. 8, 1997, pp. 43–49.