How to Incorporate Noise
Control into your HVAC System
our last newsletter we attempted to review sound basics. This article
will provide a follow-up on how to treat noise or unwanted sound by
using some basic principles when designing mechanical systems for
simple rules to follow are:
Select equipment that has low sound power levels.
Design sound sensitive spaces away from mechanical rooms. Example
Presidents office or Boardrooms next to the Mechanical room – think
about relocating one or the other if you can.
incorporate Noise control such as sound traps / attenuators,
vibration isolation in the early design stages of your system. (It
is always more cost effective to incorporate these into the system
when designing/building a new space or building, instead of after by
having to retrofit the design if a noise problem occurs.)
most sound is airborne we need to make sure we treat it accordingly by
the use of active or passive noise control.
Active Noise Control: uses an electronic device such as a speaker to
send an opposing sound wave to cancel out the undesired frequency or
Noise Control: uses acoustic medias, perforated duct liners, sound traps
/ attenuators to absorb the sound energy and minimize the amount of
sound energy that may travel to the space.
are a number of paths that sound can travel through and they are
described below in the figure. We will go through each of the paths and
present solutions on how to treat each type.
A: Structure-borne path through floor.
Vibration – treat the Sound Source with Vibration Isolation.
B: Airborne path through supply air system.
is noise that travels down the duct from the sound source. Using a
Sound trap/attenuator installed in the duct between the source and the
receiver will reduce the noise.
C: Duct breakout from supply air duct.
This is noise that
breaks out or radiates from the ductwork, escaping from the duct as the
noise travels down the duct run. This can be treated by using such
products as loaded vinyl, heavy gauge ductwork or duct wrap.
D: Airborne path through return run air system.
Similar to supply air system: This is noise that travels down the duct
from the sound source. Using a Sound trap/attenuator installed in the
duct between the source and the receiver will reduce the noise.
E: Airborne path through mechanical equipment room wall
is noise that travels through the wall from the adjacent mechanical
space. Typically designing the wall to meet a specific STC rating
(Sound Transmission Class) will help minimize noise from transmitting
through the wall cavity.
to Determine if you Need Noise Control:
are a variety of programs available by Noise Control product
manufacturers that can help you do an analysis on your HVAC System to
determine if you need sound attenuation. These programs are developed
around ASHRAE’s guidelines presented in their handbook. To do an
acoustic analysis for your HVAC System has been simplified with these
programs that have been developed.
Things to keep in mind:
Look for the shortest runs on your system from your sound
source; these are the runs that you base your analysis on.
Look at both the supply and return runs of your system.
Where is the ductwork running? Over a noise sensitive
space such as an office or operating room – breakout of noise through
the duct may be a concern.
is a screen shot of software that has been developed by VAW Systems Ltd.
for doing an acoustic analysis.
Systems Ltd. COMSIL Selection Software – Sound Calculation Module
the example shown here you can see the overall sound powers of the sound
source in each of the octave bands and the different icons that
represent the equipment and ductwork in the HVAC System.
different components are put in to represent the system being analyzed
there are also insertion losses (or attenuation values) associated with
each of them. Showing the reduction of sound as we progress down the
duct run finally ending up at the occupied space that we are doing the
analysis on. In this example here we are targeting for an NC of 35 in
the occupied space. Based on the layout of the existing system we know
our estimated NC in the space is 62. This lets us know what level we
are currently at and where we need to get. So we can do one of two
things at this point, select a quieter fan if we can, or select a sound
attenuator to take out the rest of the noise that will be present in the
example shows the current NC to be 62; it also shows the required
Insertion Loss (I.L.) to get to NC 35 in the occupied space based on
each of the octave bands.
to Select the right Type of Sound Trap / Attenuator:
here we can select a sound trap / attenuator using the software and also
determine the best location to place the sound trap / attenuator and
type to use based on our duct, whether it be round or rectangular.
Typically you will want to place the sound trap / attenuator as close to
the sound-source as possible if this can be done in some instances this
is not possible. By doing this you not only ensure the shortest run is
attenuated and the NC in that space is met, you have also attenuated the
rest of the main run and any branches after that. Unless you have
additional sound sources built into the duct system such as a fan
terminal box. Then you will also have to check those.
this example we are going to place the sound trap in the branch right
before we enter the occupied space. Based on the design of the system
we will select a rectangular sound trap to suit. The location of the
sound trap is highlighted in yellow below.
can see that by incorporating the silencer into our analysis that we
have now met/exceeded our requirement of NC 35 in the occupied space.
In this case we’ve used a rectangular sound trap, below are a variety of
others that are available for a variety of applications.
are a variety of sound traps available:
Rectangular, Elbow, Circular using acoustic medias to absorb the
No Media types that do not use acoustic medias to absorb the sound
energy, instead they use chambers to trap the energy to cancel out
the sound wave.
Custom specific sound traps to suit the needs on your specific
are a couple of pictures of some standard sound traps.
Rectangular Sound Trap Elbow Sound Traps with circular
Circular Sound Traps
not all applications are as straightforward as the example we ran
through when it comes to placement or type of sound trap. Sound traps
can be typically installed anywhere on the system, provided that it is
the best location and orientation and have also taken additional system
effects into account for the systems static design.
more information on sound attenuating solutions, please visit
Semco or contact your Michigan Air Products sales representative.