Acoustic Liner inside ducts connected to fan coil units 7

[mechanical3010]

The specs call for acoustic liner for 3 meters of duct connected to equipment. Applying acoustic liner internally in the small sizes of duct that are within 3 meters of fan coil units is not convenient. Can sound be attenuated by applying the acoustic insulation on the outside of the ducts given that there is already thermal insulation on the outside of the ducts ?

 

[GMcD]

Nope. The fan-coil noise will just reflect down the inside of the hard duct and you'll hear the fan running at a diffuser even 25 feet away from the fan-coil. Do it right, and do it once.

 

[mechanical3010]

The diffuser in question is not installaed directly to the hard (GI) duct. the take offs for the diffusers from the main metal duct is by flexible duct with its own sound attenuating lining that ends in a plenum box where the diffuser is installed. Since the specs calls for acoustic liner for the first 10 feet of ducting starting from the equipment, the idea of doing it from outside occured from the observation that special sound attenuating paints are applied on the outside of chutes (such as garbage chutes)running many stories of the building and indeed there is hardly any noise heard due to the effectiveness of this method. So the question is whether outside application of acoustic liner has any sound attenuating effect or not.

 

[GMcD]

Acoustic insulation applied to the outside of the duct "may" attenuate radiated noise that comes out of the fan discharge connection through the duct, but it does nothing for the fan noise that is reflected down the inside of the hard duct that will be heard from the air terminals. Even if there are flex connections from the hard supply duct to the air terminals, that will likely not be enough to attenuate the fan noise at the air terminal, depending on the fan-coil selection, and fan speed.

Sound attenuating coatings applied to the outside of a garbage chute is meant to reduce the impact noise and sliding/rattling noises from objects travelling down inside the chute-a completely different type of noise generated compared to a fan-coil air noise situation.

If the specifications call for acoustic lining, then there must be a darn good reason for it.

 

[berkshire]

The acoustic liner inside a duct breaks up and attenuates the sound waves travelling inside the duct, if you are worried about chunks of liner coming off, put an expanded mesh retainer over it. Putting stuff on the outside does not do anything for you. If you have ever stood at the trash compactor end of a garbage duct, you can hear stuff being thrown into it 20 stories up.
If you need to keep the cross section of the duct, increase the size by the thickness of the liner.
If you leave that duct smooth on the inside, you will hear every squeak and rustle off that fan, just like you were using an ear trumpet.
B.E.

 

[mechanical3010]

Well the verdict of all respondents in different words is the same namely that the liner must be applied where it belongs viz. inside the ducting. Would it be effective enough if only 5 instead of 10 feet of liner is applied and could it be applied say after a few feet of the fan or must it originate immediately at the fan?

 

[KiwiMace]

5 ft of liner will absorb half the noise of 10 feet of liner. Are you prepared to challenge the designer that half the attenuation would be effective enough?

Shifting it out a few feet may allow noise to be radiated from that part of the duct, with the effect as discussed above.

Generally, this treatment is typical of noise sensitive specifications, and I wouldn't consider the specification to be over and above.
Further to Berkshires comment above - any normal spec will define the duct size indicated on the drawings as clear internal dimensions, so liner adds to overall dims.

 

[RayWK]

We had an ugly noise problem once in an office. The takeoff was close to the fan, there was no liner in the main duct, and all of the above comments applied. Rather than lining the main duct, though, we lined a single 90 degree elbow feeding the office. It solved the problem completely. Does your installation have a 90 degree elbow anywhere near the fan discharge? If so, you may be able to get your engineer to go for this as an equivalent, as long as it does not violate a specifice client requirement in the contract.
Strangely, the other offices downstream of this office did not have the noise problem, which implied to us that, if you have enough twists and turns in the ductwork, the noise problem abates on its own. Any others out there with similar experiences?

 

[AnotherEllis]

Out of curiosity, why do you want to talk the engineer out of their spec? Did the contractor not get the spec before the project was bid on? Is the owner to get a credit? Will the contractor take liability for any end user complaints?

I have to say, trying to not do something as designed and bid on because it is not convenient or easy would be unacceptable to me as the EOR. If you do go back looking for alternates, at least offer some other method for consideration. I like these for sound attenuation:

http://www.ruskin.com/catalog/default.aspx?id=13642

Of course, by the time you get these accepted on a performance basis and get the quote from Ruskin... you'll be out some bucks (and time).

PS: just re-read this and I sound like an old man. Swear to god I'm in my mid 30s!

http://www.ellisconsultingengineers.com

 

[berkshire]

To all,
If I did not know better, I would think that mechanical3010 already has these ducts installed without the liner, and is trying to find a way out of the situation he has gotten himself into.
B.E.

 

[KiwiMace]

If that is the case, mechanical3010 has the choice of fixing it or... Nope that's it. I might have said engage their own acoustic consultant to prove the design, but you may as well go out today and start to lower the ceiling.

 

[mechanical3010]

Berkshire has hit the nail on the head

 

[mechanical3010]

RayWK
There are branches from the main duct at 90 degrees to the mains. These branch connections are not by means of elbows but by 45 degree entry. In these branches are VCDs and then the flexible duct that connects to an acoustically lined plenum box that houses the diffuser.

 

[mechanical3010]

The photo of the coming from FCU that should be lined (as per above consensus) 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

 

[RayWK]

Your lined plenum box may just do the trick, as long as the air (and sound) has to bounce off at least one wall before exiting the diffuser. However, it is a gamble. If it doesn't quite work as well as you hope, guess who get the blame and the bill?!! (Hint: it's you) You need everyone to sign off on it right now. Otherwise, you will be money ahead just taking down the main duct and lining it.

 

[mechanical3010]

THE ATTACHED PHOTO ILLUSTRATES ANOTHER EXAMPLE OF HOW THE DUCT FROM FCU WILL TERMINATE IN THE CEILING DIFFUSER. HOPEFULLY THE AIR/SOUND WILL DO THE BOUNCING AROUND IN THE ROUND FLEXIBLE THAT WILL BE CONNECTED TO THE ROUND NECK SHOWN IN THE PHOTO AND THIS FLEX DUCT ALSO HAS ACOUSTIC PROPERTIES (I HOPE).

 

[marcoh]

If it is an existing installation try making the flex ducts longer, even put in a 360 degree loop, which can be very effective.

Also check the fan, can the fan speed be reduced? Ensure the index run is 100% open and the air flow is not being throttled using balancing dampers.

 

[mechanical3010]

To decide how much of the duct to take down for lining it, the question arises about the minimum recommended lenght of supply and return ducts that must be provided with acoustic liners for both the Air Handling Units and Fan coil units.
Furthermore, is it very important or effective in attenuating sound to line the ducted return to the Air Handling Units?
Thanks