Basic Documentation

Chapter 4–HVAC Sound Attenuation
As previously discussed, sound travel is independent of the direction of airflow. Therefore, all
calculations involving sound generation or attenuation apply to the exhaust portion of an
HVAC system as well as to the supply side.
Plenums
Plenums that are constructed of concrete will have virtually negligible attenuation effect on
sound generated by HVAC components. Plenums of unlined sheet metal will provide only a
little more attenuation effect than concrete. On the other hand, plenums that are fully lined
with at least two inches of sound adsorbing material can provide very significant sound
attenuation. The specific analysis of a given plenum is very complex from an acoustical
standpoint. Reference books provide mathematical procedures to calculate the attenuation of
certain plenum configurations, however these are very time consuming and are still only an
approximation. In addition, since there are almost endless arrangements possible with
plenum designs, it is extremely difficult to come up with precise results regardless of which
calculation procedure is used.
For practical considerations, it is best to assume that an unlined sheet metal plenum will have
a minimal effect on fan sound attenuation and therefore one can normally disregard its effect
on the fan sound generation. (Note that unlined sheet metal plenums are typically used in
centralized laboratory exhaust systems. However in laboratory exhaust systems, the
relatively long duct runs and large number of junctions usually provides adequate attenuation
of exhaust fan sound for the majority of areas served by such systems.)
On the other hand, supply fans discharging into fully lined plenums with 2-inch thick (or more)
sound absorbing material will typically reduce the low frequency (63 Hz to 125 Hz) sound
power by at least 5 dB, and will also reduce the upper frequencies (2,000 Hz to 8,000 Hz) by
at least 15 dB or more.
Figure 14 gives the sound absorption coefficient of different plenum materials. The higher the
plenum sound absorption coefficient, the greater will be the sound attenuation. Note how the
attenuation effect of the thicker fiberglass type liners is maximized at the lower frequencies.
Table 13 also provides specific values of plenum sound absorption coefficients.
Table 13. Plenum Lining Material vs. Sound Absorption Coefficient.
Material 63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
Bare Concrete 0.01 0.01 0.01 0.02 0.02 0.02 0.03 0.04
Bare Sheet Metal 0.04 0.04 0.04 0.04 0.05 0.05 0.07 0.09
1-in. Thick Fiberglass 0.02 0.03 0.22 0.69 0.91 0.96 0.99 1.00
2-in. Thick Fiberglass 0.18 0.22 0.82 1.00 1.00 1.00 1.00 1.00
3-in. Thick Fiberglass 0.48 0.53 1.00 1.00 1.00 1.00 1.00 1.00
4-in. Thick Fiberglass 0.76 0.84 1.00 1.00 1.00 1.00 0.97 0.91
48 Siemens Building Technologies, Inc.