Basic Documentation

Chapter 6–Minimizing HVAC Sound
Sound Attenuation Devices
It is often necessary to add sound attenuation devices when it is determined that a sound
level cannot be adequately attenuated by the duct system itself or the HVAC system
configuration. Noise attenuation devices are divided into two categories; active and passive.
Active attenuation involves using state of the art technology to generate opposing waves of
sound energy that are intended to cancel the offensive sound wave energy and thus
significantly reduce or eliminate the resulting offensive sound. Passive attenuation is based
upon the absorption of sound energy (and its conversion into heat) by acoustical material,
typically applied as liners, baffles, or insulators.
Passive Sound Attenuation Devices
Passive sound attenuation is the most prevalent method of sound attenuation since it
involves relatively low cost materials and common design and installation methods.
Linings
Internal duct lining and plenum lining is perhaps the most effective means of attenuating
sound. Note that the lining must be on the inside of the duct and must be at least one-inch
thick to be effective. Crosstalk, which occurs when room sound travels to another room by
means of a common duct connection, can mostly be eliminated if the interconnecting duct is
lined.
Note that it is important that duct lining not be allowed to become wet. Otherwise, it must
invariably be removed and replaced since once it becomes moist, it is virtually impossible to
sanitize it against the bacteria growth that inevitably occurs.
Although fiberglass has been the most prevalent lining composition, alternate materials such
as fibrous metal is also available. Fibrous metal lining differs from fiberglass in that it is not
applied as a soft thick material, but rather it is compressed into a thin sheet. Further, fibrous
metal can be tuned to closely match the frequency needing attenuation by altering its
permeability, texture, thickness and the space between the duct wall and the material.
Duct Silencers and Attenuators
These devices, also known as sound traps, are passive sound attenuation components
designed for inserting into duct systems. Configurations are available for standard sized
round and rectangular ducts.
They offer only limited attenuation in the low frequency (125 Hz and below) octave bands,
and moderate attenuation at the high frequency bands. Their maximum attenuation is in the
mid frequency (1,000 Hz) octave bands. Apart from the cost of silencers, they require a
certain amount of physical space and will create an additional duct system pressure drop.
Be sure to follow the manufacturer’s instructions regarding the proper installation and location
for a duct attenuator or silencer. Typically, there should be a certain number of duct
diameters (1.5 in. to 2 in.) between the discharge of a fan and the silencer as well as
between the silencer and other duct elements (elbows, etc.) for proper functionality.
100 Siemens Building Technologies, Inc.