Product data

50
Acoustical design
Sound power levels represent the sound as it is produced
by the source, the WSHP unit, with no regard to attenua-
tion between the source and the space. Acoustical design
goals are necessary to provide criteria for occupied spaces
where people can be comfortable and communicate effec-
tively over the background noise of the air-conditioning
system and other background noise sources.
Acoustical design goals are desirable sound pressure lev-
els within a given conditioned space and are represented
by noise criteria (NC) curves. The NC curve levels repre-
sent a peak over a full spectrum of frequencies. A high val-
ue in a low frequency band has the same effect on NC level
as a lower value in a high frequency band. It is important
that sound levels be balanced over the entire spectrum rel-
ative to the NC curve. The lower the NC criteria curve, the
more stringent the room acoustical design must be to meet
the design goals.
It is important to know how to convert NC levels
from the unit ratings in terms of sound power (Lw). This
conversion depends on the specifics of the acoustical envi-
ronment of the installation.
The resulting calculations are compared to the NC curve
selected for the area to assess the acoustical design.
Some of the factors that affect conversion of sound
power to sound pressure and consequent NC level include:
• type of acoustical ceiling
• use of metal or flex duct
• absorption in the occupied space
• location in the occupied space
• open or closed layout plan
• use of open or ducted returns
• orientation of unit to occupant
• use of lined or unlined duct
WSHP sound control
The analysis of the projected sound level in the conditioned
space caused by a WSHP unit located in a ceiling plenum is
quite involved. The key is to have good sound power rat-
ings (Lw) in dB on the equipment to determine the sound
attenuation effect of the ductwork, ceiling and room. In
combination with utilizing standard Aquazone™ equipment
attenuating features or the advanced mute package fea-
tures, suggestions for horizontal and vertical unit sound de-
sign are provided to design around the WSHP units.
Horizontal units
Use the following guidelines for layout of Aquazone hori-
zontal units to minimize noise:
1. Obtain sound power ratings in accordance with latest
standards from manufacturers to select quietest
equipment.
2. Do not locate units over a space with a required NC
of 40 or less. Instead, locate units above less sensitive
noise areas such as above or in equipment rooms,
utility closets, restrooms, storage rooms, or above
corridors.
3. Provide at least 10 feet between WSHP units to avoid
the additive effect of two noise sources.
4. Provide an acoustical pad underneath the WSHP unit
in applications where the unit must be mounted
above noise sensitive areas such as private offices or
conference rooms. The pad attenuates radiated noise.
Be sure the pad has an area at least twice that of the
WSHP footprint.
5. Maximize the installed height above the suspended
ceiling.
6. Be sure the WSHP unit is located at least 6 feet away
from any ceiling return grille to prevent line-of-sight
casing noise to reach the space below.
7. Suspend the WSHP unit from the ceiling with hang-
ers that utilize spring or neoprene type isolators to
reduce vibration transmission.
8. Utilize flexible electrical connections to the WSHP
unit. DO NOT USE NOT RIGID CONNECTIONS.
9. Utilize flexible loop water and condensate piping con-
nections to the WSHP unit.
10. Use a canvas duct connector to connect the WSHP
discharge to the downstream duct system. This
reduces vibration-induced noise.
11. Provide acoustic interior lining for the first 20 feet of
discharge duct, or until the first elbow is reached. The
elbow prevents line-of-site sound transmission in the
discharge duct.
12. Provide turning vanes in ductwork elbows and tees to
reduce air turbulence.
13. Size the sheet metal supply duct with velocities no
greater than 1000 fpm.
14. Ensure ductwork is rigid.
15. Use round duct whenever possible to further reduce
noise.
16. Allow at least 3 equivalent duct diameters of straight
duct upstream and downstream of the unit before
allowing any fittings, transitions, etc.
17. Seal all penetrations around duct entering the space.
18. Provide a 4-ft run-out duct made of flexible material
to connect a diffuser to the supply trunk duct. The
flex duct provides an “attenuating end-effect” and
reduces duct-transmitted sound before it reaches the
space. Typically a 6 dB sound reduction can be
accomplished with the use of flex duct.
19. Locate the run-out duct balancing damper as far away
from the outlet diffuser as possible. Locating the
balancing damper at the trunk duct exit is the best
location.
20. If return air is drawn through a ceiling plenum, pro-
vide an acoustically lined return duct elbow or “L”
shaped boot at the WSHP to eliminate line-of-sight
noise into the ceiling cavity and possible through ceil-
ing return air grilles. Face the elbow or boot away
from the nearest adjacent WSHP unit to prevent addi-
tive noise.
21. Do not hang suspended ceiling from the ductwork.
Application data (cont)