Introduction to Acoustics

406 Part C Architectural Acoustics
Part C 11.4
Exhaust
fan
Noise
radiated to
community
Return
duct
Supply
fan
Condenser
fan
Supply
duct
Return air
noise path Structureborne
path Supply
air
Room ceiling
noise path
Airborne
casingradiated
path
Compressor
Roof
Curb
Fig. 11.26 Common noise paths for a typical rooftop HVAC
unit
Mechanical Equipment
Common mechanical equipment associated with HVAC
systems includes pumps, compressors, chillers, generators,
and air handlers. Rotating components of these
pieces of equipment, such as gears and fans, generate
most of the noise that causes concerns in buildings. Rotating
mechanical equipment typically generate tones,
their frequencies being associated with their rotational
speeds. When mechanical equipment is housed in rooms
within buildings, its associated noise can affect rooms
throughout a building. When mechanical equipment is
placed on rooftops or slabs outside buildings, its associated
noise can also affect the surrounding communities.
Figure 11.26 shows noise sources and paths for a typical
rooftop HVAC unit.
Mechanical equipment not only generates noise in
buildings, but also generates vibrations that can propagate
throughout a building’s structural members if not
properly isolated. These vibrations can excite building
members far from the sources and cause remote building
components to rattle and generate their own noise.
Duct-Borne/Airflow Noise
Air is typically carried throughout a building using a system
of ductwork. Ducts carry the tones generated by fans
and they cause additional noise by inducing turbulence
in the airflow. Some of this noise is carried through the
ductwork to rooms in buildings (out of grilles in each
SPL in increments of 10dB
40 63 100 160 250 400 630 1000 1600 2500 4000 6300
31.5 50 80 125 200 315 500 800 1250 2000 3150 5000 8000
One-third octave band center frequency (Hz)
Fig. 11.27 Average transportation noise spectrum shape
(adapted from data in ASTM E 1332)
room) and some of it is radiated directly from the duct
walls (known as breakout noise).
11.4.2 Plumbing Systems
The most common plumbing system noise sources are
water flowing through pipes and noise radiating from
the walls of pipes.
11.4.3 Electrical Systems
The most common electrical system noise sources are
transformers and noise radiated from associated conduit
(by carrying vibrations that excite walls).
11.4.4 Exterior Sources
The most common exterior noise sources that affect
building inhabitants are those associated with
transportation (such as vehicular traffic, rail, and
aircraft), industrial operations, and mechanical equipment
from nearby buildings or mounted outside the
same building. Figure 11.27 shows an average spectrum
shape for transportation noise sources used by
ASTM E 1332. This shows the concentration of
sound in the low frequency (below 500 Hz) range,
emphasizing the need for exterior-wall noise control
design measures that effectively address low
frequencies.