TM 5-805-4/AFJMAN 32-1090
moderate (approximately 1,000 FPM). Manufac-
in one muffler both active cancellation and passive
tures provide dynamic sound insertion losses for
dissipative elements to attenuate the low and high
various flow velocities, when the air flow and
frequencies respectively. Thus, active cancellation
sound are in the same direction (supply) and when
may find application in the future in the HVAC
the air flow and sound are in opposite directions
industry, but its cost and operation will probably
(return).
limit its application to situations where the space
The manufacturers of these attenuators can also
for the use of conventional dissipative lining or
provide polymer sheeting to enclose the sound
mufflers is not available.
absorptive glass fiber packing for cleanliness and
7-5. Procedure For Calculating Noise Control
resistance to certain chemicals. Certain manufac-
Requirements For An Air Distribution System.
tures also make attenuators without any sound
absorptive packing (i.e. "packless") for systems
a. The procedure for calculating the noise con-
serving spaces that must maintain very clean
trol requirements for an air distribution system
environments. The insertion losses for these units
involves six steps:
are typically somewhat less than for the normal
(1) Selection of noise criteria or goal for the
mufflers, and the manufacturers should be con-
space(s) served.
sulted regarding their specific acoustic perfor-
(2) Estimating sound power level of sources.
mance.
(3) Estimating the insertion loss of the duct
f. Attenuators using active cancellation. Active
distribution system applicable to each source to
sound attenuation as a means of noise control has
arrive at the sound power at the air outlet(s) in
in recent years moved from the laboratory to a
the space(s) served.
growing number of applications including the con-
(4) Determine the total sound power within
trol of fan noise in ducted systems. Active sound
the room from all the sources.
attenuation involves the use of an auxiliary sound
(5) Determining the "Rel SPL" for the space
source to generate a sound wave that interferes
served and subtracting that from the sound power
and cancels an unwanted sound wave. This system
levels, from previous step, to obtain the sound
of cancellation is limited to plane wave conditions,
pressure levels in the space.
which means that the wavelength of the sound to
(6) Compare calculated sound pressure levels
be cancelled must generally be greater than the
in each octave band with the criteria to determine
largest dimension of the duct. While this is not a
noise control requirements in dB.
problem for most air distribution systems, it does
b. To carry out step (1) it is necessary to know
generally limit the frequency range for application
the function of the space served, and for that
to below about 500 Hz. Recently, with the develop-
function such as a conference room select the
ment of adaptive, digital signal processing, the
appropriate RC or NC criteria, to serve as the
application of this method has been extended to
noise control. To carry out step (2) for a constant
cancelling broad band noise and thus is not now
volume system essentially means determining the
limited to cancelling only tonal sounds. Finally, in
sound power level, in each octave band, for the
some applications it has been proposed to combine
fan, and for a diffuser, or grille. These data can
Table 7-4. Power Level Loss at Branches.
Division (dB)
Division (dB)
B/T
B/T
10
1.00
0
0.10
11
0.00
1
0.08
12
0.63
2
0.063
13
0.50
3
0.05
14
0.40
4
0.04
15
0.32
5
0.032
16
0.25
6
0.025
17
0.20
7
0.02
18
0.16
8
0.016
19
0.12
9
0.012
7-7
Previous Page
