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How to Incorporate Noise Control into your HVAC System

In 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 occupied spaces.

 

Some 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.)

 

Since 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.

 

Product ImagePassive 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.

 

 

 

There 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.

 

                   

 

 

Path A: Structure-borne path through floor.

Vibration – treat the Sound Source with Vibration Isolation.

 

Path B: Airborne path through 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.

 

Path 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.

 

Path 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.

 

Path E: Airborne path through mechanical equipment room wall

This 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. 

 

How to Determine if you Need Noise Control:

 

There 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:

q       Look for the shortest runs on your system from your sound source; these are the runs that you base your analysis on.

q       Look at both the supply and return runs of your system.

q       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.

 

Below is a screen shot of software that has been developed by VAW Systems Ltd. for doing an acoustic analysis.

VAW Systems Ltd. COMSIL Selection Software – Sound Calculation Module

 

In 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.

 

As 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 occupied space.

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. 

 

How to Select the right Type of Sound Trap / Attenuator:

 

From 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.

 

In 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.

You 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.

 

There are a variety of sound traps available: 

  • Rectangular, Elbow, Circular using acoustic medias to absorb the sound energy. 

  • 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 project.

 

Below are a couple of pictures of some standard sound traps.

 

          

     Rectangular Sound Trap                   Elbow Sound Traps with circular connections

 

               Circular Sound Traps

 

 

Now 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.

 

For more information on sound attenuating solutions, please visit VAW Systems, Semco or contact your Michigan Air Products sales representative.