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Associate

Join Date: Dec 2010
Posts: 34

Sound Attenuation in Ducts without Acoustic Liner

01/07/2011 11:09 AM

The photo of the duct coming from FCU should have be lined (but was not) splits to a branch duct for the adjacent room (shown going to the left in the photo) and the round VCD connects to the flexible duct that will supply the nearby Ceiling Diffuser. Is there a good likelihood that the acoustic lining in this planned plenum box (that will house the ceiling diffuser) effectively attenuate the noise that might find its way to it? And by the way the round VCD does not look that great either. The 45 degree boot clinched into the duct followed by rectangular VCD and then the transition from square to round might have been easier on the air, but the location of the diffuser is fairly close to the round VCD. Of course this difficulty too could have been circumvented by tapping for this Ceiling Diffuser a little more upstream! Any judgments about this would also be appreciated

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Guru

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#1

Re: Sound Attenuation in Ducts without Acoustic Liner

01/07/2011 11:17 AM

Seems to be a popular subject today. Elastomeric Paint as Duct Sound Dampener

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Associate

Join Date: Dec 2010
Posts: 34
#2
In reply to #1

Re: Sound Attenuation in Ducts without Acoustic Liner

01/07/2011 2:33 PM

ANOTHER EXAMPLE OF HOW THE DUCT FROM FCU WILL TERMINATE IN THE CEILING DIFFUSER (RE ACOUSTIC LINER MISSED)

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Guru

Join Date: Jun 2008
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#3

Re: Sound Attenuation in Ducts without Acoustic Liner

01/07/2011 11:10 PM

Sound attenuators are used to keep the noise level within the acceptable limit. There are few applications where noise level is considered as a second priority. Site conditions could be one of the reasons.

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Guru
Engineering Fields - Mechanical Engineering - New Member India - Member - New Member

Join Date: Aug 2008
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#4

Re: Sound Attenuation in Ducts without Acoustic Liner

01/08/2011 3:29 AM

It is not clear what you want. Please make small sentences. What is present noise level & what is acceptable level, How noise is generated, What is full form of VCD & FCU?

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Guru

Join Date: Jun 2008
Location: Oman
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#5
In reply to #4

Re: Sound Attenuation in Ducts without Acoustic Liner

01/08/2011 7:53 AM

I presume FCU -Fan Coil Unit and VCD -Volume (Variable) Control Damper

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Anonymous Poster
#6
In reply to #4

Re: Sound Attenuation in Ducts without Acoustic Liner

01/08/2011 7:58 AM

Sorry, I said it all in one breath. I will break it up as follows:

FCU (Fan Coil Units)

VCD (Volume Control Dampers)

The acoustic lining inside the ducts from the fan coil units to the branches is short of the specified 10 feet. It is in the order of 5 feet only.

The flexible duct from the branches connects to ceiling diffusers via acoustically lined plenum boxes.

The question: Will the given arrangement as described in words and as depicted in photo is likely to stop the noise from finding its way in the patient sleeping areas where:

Patient Room Acceptable Noise levels 30 – 35 dB.

Field Measurements of Noise generated by Fan Coil Units in patient rooms are yet to be carried out.

Thanks

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#7

Re: Sound Attenuation in Ducts without Acoustic Liner

01/09/2011 6:30 PM

Noise is air turbulence, straight t/o creates turbulence, duct liner is a petri dish and I never put it or flex in a health care facility. Main duct velocity should be 1200-1300fpm on a CV system, branches 600 max and discharge at diffuser 400-450. If an air distribution system is noisy it is always a funtion of design and/or installation. This the reason there is a requirement for submisson of shop drawings by installer to engineer for approval prior to fab and install. The safe repair in this situation is to move the tap and replace with 45' tap and alow room for 8-10 duct diameters before turn into diffuser. Intent is to establish laminar airflow at proper velocity. You might also raice the nc number on the diffusers.

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Associate

Join Date: Dec 2009
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#8

Re: Sound Attenuation in Ducts without Acoustic Liner

01/10/2011 10:35 AM

Sounds like Conwaymech knows what he's talking about.

Only way to know for sure how much sound will occur is to finish the duct installation and run it BEFORE installing the ceiling when modifications are less costly.

Has anyone seen a Helmholtz Resonator used in HVAC to cancel out certain frequencies? They are used extensively in the auto industry to silence intake noise in engine induction systems.

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Participant

Join Date: Oct 2013
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#9

Re: Sound Attenuation in Ducts without Acoustic Liner

10/24/2013 2:11 PM

In regards to the round takeoff..............

I have a comment about the round takoff. Because the damper blade is 90 degrees to the airstream, it is hurting the system. I was performing some testing and balancing on a building in the late 70's. For some reason, I wasnt getting good airflow to the diffusers. There were 8" round collars with a dampers that slightly extended into the brach duct similar to what you show. There were also airscoops on these collars (not a big fan on airscoops, but it was a big thing back then). Because of the airscoops, there was only one way to install these collars. Long story short, because the damper blade was 90 degrees to the airflow, most of the air was only entering the first half of the dampered opening. The damper blade was causing the air to "ricochet" off the damper blade, thus limiting the amount of air entering the far side of the dampered opening. This effect was magnified with the larger diameter collars. Since the collars were basically all the same length, the larger the diameter, the more of the blade entered the airstream. After having to prove this to the manufacturers, the manufacturers revised their design of the scoop and damper blade. Of course this problem only occurs with collars having scoops. But in my time, I have witnessed many occurrances where the mechanic opts to install the collars incorrectly, with the dampers 90 degrees to the air flow, as shown in your picture. Just food for thought.....

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