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Associate

Join Date: May 2011
Posts: 30

Air Handling Unit Noise Attenuation

06/15/2011 7:44 AM

Is it essential to provide the acoustic liner in both the supply and return ducts. The return duct in question is merely a bellmouth that draws air from the open plenum above the false ceiling and routes it to the Air Handling Unit at the roof. So the possibility of sound finding its way from the return duct appears very limited.

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Guru

Join Date: Oct 2008
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#1

Re: Air Handling Unit Noise Attenuation

06/15/2011 9:16 AM

You might want to look at some of these, since this is not your first question on the subject. Sooner or later, you'll be able to figure this out on your own if you do. That should save you some time. There's nothing simple about acoustics.

  1. Acoustic Analyzer™
  2. [PDF] HVAC Acoustic Fundamentals
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Associate

Join Date: May 2011
Posts: 30
#2
In reply to #1

Re: Air Handling Unit Noise Attenuation

06/15/2011 9:41 AM

Thanks a lot for the links. After having waded through helmholtz resonators and a great deal of acoustical analyses mostly in my school years, the purpose of the question was not to go back into the past. The intent was to get a general feel from professionals with sufficient field experience about skipping the liner in the return, given that it the occupancy is not a classroom and are patient bedrooms.

Any feedback will be appreciated and will provide the direction for further closer look and analyses. Thanks

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Power-User

Join Date: Jun 2006
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#3

Re: Air Handling Unit Noise Attenuation

06/15/2011 10:44 PM

Hi verycool,

On my simple understanding that the acoustic liner serves not only as noise attenuation but also as insulation to avoid from condensation, since that the air passing thru the return duct temperature is not as cold as the supply ducting, then the thickness could be lesser or sometimes nothing at all especially if the temperature inside the drop ceiling is not hot as we know condensation caused by hot and cold reaction. return duct normally should be insulated, I would say you mustn't consider it only as noise attenuation but also for the purpose of insulation.

Roman

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Guru

Join Date: Jun 2010
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#4

Re: Air Handling Unit Noise Attenuation

06/15/2011 11:17 PM

After installing a 10ton HVAC split unit in a Doctor's house for his Master Bedroom Suite we had to have a Balancing Company come in and balance the airhandler as it was so noisy you could hear it throughout the whole house.

They came equiped with sensitive sound equipment and attached digital microphones to the unit in various places, printed out charts of the various frequencies of sounds emanating from the unit.

They then proceeded to attach rubber sound dampening sheets to the outside of the metal housing, replaced the belt pulleys with lathe turned and balanced pullies (double squirrell cage blower fan) and balanced the blades of the two squirrel cage fans.

They normally do this sort of thing for hospitals but were capable enough to balance and blueprint internal compustion engines also. Very adept at all types of balancing of moving machinery.

They did nothing to the metal ductwork and after they were done you could barely tell the air handler was running even standing right next to it and there was no sound at all in the bedroom, sitting room or bath.

Point is that the sound comes from the air handler...ductwork is a non-moving part and contributes nothing to the noise level.

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Guru
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#7
In reply to #4

Re: Air Handling Unit Noise Attenuation

06/16/2011 10:22 AM

ductwork is a non-moving part and contributes nothing to the noise level.

What absolute rot!

How does an organ work? A recorder? Oboe? All these use "ductwork" to amplify pressure waves. Your HVAC ductwork can do the same thing...or it can attenuate it if it's designed properly. The same thing happens in vehicle intakes and exhausts (my particular area). Lots of work is done by OEMs to reduce intake noise as well as exhaust noise.

The pressure waves will travel both ways along ducting from the source, so you will hear the unit from the intake end....at 340 m/s those sound waves travel faster than the incoming air flow. If your unit is noisy then one way to deal with it, if you've a high frequency noise, is to use absorpative linings.

In addition, you may get structure borne noise radiated by the panells of the ducting. This can be dealt with by adding linings to change natural frequencies, or bending panels to increase stiffness and thus increase natural frequency to above the driving frequencies (and harmonics). Ever noticed that large HVAC units have X scored/stamped in their panels - stiffeners.

As your tale demonstrates, nothing beats removing the noise at source - if there's nothing there to start with, then there's nothing to be amplified. If the OP can't take this approach, then, depending on the actual source of the noise and its frequency, lining the intake will probably help.

I can't be more definite - as lyn said, in acoustics, there's nothing simple.

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Guru

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#5

Re: Air Handling Unit Noise Attenuation

06/16/2011 7:00 AM

Generally acoustic treatment is provided in supply air duct from AHU,Plenum. Acoustic treatment in return air duct is not required. Velocity in supply air is higher than return air duct. However depending upon design velocities and the noise level requirement decision can be taken.

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Guru

Join Date: Jun 2010
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#6
In reply to #5

Re: Air Handling Unit Noise Attenuation

06/16/2011 7:37 AM

If the ductwork to all the supply grills is sized properly based on room volume size and heat load and the grills are of the correct size and configeration and number there should be no noise from air flow.

You should only feel the air flow, not hear it otherwise somewhere, something was not designed correctly by the engineer.

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Associate

Join Date: May 2011
Posts: 30
#8
In reply to #6

Re: Air Handling Unit Noise Attenuation

06/16/2011 1:02 PM

You see Lyn! We are still neither here nor there despite all the tons of theory and references.

Field experience is the bird of a different feather altogether.

I hope there is more useful feeback to put the issue on track!

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129CBRider (2); 786verycool (2); English Rose (1); lyn (1); mrswamy (1); Roman (1)

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