TM 5-805-4/AFJMAN 32-1090
CHAPTER 7
AIR DISTRIBUTION NOISE FOR HEATING, VENTILATING, AND
AIR CONDITIONING SYSTEMS
7-1. Introduction.
terminal units the most direct path is often sound
radiating from the case of the unit and traveling
In this chapter consideration is given to the sound
through a ceiling, usually acoustical, direct to the
levels resulting from the operation of Heating,
observer in the space being served. The attenua-
Ventilation and Air Conditioning (HVAC) systems
tion of a typical ceiling increases slightly with
in buildings. Information is provided on the most
frequency, and thus the typical noise of an air
common HVAC equipment found in many commer-
terminal unit in an occupied space will tend to
cial office buildings, how sound is propagated
shift downward by an octave to have its highest
within ducted ventilation systems and the proce-
sound pressure levels in the octaves with center
dure for calculating sound levels in rooms from
frequencies at 125, 250, and 500 Hz. Thus, in
ventilation systems.
summary, when a system is designed to achieve
7-2. General Spectrum Characteristics Of
good acoustical balance among the various sources,
Noise Sources.
fan noise will control the noise level in the low
frequency range, air terminal units will control in
The most frequently encountered noise sources in
the mid-frequency range, and air outlets will
a ducted air distribution system designed to de-
control in the high frequency range.
liver a constant volume of air are fans, control
dampers, and air outlets such as diffusers, grilles,
7-3. Specific Characteristics Of Noise Sources
and registers (return air grilles with dampers). In
a. Fans. To determine the requirements for
a variable volume system terminal units, such as
noise control for a ducted air distribution system
variable air valves (VAVs), fan powered air valves,
one of the primary requirements is to determine
and mixing boxes, are an additional frequently
the octave band sound power level of the fan noise
encountered noise source. Operation of any of
at the discharge and intake duct connections to a
these identified noise sources can result in noise
fan. These sound power levels can be determined
generated over most of the audio frequency spec-
by a methodology described in appendix C, or
trum. Typically, however, centrifugal fans gener-
obtained from a fan manufacturer for the specific
ate their highest noise levels in the low frequency
application and this is generally the preferable
range in or below the octave centered at 250 Hz.
method. It should be noted that the method given
Diffusers, and grilles, however, typically generate
in appendix C yields the sound power level for a
the highest noise levels in the octaves centered at
fan selected to operate at its maximum efficiency,
1000 Hz, or above. In between these low and high
however the ASHRAE method suggests a correc-
frequency sources, terminal units produce their
tion factor, "C" on table C-13c, for off-peak opera-
highest noise levels in the mid-frequency range in
tion at various fan efficiencies. With a system
the octaves centered at 250, 500, and 1000 Hz
designed to deliver a constant volume to a space it
bands. In addition to these frequency characteris-
is usually possible to operate a properly selected
tics, the normal sound propagation path between
fan at or near its maximum efficiency. However,
the various system sources and an occupants of the
for a variable volume system, with a fan operating
space served influences the typically observed
at a constant speed, the static efficiency will
spectra. Thus, the fans in a system are typically
generally be significantly below its maximum
somewhat remote to an observer, and the fan
static efficiency. Thus, for variable volume systems
sound is attenuated by the properties of the path
the adjustment to the power level for operating
including noise control measures. However, this
efficiency is very important. Variable speed drives
path attenuation is greatest in the mid-, and high
allow the fan to operate at or near the peak
frequency range, and thus the noise reaching the
efficiency for different air quantities and static
receiver will primarily be in the low frequency
pressures. In this instance the fan efficiency can
range as a result of both the source and path
be maintained near its maximum, and the sound
characteristics. With diffusers and grilles, how-
power levels are reduced as the air quantity
ever, there is little or no opportunity to provide
delivered and the static pressure are reduced in
attenuation between the source and the receiver,
accordance with equation C-5. In order to use
and thus the high frequency noise of the source
equation C-5 and table C-13c it is necessary to
alone determines the spectrum content. With air
7-1
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