• Select duct silencers that do not significantly increase the required
fan total static pressure. Selecting silencers with static pressure
losses of 0.35 in. of water or less can minimize regenerated noise
from silencer airflow.
• Place fan-powered mixing boxes associated with variable-volume
air distribution systems away from noise-sensitive areas.
• Minimize flow-generated noise by elbows or duct branch takeoffs
whenever possible by locating them at least four to five duct
diameters from each other. For high-velocity systems, it may be
necessary to increase this distance to up to 10 duct diameters in
critical noise areas. Using flow straighteners or honeycomb grids,
often called “egg crates,” in the necks of short-length takeoffs that
lead directly to grilles, registers, and diffusers is preferred to
using volume extractors that protrude into the main duct airflow.
• Keep airflow velocity in the duct as low as possible (1500 fpm or
less) near critical noise areas by expanding the duct cross-section
area, which will reduce potential flow noise associated with turbulence
in these areas. However, do not exceed an included
expansion angle of greater than 15°; the resulting flow separation
may produce rumble noise.
• Use turning vanes in large 90° rectangular elbows and branch
takeoffs. This provides a smoother directional transition, thus reducing
turbulence.
• Place grilles, diffusers, and registers into occupied spaces as far as
possible from elbows and branch takeoffs.
• Minimize the use of volume dampers near grilles, diffusers, and
registers in acoustically critical situations.
• Vibration-isolate all reciprocating and rotating equipment connected
to structure. Also, it is usually necessary to vibration-isolate
mechanical equipment located in the basement of a building as
well as piping supported from the ceiling slab of a basement,
directly below tenant space. It may be necessary to use flexible piping
connectors and flexible electrical conduit between rotating or
reciprocating equipment and pipes and ducts that are connected to
the equipment.
• Vibration-isolate ducts and pipes, using spring and/or neoprene
hangers for at least the first 50 ft from the vibration-isolated
equipment.
• Use barriers near outdoor equipment when noise associated with
the equipment will disturb adjacent properties. In normal practice,
barriers typically produce no more than 15 dB of sound
attenuation in the midfrequency range.
Table 1 lists several common sound sources associated with
mechanical equipment noise. Anticipated sound transmission paths
and recommended noise reduction methods are also listed in the
table. Airborne and/or structure-borne sound can follow any or all of
the transmission paths associated with a specified sound source.
Schaffer (1991) has more detailed information in this area.
EQUIPMENT SOUND LEVELS
Accurate acoustical analysis of HVAC systems depends in part
on reliable equipment sound data. These data are often available
from equipment manufacturers in the form of sound pressure levels
at a specified distance from the equipment or, preferably,
equipment sound power levels. Standards used to determine
equipment and component sound data are listed at the end of this
chapter.
When reviewing manufacturers’ sound data, require certification
that the data have been obtained according to one or more of the relevant
industry standards. If they have not, the equipment should be
rejected in favor of equipment for which data have been obtained
Table 1 Sound Sources, Transmission Paths, and Recommended Noise Reduction Methods
Sound Source Path No.

ASHREA BOOK A47
有详细介绍如何做噪音计算，和控制

25DB的送风口应该要求很高，