The art of noise

Engineered Systems, Feb, 2005 by Joanna R. Turpin

"A 2,500-cfm fan-powered VAV box located above a private office will create a noise problem for the occupant. That same box located in the adjacent hallway with appropriate noise control on the supply and return will not," noted Peterman.

It's usually not a good idea to install fan-powered VAV boxes over conference rooms and private offices. Open offices with cubicles, hallways, and bathrooms are usually areas that can absorb higher sound levels. It works out well if a specification has different maximum noise levels for each assigned space. Simply stating that a whole building must be NC 30 or 35 is not an efficient method of noise control design.

Int-Hout stated that VAV terminal units will continue to cause noise problems in mechanical specifications as long as design professionals continue to employ outdated specifications referencing long-dead standards. "The key to success is remaining current and analyzing all aspects of the job to ensure all system components integrate properly and achieve the desired performance."

To that end, he suggests that engineers consult ARI Standard 885, "Procedure For Estimating Occupied Space Sound Levels In The Application Of Air Terminals And Air Outlets." This standard provides current application factors for converting rated sound power to a predicted room sound pressure level. It also provides a repeatable and comparable method of both predicting and specifying sound levels.

LINER NOTES

Lack of duct liner is another problem that can result in a noisy system. It's true this can be a problem in constant volume or VAV systems, as every elbow and pressure reduction fitting results in noise. The question is, which type of liner will work best? A VAV terminal can be lined 15 different ways, including foil-faced insulation and several varieties of double-wall construction, according to Int-Hout.

"The appropriate selection cannot be made without really understanding the acoustical and cost implications," he said. "While the liner type in a single duct box with little insulation does not have much acoustical effect, its impact on a large-series fan terminal can be as much as an increase of 12 NC points."

Int-Hout believes that one of the best ways to reduce noise is to use a little vinyl flex duct in conjunction with duct liner. "Vinyl flexible duct is magic for reducing noise. Frankly, three or four feet of flex duct can cut noise as much as 10 or 12 NC points. Metal flex has no acoustical properties, and wrapping the duct doesn't help either."

Some people are simply reluctant to use internal duct lining, said Paoli. "With VAV systems, you're going to have noise if you can't use duct liner at the discharge of the box. If liner isn't an option, it might be possible to put a sound attenuator in the discharge of the box. This is usually a good solution in a retrofit situation."

Then again, some believe that duct liner may not always be the answer. Peterman stated that duct liner may be sufficient for non-fan powered VAV terminal units that have been selected with low discharge sound power levels. "Duct liners can reduce mid--and high-frequency noise levels but need substantial length to be effective and do very little in the low frequency region where most HVAC noise complaints are focused."