VAV systems are different from constant volume systems in that they adjust the airflow to the occupied space based on the building's heating and cooling requirements. When less heating or cooling is needed, the fan system reduces the amount of air that is supplied to the space.
VAV systems rely on control dampers and often include distributed fan boxes for the various environmental control zones. The dampers can create airflow-generated noise, which can travel into the occupied space. The terminal units used to control the airflow into the zone are often located above the occupied space, and their casing-radiated and discharge sound levels can affect the amount of noise in the occupied space.
There will almost always be noise in the space when the terminal unit is sitting above the ceiling, but how much noise is too much? That depends on numerous factors, including the application involved, ceiling height, room sound levels, occupants, and time of day. Since it is very difficult to make an existing VAV system quiet, the best advice is to make sure noise levels will be appropriate for the application during the design phase.
Measuring Noise Levels
Noise in any HVAC system can be measured as a function of frequency in decibels, typically divided into eight octave bands. That result is then compared against a criteria curve, such as the noise criterion (NC) family of curves. Karl L. Peterman, P.E., acoustical engineer for Vibro-Acoustics, notes that ASHRAE Handbook - Applications 2003 gives some guidelines for various spaces based on their use, but not based on the system type."Private offices are typically designed for NC 30 to 35, while open offices are designed for NC 40 to 45. Experience has shown that sound levels in offices above NC 45 can start to create complaints," noted Peterman.
Dan Int-Hout, chief engineer at Krueger, believes that measuring average spectra, such as dBA, typically doesn't tell the whole story. "That data is meaningless to anyone in trying to solve the problem. If you want to know what's going on, you have to take sound level as a function of frequency or octave band, and that takes a spectrum analyzer. Then you can do diagnostics."
Having said that, Int-Hout also acknowledges that classroom specifications are written in dBA, because the research on which they were based was measured in dBA. The research, and the A-scale in particular, typically focuses on speech intelligibility. Speech happens predominantly in the mid-frequencies, so that's all a specification may be concerned with. "But if you take readings in dBA there may be a huge low frequency component happening that you don't know about, because dBA is a weighted average," he added.
Indeed, some VAV systems have other problems that cannot be addressed through comparing the level against the NC curves. These include rumbles, whines, and tonal noise problems, which are typically the result of some improper design, noted Peterman.
"If the box above the ceiling has a bad motor, you're going to get a motor hum. It's something that's going to be more of a tonal type of noise, so it'll be more annoying," said David G. Paoli, acoustical engineer with Shiner + Associates, Inc., Chicago. "Sometimes the system might hiss, which also causes complaints. These types of noises usually cause more complaints than general broadband sound."
How Much Is Too Much?
Noise is definitely subjective: What's objectionable to one person may be just fine to another. Occupants may be more aware of the noise in a VAV system simply because it causes changes in the airflow, and people notice that. Constant volume systems can be just as noisy, though their sound levels are constant.One way around this problem is to use a series fan box. These VAV boxes have a fan on the discharge unit that runs at a constant speed. "These boxes have been used for 20 years, but they're expensive and they may use more energy, so they're typically found in high-end applications. But they keep the noise constant," noted Int-Hout.
Designing a system correctly is really the only way to make sure a VAV system isn't too noisy. Proper location of VAV devices and proper selection of noise control elements will help the design to meet standard background sound level criteria. When noise control or proper selection has been neglected, then there will be a greater probability for noise problems.
"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."
He added that prefabricated duct silencers are effective elements that can substantially reduce the noise levels across the audible spectrum - especially in the lower frequency region. "Silencers can be supplied in lieu of duct liner at a cost savings and to meet noise criteria that may otherwise be unachievable," noted Peterman.
This may also be a good option when certain applications, such as hospitals, do not allow the use of duct liner due to potential microbial growth. As Paoli noted, "There are such products as hospital-grade duct silencers, which are basically standard silencers with a Tedlar liner to keep the fiberglass filling from entering the air stream. Packless silencers, which have no fiberglass fill, are also a possibility. They're designed with perforations in the silencer baffles to attenuate the sound."
How Much Is Too Little?
While noisy VAV systems are a common problem, another issue is when they are too quiet. This is especially true in open-plan offices, where people don't want co-workers to hear everything they're saying. Speech privacy, which is a condition in which you don't understand enough of what else is going on to be distracted by the conversation, is a desirable condition. To achieve speech privacy, a system should be at around NC 40, but in a VAV system, that noise isn't constant.Over the years, designers have used white noise sound-masking speaker systems in the ceiling in order to keep noise levels constant. With the advent of desktop computers, those speakers were less necessary, because the fans in the computers make constant (desirable) white noise. Today, however, more people are turning to laptops, which don't make as much noise. This may result in the need for noise-generating equipment again.
VAV systems may also be too quiet if the designer-specified ductwork, diffusers, and terminal boxes that are just too big. This results in higher thermal losses, as well, because the diffusers can't properly distribute air in the space. The only way to fix this speech privacy problem may be to install a sound-masking system.
Designers definitely need to pay attention to the noise levels written into a specification. "During the design phase of open-plan offices, we won't design to NC 30 or 35 because of the speech privacy issues. If we know the client wants background sound masking, we won't design a VAV system to meet an NC 35, because there's no need for it. The sound masking system will increase that level up to NC 40 anyway," said Paoli.
Consulting the proper standard - or hiring an acoustical engineer - is the best insurance a designer can have in making sure VAV systems make just the right amount of noise. ES
